Gas Milage Question

According to the GPS i go 2 mph slower than what my speedometer says. So i auctually get more miles per gallon than what im really getting. Does anyone know how to figure in the 2mph off in my gas milage? Thanks!!!!

Ignore your speedometer when figuring gas mileage, watch your odometer. While they're both driven from the same source, they are 2 different mechanisms.

Yes, you can use your GPS to figure a calibration factor. Drive 100 miles on your odometer, and see what the GPS says for the same distance (I assume your GPS has a tripmeter.) For example, if the GPS shows 101 miles for 100 indicated on the odometer, multiply your shown mileage by 1.01 to get the real mileage to use in your mpg calculations.

I'm lazy so I'll ask if you can give me a number to multiply by...one that'll work for all you wise acres out there...

According to mileage markers on the highway...each milepost equals 0.8 miles on my tripmeter. I've been dividing my mileage by 0.8 to correct, is this right?

i also get about .8 per mile marker

Thnik about it this way. If your odometer read twice the actual mileage, you would think you've gone 200 miles every time you've actually gone 100. If you used your odometer to figure it, your actual fuel economy would be half what you thought it was, and you'd have to divide your calculation by two, or multiply it by .5 to get it right. If your odometer reads 1 mile for every .8 miles traveled, your distance will be .8 times what you think it is, and that should be factored into your economy calculation. If you do a straight mpg calculation from the readings, you need to multiply it by .8.

I haven't checked my odometer for accuracy, but I know that with the stock wheels and tires, my 95 reads about 4 mph high, and with 235/75 tires, both the 95 and the 93 are accurate, at least as to speed.