gas milage

[Moon]

I'll admit it. I'm confused.
Got a new car last year. It has an onboard computer that does all kinds of calculations ... avg. milage, number of miles before you'll run out of gas, real time milage (updates every second to show what your milage is at that given moment) and more that I can't remember now.
So, when I bought it new I was getting 24mpg according to my onboard computer. The car is a 6 cyl toyota and this was right in line with what I expected. At 10k miles I switched to synthetic. My milage didn't change right away, but after a few weeks I noticed it was up to 27mpg. And it kept cliimbing. After a couple months of running synthetic I was at 29mpg. Pretty cool.
Then I posted something about this in one of the threads mentioning that I had witnessed my milage increase with the change to synthetic. Someone asked if I had double checked the figures manually. I had not. Now, I've double checked the figures and I'm baffeled. By my calculations I'm gettting 24 mpg. My onboard computer is still saying 28mpg (yea, it dropped from the high of 29 ...I suppose it depends on driving conditions).
So my onboard computer says one figure and I manually calculate another. I've double checked this on 3 seperate tankfulls. I've reset my trip meters (both of them) but it does not appear to affect the way the onboard computer calculates the milage.
So, I do believe my milage went up with synthetic. But my onboard computer is not calculating the number as I would expect. So I'm confused.
Has anyone else experienced this iregularity with your onboard computers mpg calculations?

Thanks,
Moon of Ohio

[Gatorboy]

The mileage calculated by the computer is done by counting the amount of fuel through the injector... It tends to be very accurate.

Manually calculated mileage can be varied by...
- bladder effect
- filling level
- different pump
- different posture
- different temperature

It's like having two watches ... you never know the exact time. But I think the computer value would be more accurate than manually calculated.

[rlw]

There should be a way to reset/clear the memory of the computer so that it doesn't reuse old data in its calculations - check your manual. On my old GMC Jimmy you held down both buttons by the display for about 5 seconds till it flashed zeros.
Perhaps the computer recorded a bogus number somewhere along the line that is skewing its calculations.
If the computer is not cleared at the same time you fill the tank, the average mpg displayed is going to include your history.

[ByronBob]

Is the computer doing a rolling statistical average?

[Av8r3400]

Unless you zero the calculator out it will be calculating a rolling average since the last zero (new?).

Accuracy is something which could be debated. My wife ran her Jeep out of fuel (99 Grand Cherokee) with the computer saying 100 miles DTE (Distance to Tank Empty). I don't trust these computers more than a general "ballpark" figure.

[dmccarty]

Moon,

The MPG listed by my trucks computer is consistently off by .3-.7mpg compared to the hand calculation. The low fuel warning light seems fairly consistant in that by the time I fill up I put in about 32 gallons which leaves me with 6 gallons in the tank. The last fill up I supposedly had 75 miles or so to go before I ran out of fuel. But at the current milage I really had about 114 miles before I had to start walking.

The computer calculation is just a guide as far as I'm concerned.

Later,
Dan

[Charlie_in_TX]

There is a reason they call them 'lie meters'. I have had 3, one in a 'o3 GMC 2500hd, one in a 'o4 GMC YukonXL and one in a 'o4 Ford 250 PSD. The 2 GMs were remarkable acuate. Usually within 0.1 mpg. The Ford is a wish meter. It usually says 17-18, hand calc is always 0.5-0.7 less.

Someone mentioned that the computer is more accuate than hand calc. UH, no way, no how, ain't go'n a happen over a 3 tank average. A single tank average can be thrown off by the factors mentioned. Not over multiple continuos tanks.

I have heard many people mention higher milage from syn. I have never seen it myself. Deffinatly not the 20% you mentioned. Think about something for a second, Ford, GM, Dodge, Honda, Toyota, ect fight, scratch, claw and spend 100s of millions of dollors to add tenths of a MPG to their CAFE number. If just changing to synthetic got them 20% don't you think they would? Shoot, Ford and Honda went to 20 weight oil to get a tenth of a mpg. If synthetic gave them 0.2-0.4 they would do it.

[Bird]

Yep, one or two, or maybe even three, tanks of fuel is not likely to be an accurate average of mileage; too many variable possible. I know it isn't worth the time for most of us, but I keep a spreadsheet just for fun that has every gallon of gas ever put in my vehicles from the day I bought them. I have the odometer reading at the time, the date, the price per gallon, number of gallons, mileage for that tankful, and cumulative average mileage from the day I bought the vehicle.

[MikePA]

I track the same information except in a little book I bought called an Automotive Diary. I've kept one in every car I've ever owned.

[RalphVa]

ExxonMobil claim 4% improvement for Mobil 1. Think I'm getting around 3%.

I keep my records for all my vehicles in little books I keep in the side pocket with labels for Fuel, Oil and TBA. About every 3 or 4 months I put the fuel mileage stuff into an Excel spreadsheet. I just total up the gallons over a whole sheet and put down the mileage at the end. Mileage varies about 2 mpg between winter and summer (out of 26-34 mpg I get on my vehicles). Some of your increase is due to 1) going to warmer weather and 2) engine being more broken in.

Ralph