Ecenomical Cruising Speed of a car

thegreatchimp

Does anyone have definitve figures on the ecenomical driving speeds of cars?

I drive a 1.6 litre ptrol mazda 323. I've been trying to save money on petrol by driving at a fuel efficient speed. However I've heard many different figures for this "magic" speed. Originally I was told 70mph (110kmh), but other people swear it is as low as 56mph (90kmh) and even 50mph (80km/h). That's a pretty massive variation.

I realise there's will be a variance based on engine size and power to weight ratio, but I would have thought that calculating this would be fairly straight forward; but amazingly I haven't been able to find anything definitive on the net. Not even road authorities and car manufacturers seem to know, and posts on forums are just as varied as the above figures. Surely there's some mechanic or engine expert out there that can shed light ont this...

Any replied appreciated.Thanks

Sean

johndaman66

In a way I think your sort of answering your own question. It very much depends on the car and the engine, load on the car, gradient of the road etc etc.

A car will have a sweet spot at which economy is maximised. Above this engine speed economy will be compromised but below this engine speed you may be labouring the engine, again compromising economy but also placing addional pressure on engine bearings and possibly other components.

I couldn't tell you the sweet spot of of a 1.6 Mazda 323 but certainly wouldn't think its as high as 70 mph. If its a naturally aspirated engine I'd hazard a guess of between 50 and 60 mph approx.

Mycroft H

55MPH in my Clio. It's been the same by ±5MPH in every other car I've driven. Bearing in mind I've not driven a 6sp yet.

Definitely not 70mph, when your at that speed most of the engines effort goes into defeating air resistance. Doubling speed quadruples drag.

CJhaughey

Aerodynamics plays a huge part at higher speeds, wind resistance increases exponentially.
At twice the speed the power required to overcome the drag is a multiple of 4.

MuppetCheck

I've played around with this to aid my boredon on the motorway. For some reason, over many journeys on my route 130kph is more economical than 120kph. Even at 120kph the engine isn't laboured (just over 2k revs). It is a diesel though.

Nissan doctor

As a VERY basic rule of thumb, its normally AROUND 100kph as this is in and around the national speed limit of most countries so most manufacturers design the gearing and fuel maps around this. Also, above 100kph you start getting into the issues of wind resistance and aerodynamics etc etc.

But really, as stated above, there is no one specific 'speed' that is most economical, it depends on far too many factors. Its your control of the throttle and the application of engine load that will most aid fuel economy, not driving at a certain speed. E.G, driving down a hill at 120kph while barely touching the throttle will be vastly more economical then going up a hill at 80kph with your foot on the floor.

CiniO

Nissan doctor wrote: »

As a very basic rule of thumb, its normally around 100kph as this is in and around the national speed limit of most countries so most manufacturers design the gearing and fuel maps around this. Also, above 100kph you start getting into the issues of wind resistance and aerodynamics etc etc.

I've never had or probably even driven a car that would be more economical during constant run at 100km/h than at 80km/h.

In my Mazda 6 2.0 it seems that best fuel economy is on 5th gear at about 70 to 80km/h. Above that fuel economy goes down a bit, while over 140 km/h it starts literally sucking fuel. AFAIR at 200km/h its consuming about 18 l/100km which is about 15 MPG.
Also at lower speed fuel economy suffers, as below 70 rpm on fifth gear fall too low, and fuel consumption increses again, while obviously if you reduce to 4th, fuel economy is going to suffer as well.

So keeping it between 70 and 80 km/h seems to be the most economical option, and I have very similar experience with most other cars.

Mycroft H

There probably is cars out there Clino, I've not come across them but I'd be thinking along the lines of a 4L TDI A8 with a 6/7sp gearbox.

deckie27

My wife's car only looses about 5mpg between about 110k and 160k

CiniO

deckie27 wrote: »

My wife's car only looses about 5mpg between about 110k and 160k

What car is it?

bladespin

Often got 50mpg ish in the Alfa by keeping it around 50mph on long runs (was torture though).

deckie27

CiniO wrote: »

deckie27 wrote: »

My wife's car only looses about 5mpg between about 110k and 160k

What car is it?

Saab 95, petrol

bladespin

Great bangernomics cars now too

mickdw

Going by the instant fuel useage display giving litres / 100km, my car is happiest at 45 to 50 mph. Thats a 6 speed 1.8T petrol.

Yakuza

My current car (Qashqai+2) has an optimal speed around 100kph (1.6 petrol, 5 gears), the previous one (same make and model, 1.5 daysul and 6 gears) had a sweet spot somewhere between 100 and 110 kph. Both are fairly underpowered. Having driven several hired E92 320d and a 120d, they seem to do well up to 120kph.

CiniO

Yakuza wrote: »

My current car (Qashqai+2) has an optimal speed around 100kph (1.6 petrol, 5 gears), the previous one (same make and model, 1.5 daysul and 6 gears) had a sweet spot somewhere between 100 and 110 kph. Both are fairly underpowered. Having driven several hired E92 320d and a 120d, they seem to do well up to 120kph.

Optimal in what sense?
Are you trying to say, that your Qashqai will use less fuel if you travel steadily at 100km/h than if you were travelling at 80km/h?

vibe666

i would have imagined that (assuming no wind and a flat, straight road) the lowest speed you can maintain in your highest gear without labouring the engine would generally be the most economical?

CJhaughey

I think its more likely to be the point at which the torque curve intersects with the drag curve.
Probably 50mph for most cars and motorcycles.

Sesshoumaru

In the Leaf I can see real time energy usage, 85 to 90 kph is the energy efficient sweet spot for this car. Any higher and air resistance adds a lot to energy consumption. I don't think air resistance will differ much for most cars, so I think 90kph should be your cruising speed if you want to save as much fuel as possible.

unkel

vibe666 wrote: »

i would have imagined that (assuming no wind and a flat, straight road) the lowest speed you can maintain in your highest gear without labouring the engine would generally be the most economical?

The lowest speed in the highest gear would use less fuel than any speed higher than that. Per hour. Not per mile

anthony4335

deckie27 wrote: »

Saab 95, petrol

From 28mpg to 23mpg!

Yakuza

CiniO wrote: »

Optimal in what sense?
Are you trying to say, that your Qashqai will use less fuel if you travel steadily at 100km/h than if you were travelling at 80km/h?

Not at all. When I say optimal I mean speed (and inversely, journey time) versus fuel consumed. Basically, after roughly 100kph in my current car, my consumption starts to increase at a greater percentage than increase in speed (I say roughly, depending on whether it's just me in the car, or the whole family / luggage etc). Yes, at 80kph consumption is better than at 100kph but I don't want to spend all day driving!.

If a 10% increase in speed results in a 15% increase in consumption (going on the instantaneous readout), then I'll go back to the original speed (unless pressed for time!). It may be a bit nerdy (and meaningless in terms of real fuel savings ) but it keeps the auld brain active on my long drives down the M5 / M6 through the UK.

deman

The sweet spot in my '05 Primera 1.6 is somewhere between 95-100 kmph. It's easy to see on the dash display.

monkeybutter

thegreatchimp wrote: »

Does anyone have definitve figures on the ecenomical driving speeds of cars?

I drive a 1.6 litre ptrol mazda 323. I've been trying to save money on petrol by driving at a fuel efficient speed. However I've heard many different figures for this "magic" speed. Originally I was told 70mph (110kmh), but other people swear it is as low as 56mph (90kmh) and even 50mph (80km/h). That's a pretty massive variation.

I realise there's will be a variance based on engine size and power to weight ratio, but I would have thought that calculating this would be fairly straight forward; but amazingly I haven't been able to find anything definitive on the net. Not even road authorities and car manufacturers seem to know, and posts on forums are just as varied as the above figures. Surely there's some mechanic or engine expert out there that can shed light ont this...

Any replied appreciated.Thanks

Sean

Generally the higher the revs the more fuel is used, but you need to factor this against a longer journey. Acceleration is a killer and I doubt the Mazda is super efficient (older models anyway) at cutting back usage to zero etc when slowing down or breaking.

Having tested this rigorously, it is indeed 100k on the old Mazda 323, BH generation.

To go from 100k to circa 120k I need to up my average revs from 3200 to 4000, and this has roughly a 20-25% change in my fuel consumption across the same trip. This would be dublin to cork. So I can up the mpg from about 31 to 38.

Doing 120k would also usually mean more breaking and more acceleration over taking people etc. Whereas at 100k, I am mostly the one being overtaken

So 380-400 miles (if you are don't do any breaking) on a 10 gallon tank. So up and down on one tank or 76 euro.

The same trip takes roughly 30 mins longer, but the trip is more relaxed as I'd be chillin like Matt Dillon on Penicillin and possibly safer as I am going at slower speed and doing less lane changes.

If you can tuck in a mini convoy of cars it should also help economy but, at a sensible distance. Harder to measure this.

Zubeneschamali

monkeybutter wrote: »

Generally the higher the revs the more fuel is used, but you need to factor this against a longer journey.

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time. If you are after mpg, 100 kph is too fast in just about any car.

I'd bet 80kph will be better in every car that anyone on boards drives.

CiniO

Yakuza wrote: »

Not at all. When I say optimal I mean speed (and inversely, journey time) versus fuel consumed. Basically, after roughly 100kph in my current car, my consumption starts to increase at a greater percentage than increase in speed (I say roughly, depending on whether it's just me in the car, or the whole family / luggage etc). Yes, at 80kph consumption is better than at 100kph but I don't want to spend all day driving!.

One note here, as I think not many people realise.
Extra weight in the car obviously adds to the fuel consumption, but it differs a lot on driving conditions.

F.e in city runs where there is plenty of acceleratin and braking, it can make really big difference if you are traveling alone, or 5 people on board with planty of luggage. All because it's extra weight to accelerate and therefore extra energy needed.
Also when traveling in mountain areas, it need way more energy to push up the car up the hill if it's loaded comparing to if it was empty.

But at steady motorways speed on flat area (no mountain or hills) car load makes very little difference. As to keep constat speed, force which engine is producing must equal resistance forces. Air resistance remains the same no matter what weight you are carrying (and air resistance is the major force at motorways speeds that engine must work agains). Only what is increased if weight is bigger, is the rolling resistance, but at motorways speed that make really small difference.

So in general - if you load up your car with heavy weight to maximum, expect big MPG drop in the city or if driving in mountain regsion, while at motorways if you drive steadily in oppose to accelerating and braking all the time, MPG should remain nearly unchanged.

Top Dog

Zubeneschamali wrote: »

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time. If you are after mpg, 100 kph is too fast in just about any car.

I'd bet 80kph will be better in every car that anyone on boards drives.

I'd disagree with you based on experience. Our last car, a Laguna II 1.9dCI with the 6-speed manual returned an average of 50.2mpg when averaging 65mph compared with 47.6mpg when averaging 55mph (engine almost labouring in 6th where it was revving a little higher than you'd want in 5th for best economy). It also did the same 47ish when cruising at an average of 70mph.

CiniO

Top Dog wrote: »

I'd disagree with you based on experience. Our last car, a Laguna II 1.9dCI with the 6-speed manual returned an average of 50.2mpg when averaging 65mph compared with 47.6mpg when averaging 55mph (engine almost labouring in 6th where it was revving a little higher than you'd want in 5th for best economy). It also did the same 47ish when cruising at an average of 70mph.

Did you try what MPG would you get on 5th gear at 50mph?

Top Dog

CiniO wrote: »

Did you try what MPG would you get on 5th gear at 50mph?

Afraid the only time I did that on an extended motorway journey was with a caravan so the figures wouldn't be of any use.

Wetbench4

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

CiniO

Wetbench4 wrote: »

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

I think you didn't give a think about what you just said.

Wetbench4

CiniO wrote: »

I think you didn't give a think about what you just said.

I did, trust me. :rolleyes:

Zubeneschamali

Wetbench4 wrote: »

I did, trust me. :rolleyes:

Well, the definition of better miles per gallon means you go more miles on a gallon. Speed, gearing etc. can't change the fact that more miles per gallon means less gallons per mile.

Wetbench4

Zubeneschamali wrote: »

Speed, gearing etc. can't change the fact that more miles per gallon means less gallons per mile.

Isn't this why we have a gearbox? Like the before posters said, there is a rpm sweet spot (in top gear usually) in which you are covering the most miles per gallon. Anything above or below that is not as energy efficient as the sweet spot.

edit: also this would be different on every car and even different journeys in the same car due to the huge amount of variables.

CiniO

Wetbench4 wrote: »

I think the confusion is the fact that just because your're using more fuel doesn't mean its less economical. Obviously a car will use more fuel at 120kph compared to 100kph, but that same car might use less fuel to cover 1 mile/km at 120kph than at 100kph because of gearing and torque etc.

I did, trust me.

It's impossible that car will use more fuel at 120km/h than 100km/h and at the same time will use less fuel at 120km/h to cover 1mile than at 100km/h.

Because that's pretty much what you just said.

In that case, it will use more fuel at 120km/h than at 100km/h per what?
Per minute?
You don't calculate usage per minute.
All units we use for measuring fuel consumption refers to usage per distance. (MPG, l/100km, etc).

Wetbench4

CiniO wrote: »

It's impossible that car will use more fuel at 120km/h than 100km/h and at the same time will use less fuel at 120km/h to cover 1mile than at 100km/h.

Because that's pretty much what you just said.

In that case, it will use more fuel at 120km/h than at 100km/h per what?
Per minute?
You don't calculate usage per minute.
All units we use for measuring fuel consumption refers to usage per distance. (MPG, l/100km, etc).

Its using fuel faster but is covering the ground faster and more efficiently then it would if it were going 100.

CiniO

Wetbench4 wrote: »

Isn't this why we have a gearbox? Like the before posters said, there is a rpm sweet spot (in top gear usually) in which you are covering the most miles per gallon. Anything above or below that is not as energy efficient as the sweet spot.

edit: also this would be different on every car and even different journeys in the same car due to the huge amount of variables.

There obviously is a sweet spot for most efficient RPM, but it's only small bit of the whole thing.

Take notice, that if you are parked in neutral, to keep car at 2500rpm you need to open throttle only a tiny bit (press gas pedal only small bit).
But if you want to keep that 2500rpm in 6th gear at high spead on incline you might have your gas pedal pressed nearly to the floor.

Surely consumption will differ greatly between those 2 cases.

Also take notice, that air resistance increases squarely to speed, so by accelerating from 100 to 120 (increasing speed only by 20%) air resistance will rise by 44%. That really makes a difference.

CiniO

Wetbench4 wrote: »

Its using fuel faster but is covering the ground faster and more efficiently then it would if it were going 100.

So you are saying, that at 120km/h fuel consumption per unit of time is bigger than at 100km/h, which is actually correct, but you indicate then, that at 120km/h you might do more distance with the same amount of fuel used, which means in other words that your MPG is lower.
No - trust me - I hardly believe there exist a car which is more economical to run at 120km/h than at 100km/h. And even if there is one, no one from boards.ie has one

Wetbench4

CiniO wrote: »

So you are saying, that at 120km/h fuel consumption per unit of time is bigger than at 100km/h, which is actually correct, but you indicate then, that at 120km/h you might do more distance with the same amount of fuel used, which means in other words that your MPG is lower.
No - trust me - I hardly believe there exist a car which is more economical to run at 120km/h than at 100km/h. And even if there is one, no one from boards.ie has one

Its down to where the torque and horsepower, of the engine peak. This is the most efficient rpm that the engine runs at. My original point is that because of differences in gearing etc, this is different in each model. Anything above or below this sweet spot is less efficient. Sorry, i must be crap at explaining stuff but this makes perfect sense to me. I will try and find something to back me up.

Wetbench4

Taken from wikipedia so its still not a great backmeup link but still:

Speed and fuel economy studies


1997 fuel economy statistics for various US models
Fuel economy at steady speeds with selected vehicles was studied in 2010. The most recent study[17] indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better mileage at 65 mph (105 km/h) rather than at 45 mph (72 km/h),[17] although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 miles per hour (89 km/h) (29.1 mpg-US (8.08 l/100 km)), and gets 2 mpg better economy at 65 mph (105 km/h) than at 45 mph (72 km/h) (25 mpg-US (9.4 l/100 km) vs 23 mpg-US (10 l/100 km)). The proportion of driving on high speed roadways varies from 4% in Ireland to 41% in Netherlands.
When the National Maximum Speed Law's 55 mph (89 km/h) speed limit was mandated, there were complaints that fuel economy could decrease instead of increase. The 1997 Toyota Celica got 1 mpg better fuel-efficiency at 65 mph (105 km/h) than it did at 45 mph (72 km/h) (43.5 mpg-US (5.41 l/100 km) vs 42.5 mpg-US (5.53 l/100 km)), although almost 5 mpg better at 60 mph (97 km/h) than at 65 mph (105 km/h) (48.4 mpg-US (4.86 l/100 km) vs 43.5 mpg-US (5.41 l/100 km)), and its best economy (52.6 mpg-US (4.47 l/100 km)) at only 25 mph (40 km/h). Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph (89 km/h) vs. 65 mph (105 km/h). Their best economy was reached at speeds of 25 to 55 mph (40 to 89 km/h) (see graph).[17]
Officials hoped that the 55 mph limit, combined with a ban on ornamental lighting, no gasoline sales on Sunday, and a 15% cut in gasoline production, would reduce total gas consumption by 200,000 barrels a day, representing a 2.2% drop from annualized 1973 gasoline consumption levels.[18][a] This was partly based this on a belief that cars achieve maximum efficiency between 40 and 50 mph (64 and 80 km/h) and that trucks and buses were most efficient at 55 mph (89 km/h).[20]
However, the United States Department of Transportation's Office of Driver Research found total fuel savings of the 55 mph limit to be 1%, and independent studies found a 0.5% savings.[21]

One of example given above is the 97 celica which has two efficiency peaks on the graph and is more efficient at 105kph than at 72kph.


Here is the full wiki page: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles

unkel

Wetbench4 wrote: »

Its down to where the torque and horsepower, of the engine peak. This is the most efficient rpm that the engine runs at.

That engine and where it is most efficient is only half of the equation. The engine is in a car which suffers air resistance when on the move. That's the other half of the equation.

If air resistance was the same at 120km/h as at 80km/h then a typical car might well be more efficient at 120km/h

But it isn't. Air resistance at 120km/h is more than twice as high compared to 80km/h

Wetbench4

unkel wrote: »

That engine and where it is most efficient is only half of the equation. The engine is in a car which suffers air resistance when on the move. That's the other half of the equation.

If air resistance was the same at 120km/h as at 80km/h then a typical car might well be more efficient at 120km/h

But it isn't. Air resistance at 120km/h is more than twice as high compared to 80km/h

Yes i agree my 100/120kph examples were bad because of drag but at lower speeds 80/100kph it makes sense.

CiniO

Wetbench4 wrote: »

Taken from wikipedia so its still not a great backmeup link but still:





One of example given above is the 97 celica which has two efficiency peaks on the graph and is more efficient at 105kph than at 72kph.


Here is the full wiki page: http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles

Sorry, but according to your graph it's opposide. Celica is tiny bit more efficient at 72km/h than 105km/h.

CiniO

Wetbench4 wrote: »

Yes i agree my 100/120kph examples were bad because of drag but at lower speeds 80/100kph it makes sense.

Indeed some vehicles out of this graph, seems to be more economical at speeds 90-95 km/h than at 70-80km/h.

Wetbench4

CiniO wrote: »

Sorry, but according to your graph it's opposide. Celica is tiny bit more efficient at 72km/h than 105km/h.

No its mpg, the higher the number the better. Are you looking at the right one, its the light blue line at the top.

edit: although id say its american gallons but im not sure.

And i see what your saying now, it should 60mph v 40mph

CiniO

Wetbench4 wrote: »

No its mpg, the higher the number the better. Are you looking at the right one, its the light blue line at the top.

edit: although id say its american gallons but im not sure.

And i see what your saying now, it should 60mph v 40mph

Ahh that's right.
60mph seems to be more economical for that celica than 40mph.
But 105km/h (65mph) is not more economical than 72km/h (45mph), as least according to the graph.

Wetbench4

CiniO wrote: »

Ahh that's right.
60mph seems to be more economical for that celica than 40mph.
But 105km/h (65mph) is not more economical than 72km/h (45mph), as least according to the graph.

Ya i didn't spot that, i just took the numbers from the article. Its interesting stuff though. The jeep cherokee is only about 6mpg lower at 70 than at 40.

Seweryn

Zubeneschamali wrote: »

No, the OP is asking what speed to drive for the best MPG, they're not asking to balance mpg against time.

That is what I got as well.

And there is no family type car out there that uses less fuel at 90km/h that it does at 70km/h. Laws of physics are simple and if the car has 17 gears, just drop it to keep the engine speed at "cruising minimum".