How come every vehicle's MPG computer is off?
- Thread starter OppositeLocK
- Start date
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
Maybe
You make some good points. Unless one goes to the same pump consistently to refill, there is additional variability in the calculations.
Is it way off or not depends on a number of factors. I would think today, in the age of evaporative emissions controls, which probably also apply to gas pumps, the pumps have to meet some standard of shutting off to avoid spillage and/or vapor release. So the pumps are probably more alike than different.
But, scientifically, the best, most repeatable results are when the same equipment and methods are used.
I was approaching the problem from the idea that we have no idea how each carmaker implements the algorithm that calculates the MPG. If they don't take into consideration idling, for example, they could produce an optimistic value.
My Mazda, which I realize is an anecdote and not data, is within 1% of the hand calculated value over 50k miles of driving. So I'm not sure I buy the "it's way off" camp's argument either.
And people are bad at math, so there is that...
Now let's argue about is empty really empty? I found out two years ago that even when my Mazda says something like 7 miles to empty, there is still close to two gallons on board, unless the published value for tank capacity is off by about 7L or 1.75 gallons
Originally Posted by itguy08
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
You make some good points. Unless one goes to the same pump consistently to refill, there is additional variability in the calculations.
Is it way off or not depends on a number of factors. I would think today, in the age of evaporative emissions controls, which probably also apply to gas pumps, the pumps have to meet some standard of shutting off to avoid spillage and/or vapor release. So the pumps are probably more alike than different.
But, scientifically, the best, most repeatable results are when the same equipment and methods are used.
I was approaching the problem from the idea that we have no idea how each carmaker implements the algorithm that calculates the MPG. If they don't take into consideration idling, for example, they could produce an optimistic value.
My Mazda, which I realize is an anecdote and not data, is within 1% of the hand calculated value over 50k miles of driving. So I'm not sure I buy the "it's way off" camp's argument either.
And people are bad at math, so there is that...
Now let's argue about is empty really empty? I found out two years ago that even when my Mazda says something like 7 miles to empty, there is still close to two gallons on board, unless the published value for tank capacity is off by about 7L or 1.75 gallons
Originally Posted by itguy08
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
OP, what vehicles are in question here? I've been hearing for years how the mpg computer is optimistic--but then over the last few years I've been hearing how they have gotten much better (as attested in this thread). Maybe you have an older model?
Originally Posted by OppositeLocK
I always measure mileage like this:
- Fill to max.
- Set Trip B to zero.
- When it's time to fill up again, fill up to max, see how many miles are on Trip B compared to how many gallons I had to fill up to get to max.
I've done this countless times on all kinds of vehicles and the car's computer is always a few MPGs (2-3) better than what it is in actuality. Why is this? If it's just an estimate, I get that, but how come it's always in the favour of showing better mileage than you're actually getting?
I also wonder when people list their MPG figures, if they're actually calculating by hand, or if they just rely on the computer.
PS. I never pump after the first click, so it's not like I'm filling up the hose.
Mine isn't. It's +- 1mpg every time I've tested it, and at near 30mpg, I consider that to be pretty close to "within variance of the pump". I've even tried filling the fuel to the neck so as to KNOW EXACTLY how much fuel is in the tank each time (per the pump, at least), to remove variables of "which click" and so forth. Usually it's well under 1mpg variance. The computer has always been pessimistic, when it WAS off.
2019 CX5 GT Reserve
I always measure mileage like this:
- Fill to max.
- Set Trip B to zero.
- When it's time to fill up again, fill up to max, see how many miles are on Trip B compared to how many gallons I had to fill up to get to max.
I've done this countless times on all kinds of vehicles and the car's computer is always a few MPGs (2-3) better than what it is in actuality. Why is this? If it's just an estimate, I get that, but how come it's always in the favour of showing better mileage than you're actually getting?
I also wonder when people list their MPG figures, if they're actually calculating by hand, or if they just rely on the computer.
PS. I never pump after the first click, so it's not like I'm filling up the hose.
Mine isn't. It's +- 1mpg every time I've tested it, and at near 30mpg, I consider that to be pretty close to "within variance of the pump". I've even tried filling the fuel to the neck so as to KNOW EXACTLY how much fuel is in the tank each time (per the pump, at least), to remove variables of "which click" and so forth. Usually it's well under 1mpg variance. The computer has always been pessimistic, when it WAS off.
2019 CX5 GT Reserve
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I'm with Ws6. I suspect it varies from manufacturer to manufacturer depending on what kind of software they use. On my Mazda CX 5, it doesn't seem that accurate just between fill ups, but the "lifetime" average, which is trip odometer A that I have never reset since the car was new is accurate to within 1 MPG as far as I have been able to calculate. Close enough for me. Like any kind of statistics, the larger the sample size, i.e. 6000 miles total vs. a 300 mile tankful the more accurate the results will be.
My Raptor was off about 9% consistently, on the optimistic side. Someone here, thanks, linked how to adjust it and now it's within a tenth of a mpg over the last 4 or 5 tanks. I wonder if some of the discrepancy is related to various ethanol concentrations.
Originally Posted by Astro14
I don't buy your premise. Mine are dead on.
E.G. trip computer says 18.0 and trip odometer/gallons calculation showed 17.99. There is no statistical difference between those two.
Mine also...
I don't buy your premise. Mine are dead on.
E.G. trip computer says 18.0 and trip odometer/gallons calculation showed 17.99. There is no statistical difference between those two.
Mine also...
Mine was off by roughly 7% in my 2016 F150 on the optimistic side, and it wasn't a one time issue - that was long term after over 30,000 miles. Using engineering mode, you can adjust this to reflect reality which I've done. It now generally reads within 1.5% of where hand calculating returns - and matches the long term average over all tanks of gas.
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Originally Posted by itguy08
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
This is a very good point. The computer takes away all the variables.
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
This is a very good point. The computer takes away all the variables.
To accurately calculate the mpg's you would need to know accurately how many miles were driven since the last fill up, and know exactly how many gallons were put in. The computer can easily know how many miles are driven, but I suspect the gallons used is a little more difficult.
Maybe the computer can know fairly close how much gas was sprayed into the engine but not precisely. That could account for a few mgp's off on some cars.
Maybe the computer can know fairly close how much gas was sprayed into the engine but not precisely. That could account for a few mgp's off on some cars.
As long as one is comparing the same vehicle or the same implementation of an MPG calculator, I'd agree.
Once we are comparing different cars and/or different implementations, we simply have the possibility of different variables.
Originally Posted by PimTac
Originally Posted by itguy08
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
This is a very good point. The computer takes away all the variables.
Once we are comparing different cars and/or different implementations, we simply have the possibility of different variables.
Originally Posted by PimTac
Originally Posted by itguy08
They are not off. The hand calculation methods are the ones that are way off. You have no way of knowing how much gas you put in the tank at each fill up.
For example. You drive 200 miles to 1/4 tank and fill it up to the first click and stop. You have no way of knowing how that pump was set for the click off (we've all had those pumps that click off near immediately and those that take a while). So while it may say "F" on the gauge and 15 gallons on the pump you have no way of knowing if it was just registering "F" or topped off. Next time you do the same and put in 16 gallons at another pump. Your hand calculations stated you got less MPG but that pump could have put you in a different spot on the "F" range (say topped off).
Many dash MPG calculators are a rolling type of the last 500-1000 miles. So they don't immediately change. I know when we tow it goes down like a rick but takes a few tankfulls to go back up.
Contrast that to the MPG meter which is set by the computer which knows exactly how much fuel is used for each injector pulse and can do the calculations much more precisely.
Take an average over, say 5000 miles and they should be pretty close. For my vehicles tracked with a phone app and the dash computer over the long haul (135k on one, 80k on the 2nd and 14k on the 3rd) they are all very close to the dash.
This is a very good point. The computer takes away all the variables.
My elantra is always 2-3 mpg off. Get 32 by dash calculated is generally 29. It's consistently higher that what I actually get. Never the opposite.
The Mazda on the other hand seems to be very close.
The Mazda on the other hand seems to be very close.
'18 Accent SE with 6 speed auto computer display off 3 > 4 m.p.g. from calculation . Seems like most people believe the display from Hyundai / Kia vehicle owner reviews I've read , a bit deceptive . The base '11 Fit w/ 5 speed auto of 1 > 1.5 m.p.g.. Be nice if you could set it closer to calculated like that of Eco-Meter , ScanGuage and other similar units .
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My last two MAZDAs were withing +/- 1to2% measured vs computer. I only use the computer now. Since the sampling rate is constant I believe the most accurate computer readout is on longer constant speed drives.
As long as we(for ourselves) are filling up our vehicles the same way each time from one fillup to the next, we will be consistent with ourselves and should get accurate mpg figures. Or at least with our hard reading(phone calc or handheld calc), even if the vehicle's computer is off from our own readings.
Mine are dead nuts on.
2013 outback is pretty much dead on with hand calc.. I mean there is a built in difference.. what if the pump shuts off .25gal early?
seems to be within .5mpg of hand calculated.
my 2011 forester and 2015 forester were usually 0%-5% optimistic
2019 jeep is very close on road trips and in the summer. seems a little optimistic(3%-5%/1-2mpg) if excessive idling or winter short tripping.
seems to be within .5mpg of hand calculated.
my 2011 forester and 2015 forester were usually 0%-5% optimistic
2019 jeep is very close on road trips and in the summer. seems a little optimistic(3%-5%/1-2mpg) if excessive idling or winter short tripping.
The 2018 Jeep Cherokee is pretty accurate but always about 1 mpg more than what I'm getting. My 2012 Chevy truck actually takes about 1 mpg from the actual mileage. The 18 Mustang GT is 2 mpg generous.
A more accurate method is used for instance on my motorcycle. I can visually see exactly how much I put in the tank, reset the trip meter, and when it runs out I flip to reserve and note the mileage. Even then on the same commute every week, result vary regardless of conditions - hot or cold weather, traffic etc. Filling a car until it auto shuts off is hardly accurate. Average fuel consumption on the trip meter can also vary - if I reset to zero on every trip, I get a much lower reading than if I leave it for several weeks...then it changes less and is probably a more realistic figure than on a single trip.
