What's the best way to manually check a cars mpg?

Stallingrad

Not sure I trust the computer in the car, so what is the best way to manually check a cars mpg/litres per 100?

Hersheys

Wait until the tank is empty.

Put in a set/known amount of fuel.

Reset milometer.

When your tank empties again you'll have an idea of how many miles you've travelled per litre of petrol.

recyclebin

Hersheys wrote: »

Wait until the tank is empty.

Put in a set/known amount of fuel.

Reset milometer.

When your tank empties again you'll have an idea of how many miles you've travelled per litre of petrol.

Do this a few times as you will never know exactly how empty or full the tank is. The more times you do it the smaller the margin of error.

This is assuming the petrol pumps you use are calibrated.

Yakuza

Never let your tank go low!!!, you'll get crud from the bottom of the tank in the engine.
1) Brim the car (fill it up until you can't fill any more (you should be able to see fuel just below the rim of where the nozzle goes in).
2) Reset your trip clock.
3) Drive a few hundred miles.
4) Brim the car again. Note how many litres you buy this time to fill the car. This is a reasonably good estimate for the fuel you've consumed.
5) As you've asked for mpg divide the miles showing on your trip clock (assuming it's in miles) by (the number of litres you bought divided by 4.544) (this will give you miles per gallon).
5a) MPG = (distance)/(litres bought/4.544) or (distance/litres bought) * 4.544 - same thing

6) Do steps 1-5 again - differerent driving patterns (city / country / motorway) will give different figures.

mik_da_man

Fill the car
Set trip computer to zero
Next time u fill take note of distance travelled, divide by the amount taken to fill

Simples.

vincenzolorenzo

This way is fairly inaccurate unless you drive until the car actually runs out of petrol. The method i use is:

1. Fill thetank right to the brim. Reset odometer.
2. Drive until tank is fairly empty, doesn't have to be as far as the light coming on.
3. Fill the tank right to the brim again. Record number of miles on clock and litres of fuel to fill.
4. Use the formula ( 4.546 * Miles ) / Litres to get mpg

Terrontress

Make sure your mileometer is correctly calibrated before you do this.

650Ginge

Hersheys wrote: »

Wait until the tank is empty.

Put in a set/known amount of fuel.

Reset milometer.

When your tank empties again you'll have an idea of how many miles you've travelled per litre of petrol.

That is a very unreliable method.

Fill the tank reset the trip drive refill and work it out. Repeat at least 10 times to get better average. Once is not enough.

record all fills on fuelly.com.

Stainless_Steel

Terrontress wrote: »

Make sure your mileometer is correctly calibrated before you do this.

I wouldn't go that far.

Mint Sauce

Fill the car when the light comes on
Make a note of how much put in
Reset trip computer
Do maths just before filling again

Ie

It took 47 L to fill my car last time, by the time the red light came again it had done 650km. 650/47=13.82 km/litre

or if prefer in miles/gallons

1 UK Gallon = 4.54 litres
1 mile = 1.61 km

403.89/10.34=38.97 miles/gallon

More times you do this, the more accurate the result.

Henry Ford III

irish-stew wrote: »

Fill the car when the light comes on
Make a note of how much put in
Reset trip computer
Do maths just before filling again

Ie

It took 47 L to fill my car last time, by the time the red light came again it had done 650km. 650/47=13.82 km/litre

or if prefer in miles/gallons

1 UK Gallon = 4.54 litres
1 mile = 1.61 km

403.89/10.34=38.97 miles/gallon

More times you do this, the more accurate the result.

How empty or otherwise the tank is before you fill the car isn't important. Just ensure it's full.

Drive away, and refill. You'll know how many litres have been put in the tank, and you'll know the miles travelled.

From there it's simple maths.

Mint Sauce

Henry Ford III wrote: »

How empty or otherwise the tank is before you fill the car isn't important. Just ensure it's full.

Drive away, and refill. You'll know how many litres have been put in the tank, and you'll know the miles travelled.

From there it's simple maths.

Probably not, but would have thought the more in the tank and the more distance travelled, add to that putting the same in everytime, ie when the light comes on, would give more accurate results.

Stallingrad

I had a feeling there would be more than one way to skin this cat.

SilverBell

irish-stew wrote: »

Probably not, but would have thought the more in the tank and the more distance travelled, add to that putting the same in everytime, ie when the light comes on, would give more accurate results.

When the light comes on, you might not be anywhere near a petrol station though. So you might have to drive another bit with the light on.

Also, them sender units could have a bit of leeway, they may not come on at the exact same point each time. You are never really sure how much is left in the tank at any time.

If you brim the tank each time you will be most sure about the amount you are putting into the car.

challengemaster

Stallingrad wrote: »

I had a feeling there would be more than one way to skin this cat.

There's more than one, just any saying run the car nearly dry is idiotic and unnecessary.

slimjimmc

Yakuza wrote: »

Never let your tank go low!!!, you'll get crud from the bottom of the tank in the engine.

Considering the fuel outlet is located at the bottom of the tank where all that crud is lying can you explain why the crud doesn't get sucked up with a full tank but does with a low one. Perhaps it's just an urban myth?

Yakuza wrote: »

1) Brim the car (fill it up until you can't fill any more (you should be able to see fuel just below the rim of where the nozzle goes in).
2) Reset your trip clock.
3) Drive a few hundred miles.
4) Brim the car again. Note how many litres you buy this time to fill the car. This is a reasonably good estimate for the fuel you've consumed.
5) As you've asked for mpg divide the miles showing on your trip clock (assuming it's in miles) by the number of litres you bought. (This will give miles per litre) and divide that figure by 4.544 (this will give you miles per gallon).

6) Do steps 1-5 again - differerent driving patterns (city / country / motorway) will give different figures.

Agreed.

Terrontress

slimjimmc wrote: »

Yakuza wrote: »

Never let your tank go low!!!, you'll get crud from the bottom of the tank in the engine.

Considering the fuel outlet is located at the bottom of the tank where all that crud is lying can you explain why the crud doesn't get sucked up with a full tank but does with a low one. Perhaps it's just an urban myth?

Yakuza wrote: »

1) Brim the car (fill it up until you can't fill any more (you should be able to see fuel just below the rim of where the nozzle goes in).
2) Reset your trip clock.
3) Drive a few hundred miles.
4) Brim the car again. Note how many litres you buy this time to fill the car. This is a reasonably good estimate for the fuel you've consumed.
5) As you've asked for mpg divide the miles showing on your trip clock (assuming it's in miles) by the number of litres you bought. (This will give miles per litre) and divide that figure by 4.544 (this will give you miles per gallon).

6) Do steps 1-5 again - differerent driving patterns (city / country / motorway) will give different figures.

Agreed.

I had heard that as a car runs out of petrol, the exhaust sucks momentarily as the engine splutters, sucking dirt off the road.

It could well be urban myth.

SilverBell

slimjimmc wrote: »

Considering the fuel outlet is located at the bottom of the tank where all that crud is lying can you explain why the crud doesn't get sucked up with a full tank but does with a low one. Perhaps it's just an urban myth?

Yep!! I always wondered the same about that story, how come it gets past the filter if the tank is low, but wont if the tank is full?

Either in most tanks the tank outlet is slightly above the lowest point so the level of crud is independent of how much fuel is in the tank.

dredg

I use an app on my iphone called road trip. Every time I fill the tank I record how many litres I put in and the mileage at the time of filling and the price of fuel. It displays graphs of mpg and historical price.

Mint Sauce

SilverBell wrote: »

When the light comes on, you might not be anywhere near a petrol station though. So you might have to drive another bit with the light on.

Also, them sender units could have a bit of leeway, they may not come on at the exact same point each time. You are never really sure how much is left in the tank at any time.

If you brim the tank each time you will be most sure about the amount you are putting into the car.

If I have a long drive coming up, I would never risk going to the Red Light. If just driving around town I might.

However, like working out the MPG on a car, by filling the car when the red light comes on a few times until I get the click in the pump, I have estimated the amount left in the tank, by the amount it took to fill it again.

Yakuza

slimjimmc wrote: »

Considering the fuel outlet is located at the bottom of the tank where all that crud is lying can you explain why the crud doesn't get sucked up with a full tank but does with a low one. Perhaps it's just an urban myth?

It's what I learned from my dad and he from his dad before him!
I never really questioned it, to be honest. I'd be fairly certain that the received wisdom is there'd be some impurities in petrol (dodgy diesel notwithstanding), which would precipitate out as sediment in the bottom of the petrol tank. Sucking the last of the petrol into the fuel line will bring in some of this sediment, clogging the filter (ok, so not the engine) and will also put a strain on the fuel pump.

coolbeans

irish-stew wrote: »

SilverBell wrote: »

When the light comes on, you might not be anywhere near a petrol station though. So you might have to drive another bit with the light on.

Also, them sender units could have a bit of leeway, they may not come on at the exact same point each time. You are never really sure how much is left in the tank at any time.

If you brim the tank each time you will be most sure about the amount you are putting into the car.

However, like working out the MPG on a car, by filling the car when the red light comes on a few times until I get the click in the pump, I have estimated the amount left in the tank, by the amount it took to fill it again.

Sounds like guesswork to me. The only way I know how to do it is ky brimming tank, driving X kms, refilling and noting litres required for refill and distance covered. Do this a few times over what you would consider normal routes and there's your figure.

Fuel lights come on at various times and petrol pumps click due to variations in air pressure as well as a full fuel tank. again, this won't be consistent and is not a satisfactory way do it at all.