Better mpg when tank is filled from 1/2 full

Sam Kade

After many readings I always get better mpg when I keep the tank above half full. I always fill the tank and calculate mpg when refilling. I drive a mazda 6 1.8 petrol if I let the tank empty to the amber light I usually get between 36-40 mpg. If I fill it from half to 3/4 full I get 42-45 mpg Anyone know the reason for this?

jodaw

Is that based on (1) trip computer readings or based on (2) manual calculation based on brimming the tank and resetting the trip computer?

If (1) above maybe the computer is little out since the first quarter of the tank normally takes a little longer to use when the tank is brimmed.

If (2) then it does not make sense. The will be less weight in the car with less fuel so the MPG should not go down assuming all other things are equal. The extra MPG because of less fuel would probably be negligable.

Qualitymark

Does it work for cyclists? If I cycle when I'm half tanked do I get there sooner?

dmc17

Qualitymark wrote: »

Does it work for cyclists? If I cycle when I'm half tanked do I get there sooner?

No, you don't get there at all.

Sam Kade

jodaw wrote: »

Is that based on (1) trip computer readings or based on (2) manual calculation based on brimming the tank and resetting the trip computer?

If (1) above maybe the computer is little out since the first quarter of the tank normally takes a little longer to use when the tank is brimmed.

If (2) then it does not make sense. The will be less weight in the car with less fuel so the MPG should not go up assuming all other things are equal. The extra MPG because of less fuel would probably be negligable.

Always manual calculation as my car doesn't have a trip computer. I read somewhere that the petrol evaporates when the tank is nearly empty. As I said I have tried it out many times not just a once off calculation.

Sam Kade

Qualitymark wrote: »

Does it work for cyclists? If I cycle when I'm half tanked do I get there sooner?

Only one way for you to find out

No Pants

I had a 1.8 petrol Mazda 6. The top "half" of the tank according to the gauge was larger than the lower "half".

TheChizler

You'll only be able to get an accurate reading by brimming, how are you figuring out the consumption when you keep it below half?

Sam Kade

No Pants wrote: »

I had a 1.8 petrol Mazda 6. The top "half" of the tank according to the gauge was larger than the lower "half".

But I am not going by the gauge, I know that the first half lasts longer but that is irrelevant in this case. Manual calculation is proving better mpg when the tank isn't left go below half before refilling.

TheChizler

Sam Kade wrote: »

But I am not going by the gauge, I know that the first half lasts longer but that is irrelevant in this case. Manual calculation is proving better mpg when the tank isn't left go below half before refilling.

If you're gong by the petrol light then you're going by the gauge, which you're never going to get an accurate reading from.

Sam Kade

TheChizler wrote: »

You'll only be able to get an accurate reading by brimming, how are you figuring out the consumption when you keep it below half?

It's brimmed from half, I keep it full not half full. I also tried it to the amber then fill and always get better mpg when I don't let it go below half full.

Truckermal

What happens if you are pessimistic because then your tank will be half empty..:pac:

Sam Kade

TheChizler wrote: »

If you're gong by the petrol light then you're going by the gauge, which you're never going to get an accurate reading from.

I use the most trusted method brim the tank reset trip meter then calculate mpg when refilling.

Toyotafanboi

i think the first half last longer because if you're like me when the pump clicks out the first time to tell you the tank is full, you still persist and get like another €2 in there until it is nearly spilling out onto your shoes by filling the filler neck etc too. i find my car does like 60-80miles before the needle leaves full, then i may only get say 120 miles out of the next indicated full quarter, i always put this down to the extra litre or three in the filler neck.

Sam Kade

Toyotafanboi wrote: »

i think the first half last longer because if you're like me when the pump clicks out the first time to tell you the tank is full, you still persist and get like another €2 in there until it is nearly spilling out onto your shoes by filling the filler neck etc too. i find my car does like 60-80miles before the needle leaves full, then i may only get say 120 miles out of the next indicated full quarter, i always put this down to the extra litre or three in the filler neck.

That's only relevant when using the gauge which I don't so the extra petrol in the neck of the tank is still accounted for.

jodaw

You are brimming it so you are always getting accurate MPG. So in reality I have no clue how you are getting such a difference by keeping the tank half full.

In reality you should get more MPG by running to empty but i imagine a very tiny amount.

I have always brimmed my tank for convenience rather than anything but out of interest I am now testing keeping it as low as i can for the next few months. I brimmed it once and let it run to almost empty. I will now add in 20 euros and a time for the next while and then brim it again. I am expecting a slight increase in MPG over that time but i will wait and see.

I assume you are always using same garage/fuel and fill to first click?

Renegade Mechanic

Simple rule here: The less fuel you have in your tank, the more mpg you get. I have a 70 ltr tank so thats about 70kgs of fuel when full. Half a tank is only 35kgs so I now have 35kg less to haul around. It does make a bit of a difference alright

jodaw

Renegade Mechanic wrote: »

Simple rule here: The less fuel you have in your tank, the more mpg you get. I have a 70 ltr tank so thats about 70kgs of fuel when full. Half a tank is only 35kgs so I now have 35kg less to haul around. It does make a bit of a difference alright

Yes you would think but the OP is saying the opposite. It's a head scratcher since they are using the most accurate way to measure MPG.

Sam Kade

Renegade Mechanic wrote: »

Simple rule here: The less fuel you have in your tank, the more mpg you get. I have a 70 ltr tank so thats about 70kgs of fuel when full. Half a tank is only 35kgs so I now have 35kg less to haul around. It does make a bit of a difference alright

My car seems to be the exception to the rule

jodaw

Sam Kade wrote: »

My car seems to be the exception to the rule

Are you driving the same trips and using the same driving style? Do you regularly check tyre pressure?

dmc17

When you top up from 3/4 full you're not going to get as many miles as you would if you topped up from near empty. Therefore your calculations are most likely going to based on a different range of driving conditions and there will be variations as a result.

mickdw

I cannot think of any logical reason why you would get better mpg when brimming the tank.
Can you give a more detailed explanation of exactly the procedure you use in both cases to get mpg?

Are you buying fuel in same place in both cases?

Sam Kade

jodaw wrote: »

Are you driving the same trips and using the same driving style? Do you regularly check tyre pressure?

Same trips and same driving style. Yes tyre pressure kept correct and serviced on time as I diy.

Renegade Mechanic

Sam Kade wrote: »

My car seems to be the exception to the rule

My apologies, I read your op wrong
Now that is strange alright

jodaw

Sam Kade wrote: »

Same trips and same driving style. Yes tyre pressure kept correct and serviced on time as I diy.

Sounds legit. Think you may have a car that can defy physics;). Have you checked is there a button that can allow it to fly?

Sam Kade

dmc17 wrote: »

When you top up from 3/4 full you're not going to get as many miles as you would if you topped up from near empty. Therefore your calculations are most likely going to based on a different range of driving conditions and there will be variations as a result.

I get around 200 miles from full to 3/4 so there should be plenty variation there.

TheChizler

mickdw wrote: »

I cannot think of any logical reason why you would get better mpg when brimming the tank.
Can you give a more detailed explanation of exactly the procedure you use in both cases to get mpg?

Are you buying fuel in same place in both cases?

Can you explain exactly your testing procedure? And roughly how many times you've done it?

Sam Kade

mickdw wrote: »

I cannot think of any logical reason why you would get better mpg when brimming the tank.
Can you give a more detailed explanation of exactly the procedure you use in both cases to get mpg?

Are you buying fuel in same place in both cases?

Fuel is bought in the same place, I always brim the tank only difference is that I sometimes fill from 1/2 to 3/4 full to brim other times from amber light to brim. I only use one calculation brim set trip meter get reading at next fill and convert litres to gallons L/4.45, miles/gallons.

dmc17

Sam Kade wrote: »

I get around 200 miles from full to 3/4 so there should be plenty variation there.

Plenty, but not as much as from near empty, thus there will be variations

Sam Kade

dmc17 wrote: »

Plenty, but not as much as from near empty, thus there will be variations

But if I go from 3/4 to brim drive 200 miles then brim again and drive another 200 miles etc. there will be plenty variation same as going from full to empty.

dmc17

Sam Kade wrote: »

But if I go from 3/4 to brim drive 200 miles then brim again and drive another 200 miles etc. there will be plenty variation same as going from full to empty.

The bigger the range of miles you measure the mpg over, the more accurate it will be. Therefore, if you measure it over a full tank you are more likely to get a more true reading than measuring it over the first 1/4 of the tank.

Sam Kade

jodaw wrote: »

Sounds legit. Think you may have a car that can defy physics;). Have you checked is there a button that can allow it to fly?

TheChizler

When you brim it from 3/4 full are you planning on going on longer journeys and so fill it sooner? If you then go on long journeys which are probably at higher speeds you'll get better efficiency.

jodaw

dmc17 wrote: »

The bigger the range of miles you measure the mpg over, the more accurate it will be. Therefore, if you measure it over a full tank you are more likely to get a more true reading than measuring it over the first 1/4 of the tank.

Yes but if he is driving the same trips and same driving style to get a definitive answer he would drive for maybe 2000miles and when thank gets to 3/4 refill. Get the average MPG over 2000miles.

Then do exactly the same but allow tank to go to empty and refill. Measure average over 2000miles. This should give an accurate comparsion over large mileage.

If he plots a chart for each test then that chart should trend up when refilling from 3/4 and trend down when filling from empty. Logic states the opposite. If the chart is all over the place then there is something causing variations.

COYVB

I think the answer to the question is perception rather than reality. The top "half" of the tank in this case clearly holds more fuel than the bottom "half", but the fuel indicator fails to factor this in.

It's not providing more mpg at all, it's just reporting that it's using less fuel than it actually is for that section of the tank

jodaw

OP ask them mythbuster nerds to come take your car away and test it. I would watch that episode because I am intrigued to know what the deal is.

jodaw

COYVB wrote: »

I think the answer to the question is perception rather than reality. The top "half" of the tank in this case clearly holds more fuel than the bottom "half", but the fuel indicator fails to factor this in.

It's not providing more mpg at all, it's just reporting that it's using less fuel than it actually is for that section of the tank

But OP is measuring it accurately. They are not just saying i reckon this or my guage says that.

Sam Kade

jodaw wrote: »

OP ask them mythbuster nerds to come take your car away and test it. I would watch that episode because I am intrigued to know what the deal is.

Great idea

Sam Kade

jodaw wrote: »

But OP is measuring it accurately. They are not just saying i reckon this or my guage says that.

Exactly

mickdw

It really is a puzzle and from your posts you appear to be doing the measurement properly.
I can only really think that subconsciously you are driving in a manner that makes this happen.
Could be something like this - each time you fill you drive in a careful fue efficientl manner but gradually get careless. When you fill more often from half full you end up driving carefully for the entire fills whereas when you let it low, you drive carefully for say half the fill them drive less fuel efficiently as you near the bottom possibly made even more noticeable by a subconscious will to prove the notion.

sandydan

mickdw wrote: »

It really is a puzzle and from your posts you appear to be doing the measurement properly.
I can only really think that subconsciously you are driving in a manner that makes this happen.
Could be something like this - each time you fill you drive in a careful fue efficientl manner but gradually get careless. When you fill more often from half full you end up driving carefully for the entire fills whereas when you let it low, you drive carefully for say half the fill them drive less fuel efficiently as you near the bottom possibly made even more noticeable by a subconscious will to prove the notion.

according ot one "expert" i know just filling with required amount of petrol gives most fuel economy and says research proves it, puts €20 euro in every time red light comes on except if going on long journey, then fills it and adjusts tyre pressure; drives me nuts:p