calculating mpg?
 

I changed tire sizes. Went to a 315/70/17 from the stock 265/70/17 I think. maybe it was a 265/75/17. How do I figure my fuel mileage now that my odometer reading is off? Any input would be great. I feal as if I have seen a decrease in fuel mileage.

Speedometer Difference: 8.739% too slow.

Actual Speed: 60 mph Speedometer1: 55.1 mph

Diameter Difference: 8.04%

This should give you a starting point, this is the difference between the stock 265/70R/17s and BFG 315/70R/17s.

get a gps unit

I used the trip odometer & highway mile markers. I think it works best to go 100 miles if you could. But even 10-20 miles would give you an accurate reading. My stock truck will show a little more than 10.1 when the mile markers are 10 miles. Here is the math I used for my truck , Mile Marker/truck odometer=calibrated ....EXAMPLE 10/10.2=0.9803, then multiply your truck odometer distance by 0.9803 to get actual. When I multiplied the indicated MPH by the calibrated number I would get true MPH (verified by GPS). I just made a Excel spreadsheet for my palm pilot so I don't have to think about it......... I know, I know, don't eye roll me too much [laugh] .

I figure I am off about 9% using the mile marker method so I multiply the distance driven from my odometer by 1.09 then divide by the amount of fuel used.

Believe it or not my overhead is actually really close.

one thing to remeber when it says your doin 65 you are well over 70 actually almost 75 mph so be careful with smokey bear

I almost forgot. Using RamWheelsBy4's numbers, use 60/55.1=1.0889. Then multiply your miles or MPG by 1.0889 to get actual. Hope I'm helping & not confusing

Re-Calibrate
 

If you're concerned you should spend the money and go to the dealership or a mechanic and have the speedometer re-calibrated. Then you won't have to worry about math on the fly...

true point he has an 04 so the dealer should be able to recalibraate it.

Thanks fellas. Might go to the dealer here in a week or so. It will give me an excuse to go Uncle Lee's (hunting store). [guitar]