dmcentee

Hi,

The on board computer display in the tachometer has a readout for MPG - but it reports numbers that are pretty far off from actual fuel consumption. On a recent trip it read ~20mpg; but when I refilled the tank my actual was ~25 mpg.

Is this 20% error typical ? Is there a calibration procedure for the OBC?

Thanks

Svaha

When I first got my car and did the math, my OBC was spot on. I live at the opposite end of the fuel consumption spectrum however (14 mpg), don't know if that could effect readings. Try driving with your right foot pressed completely to the floor for a tankful and see how you do!

jimq

Did you reset it between fills? If not it will give you a average between resets.

cowtown

I've got an OBC retrofit, and it's usually within 1mpg of the calculation when I refill. It's usually on the pessimistic side (reporting lower than actual MPG). I get around 20-22 in mixed driving.

There's no calibration that I know of.

You have stock tire sizes on the car?

Coleman

Okay, dumb question, what is the best way to measure your gas mileage? I dont have the OBC. Do I just visually try to get the needle to a hash mark on the gauge and fill the tank, record the miles and do the math to the same hash mark on the gauge, or is there a trick? I'm thinking that there is quite a bit of margin of error by trying to line the needle up..

Coleman,

jimq

fill it up. record milege. drive it until you need gas. fill it up. record mileage. subtract mileage. divide by how many gallons you bought and there ya go

Coleman

Jim, the error is going to lie in the "drive until you need gas" stage.. that is why I was going into some detail about errors on the accuracy of reading the hash marks on the fuel gauge.. if my reading of the needle is off a bit, it's going to effect the MPG..

Coleman-

dmcentee

Coleman -
You're not using the gauge or the needle. As Jim says - fill the tank, drive a few hundred miles, refill the tank. Note the actual gallons used and the miles driven and divide.

ehb86

Where the needle is has no effect on the calculation. Just fill the tank, ignore the needle, put fuel in until it will take no more. Note your mileage. Drive until you feel like putting fuel in. Fill it up, ignore the needle, put fuel in until it will take no more, note how many gallons you put in. Note mileage. Subtract previous mileage reading from present mileage reading, divide by the number of gallons you put in the last time, that is you mpg average. Ignore the needle. Repeat as necessary. That should do it and be pretty accurate.

Floodj

I don't understand why you guys are making this so complex. Like dmcentee says, use your mileage from your trip odometer divide it by the actual number of gallons from the pump. The only trick is to make sure you reset your trip odometer between fill-ups. If you want to know what kind of mileage your car gets in a particular situation, say around town only, drive around town and refill before taking a trip on the highway. The actual numbers of miles driven is irrelevant as long as you reset the trip odometer between fill-ups. We can talk about significant figure if you want if are worried about accuracy.

dmcentee

I'm just trying to figure out why my OBC has a 20% error :-(

Toga

Do you have the wheels and type of tires the car had when leaving the factory?Different wheels or tires with a slightly different diameter may be? Is the tacho reading correct?

JM993

Did you do a OBC retrofit? I did and my fuel consumption is also way off (every other measurement is accurate). My theory is that my OBC tach (which is from a turbo) is not calibrated to my NA engine.

Just a theory.......

fast_freddy

Mine is spot on.