Did my 1st calculation of fuel mileage today and found that despite an indicated average of 20.0 mpg (which I had reset at beginning of tank), I actually had achieved 21.2 mpg for the tank by my calculation.

I realize there are many variables and that true mpg can only be determined over many tankfuls due to variation in auto-shut-off point, flow rate, etc. Still, I am curious whether anyone else has noted a favorable discrepancy between indicated mileage versus calculated?

Does this mean my car is already set up for my swap-out to a 3.55 or 3.73 rear?

 

Everyone so far has had about the same amunt of error as you, between 1 and 2 MPG short of the actual measured economy.

I think Ford went very conservative on the programming for the fuel economy average in the PCM.

Better to underestimate the average, makes everyone happy when it turns out better than what the computer shows.

 

Mine was spot on at last fill up at 24.6 MPG (combined, '10 V6). Last couple tanks the computer was high by something close to 1MPG.

 

Both of my cars are always within 1 mpg.

 

Wannabe, Do you have any mods done to your car?
Mine is only getting a combined of 19.5 MPG(Combined) and I know it is already broken in since I have almost 21K on the car.
I ordered a programmer this last week in hopes it will help with mileage.

 

Mine is always within .5 - 1 MPG less than it figures out with the calculator. Computer said 25.7 last fill up and figured out at 26.5.

 

Good to know. The MPG indicator in my buddy's MINI reads 3-4mpg higher than the actual mileage..

 

My first fillup (23 miles to 215 miles) it was way off. Said I was getting 9 mpg, when I did the calc it was actually around 17. Next fillup, 215 to 441, it said 14 mpg. When I did the calc it was about 18.

So it seems to be getting better, but it was way off at the beginning.

PS - This is both city + highway driving in a new 5.0 GT, and being VERY aggresive with the throttle
So not too bad really.

 

I tried almost an entire tank granny shifting and the thing reads.........12.5 mpg. Wtf.

I didn't check my ACTUAL mpg, cause I forgot. Doh. But that can't be right.

 

No mods. I'm at 4,000 and a few miles, and the fuel economy seems to be pretty stable now.

 

It is good to read that the mpg calculator is pessimistic - much better to be pleasantly surprised than to be disappointed. Case in point; the mpg calculator in my 2008 Dodge Ram ALWAYS shows 1 - 1.5 mpg more than the actual mileage ends up being.

 

I have checked mine over several tankfulls -- routinely reset the MPG and write down the gallons and miles every time I fill up -- and it has always been within a few tenths, either high or low, I think the worst it has been off is 0.4 MPG, IIRC . . . this is on a 2010 GT with about 10K miles on it

 

My MPGs are always lower by .5 to about 1.5 MPGs on the car than with my own calculations.

 

Thanks, good to hear my 1.2 mpg pessimism of my on-board indicator is fairly average.

My erstwhile 1999 BMW 540's on-board indicator was always about 1 to 1.5 mph optimistic. Still, that car (6MT trans) got great gas mileage too, even when corrected by the calculation (DOHC 4.4L V8, 282HP/325 lb-ft, 3750-lb curb weight). Would do 23 mpg combined and close to 30 on pure highway, about the same as my next car, the 2006 Honda Accord V6. I'll be happy if this ballsy 5.0 is within 2 mpg or so of those cars, and that seems to be the case.

 

You need to reset the computer...Looks like you're calculating an average form the day of its manufacturer. For a long term lifetime average just leave it, but if you want accurate, real time data, reset regularly.

 

The thing that I don't get is all these 'hand calculations' are always such nice numbers like, oh- I went 425 miles on 14.5 gallons = 29.31 mpg.... Who's to say it's not 423.28 miles on 14.672 gallons = 28.8495.

It's so trivial. Who cares? The system in the car is more than adequate to give you real representation. Like the above user said, reset regularly if you want to see real time.

 

Reset it and see what happens. I can get 29 MPG in pure freeway cruising.

 

Hmmm... my V6 barely gets 29 MPG in "pure freeway cruising" . . . Perhaps I cruise faster.

 

As mentioned in my original post, I reset the avg fuel economy meter when I filled the tank that I was measuring by calc.

OTOH, I see your point about the potential calculation error, that's why I also acknowledged in my original post that it would be more accurate to measure the average over at least 3 or 4 tanks of fuel, which would tend to mitigate this measurement error. I think the main thing is that it's close, which it it certainly is.

My main goal was just to get a feel whether anyone else had noticed the meter being pessimistic. If there was truly just a measurement error in play, I would have expected to see as many people say their meter read high (optimistic) as low (pessimistic). Since it appears more people said "pessimistic", I think it's a fair conclusion to say that there's likely some degree of conservative error built into the avg fuel consumption display/meter.

 

I just did a 340 mile drive today from CO to KS, and this mileage puts me at about 800 since the new stroker kit went in. The computer showed me as 29.3 MPG and my calculation was 26.25. It used to be a lot closer, but the computer was powered down for 3 months and this is a new tune load. Still, not bad for an engine that JDM is expecting to put down 525-550. I'll know for sure in a couple of weeks after they dyno it.