: Dont believe your MPG computer!

03-12-06, 09:50 AM
This past tuesday I went on a one day trip to pittsburg, PA with a friend to buy a car.... Instead of taking my Q45, we took my parents pontiac grand prix. Its a 2005 with about 15,000 miles (which has only seen one oil change thanks to the trusty oil life monitor ;) )....

I have always been skeptical about those MPG calculators nad how accurate they were... so i figured id reset the computer at every gas fillup and calculate it the old fashioned way (miles/gallon).... Not surprisingly, the computer calculated almost 2MPG MORE than what I actually got. Over 4 tanks of fuel on all highway driving, it calculated about 23MPG each time, and I never saw over 21MPG.... i think 21.5 might have been the highest.. I did not reset it in between the 3rd and 4th tank, and by that time it was well over 24MPG.....

03-12-06, 12:13 PM
Yeah mines usually off by about 2 mpg. For example, when it says 18.9 MPG AVG, its about 16.5 or so. And my fuel guage is off by about two gallons, when it says 5 gallons left..I have 7 left.

03-12-06, 01:53 PM
The one in the 525 was dead-on accurate.

03-12-06, 02:14 PM
The Fuel Data thing-a-mabob in my Coupe Deville is pretty accurate, archaic, but accurate.

Jonas McFeely
03-12-06, 02:21 PM
The fuel data thing in my brougham is fairly accurate.I never really look at the mpg im getting,but i do always have the distance thing on,that tells me how many miles i can go til next fill up.I find that that is pretty accurate.Though when it gets to 40 and below it just says L,and i try not to push it beyond that.

03-13-06, 12:18 AM
Mine, in my 94 STS, always was within a half mpg or so...I kept records of every fill up for six months...date, fuel taken on, money spent, price per gallon and miles travelled since last fill up...and it was always very close...but I can easily see how some of them could be off by a bit!

03-13-06, 04:08 AM
The one on my ETC has been accurate within a percent or two now that the FPR is fixed. Frankly I'm more likely to distrust the gas pump; the MPG computer isn't what's charging you.

03-13-06, 02:17 PM
Odd that this comes up. I got in my car Saturday and it had a very hard start. It died three times. The estimated fuel range and the avg mpg were zero when it finally decided to start. The info shown on the bar under the gauges read accurately though.

03-18-06, 04:01 AM
It's difficult to say which is most accurate.

Becuase if you fill teh tank, reset the trip meter, use 3/4s of a tank or more and then fill it back up and see how many miles you covered (and how many gallons you used on your receipt to top it back up) the calculation is flawed becuase the trip meter uses the speedo to calculate how far you've travelled and your speedo isn't accurate. It's as much as 10% optimistic which means your distance travelled is up to 10% higher than it really is. Plus, you can never be sure to to the tank back up to exactly where it was.

The real number is probably somewhere between the 2.

Jonas McFeely
03-19-06, 06:19 PM
Ok,just for fun last night,i was using my MPG data thingy after taking a friend home.I pressed the Instant Average button,it was a steady 20-22 mpg at a 45 mph cruise.Then when i dropped down to about 30 mph,it was getting like 45mpg and would jump up to 60mpg! Damn primative crap! I could only dream of it actually getting that...

03-20-06, 06:21 AM
Actually the instant MPG thing is a fairly accurate algorithm but it's based on some reasonably innacurate data.
If you are going downhill and your foot is off the gas, the engine is idling (depending on what your transmission is doing) so if you are going 60 mph and your engine is idling you don't think it's possible that you are getting 60 mpg?
Also try looking at the instant meter when you are sitting still... I bet it says 0 mpg.
That's because you are burning gas and the car is not moving. No one takes idling time into account when they figure mpg. (yes I know how it's calculated, but what I mean is everyone cries about their mpg but they don't consider how long they were sitting still)
If you warm up your car in winter, sit in traffic and idle in the drive through you are still burning gas but you aren't going anywhere.

Those trip computers are made to help you keep track of about how far you have left and in fact when the gage gets to a certain point it changes from a number to "low" because the manufacturer doesn't want you to have "faith" in those little displays.
They are only as accurate as the fuel level sensor, since that is the only way the computer has to know how much fuel you have. If the sensor is damaged/worn it will drastically effect those calculations. The fuel level itself is not meant to be an accurate reflection of exactly how much fuel is in your tank. Afterall even if it reads gallons (for those of you with digital displays), it's only accurate to 1 gallon increments, there are no decimal places.

You guys would get along better with my dad, he also thinks that where the needle is in relation to the E is an accurate way to determine how much fuel he has left. He left us stranded more than once.
He taught me two things.
Fuel gages are meant to be an indicator and not a precision instrument.
When the fuel gage reads less than 1/4 tank, fill it up.

Jonas McFeely
03-20-06, 09:15 PM
Cool.That makes sense.Thanks.

03-22-06, 03:10 PM
Just calculated mine: Computer said 18.0, calculator said 17.9. Not too shabby for a 175K mile '90 SDV...

03-23-06, 04:34 AM
I remember once when my dad had a Ford Mondeo (Contour).
The fuel computer counted down to a range of 0.1
When it hit 0.0 the car spluttered and died.

That surprised me as they shouldn't be that accurate but I guess we got lucky.
Well, unlucky, but you know what I mean...