BobF

Without getting too technical, my odometer is off 2 MPH, where my GPS shows 50 MPH and where the speedometer shows 52 MPH. That 2MPH difference converts to a simple 4% offset (on a 100 percent scale). My odometer shows that I am traveling more miles than I actually am. You can adjust the following data depending on your GPS readings for your situation. Just use 50 mph on your GPS as an easy conversion mark. Just double your resulting difference. If your speedometer shows 48, then, you have a 2 mph difference, which becomes a 4% negative offset. (if your odometer is showing less miles then you actually are traveling see further down post).

To calculate the true MPG

1. Fill up at a gas station, record mileage or reset odometer.
2. Run near empty, fill up. Record mileage again or record odometer.
3. Subtract mileage from #1 from #2, write down if needed, or just use the odometer readings.
4. Now at this point, take that reading, and subtract or add your '4 %' or .04 or whatever offset you are using (plus or minus)

Example:

Total odometer reading for trip = 320 miles times .04 = 320 x .04 = 12.8 and then subtract that from 320 = 307.2 total corrected trip miles

5. Take 307.2 and divide by the number gallons on fill up: 19.2 Gallons

307.2 / 19.2 = 16.0 mpg

And there you go. TRUE 'average' MPG.

For a positive offset, such as negative 6 %, (exp. gps = 50mph and speedo = 47, so your odometer is showing less miles then you have actually traveled)

320 miles x .06 = 12 and then Add that to 320 = 332 total corrected trip miles.

Another mathematical way that my buddy showed me is below and you can use it for any sets of numbers.

Quote:

If the speedo reads 52 and the gps reads 50, and the gps is correct, I would use a ratio as a correction: 50/52 x odo should give corrected miles. (50/52 = 0.96153846) so use 0.96153846 as your correction because the actual miles is less than what the odometer displays, so multiplying any odometer reading by 0.96153846 will give you what the gps would have given.

The speedo is reading high, so the actual miles are lower than what the odometer displays. Using your method, you would want to subtract your correction from the odo reading, to get actual miles.

Using my method, you just multiply the odometer reading by 0.96153846 to correct the odometer reading.

Algebraic expression
(gps/odo) x odo = gps.
0.96153846 x 320 = 307.692307 mi
307.692307/19.2 = 16.025641 mpg

Simplified: 0.96 x odo / gal = mpg

Unquote.

For all: I see a lot of numbers slung around the forums along with different tire sizes, gearing, etc. But it would really help if the numbers were 'close' to being accurate for comparisons. Use whatever method above you feel comfortable with.
Note: Even if you don't have a dedicated GPS, you or your friends may have a smart phone with a GPS built-in, download any simple GPS app and use its readings. For safety, try and have a friend with you to read the GPS, so you don't get distracted while trying to keep the speed steady.
Happy Jeeping,
Dylan (PM me if anything is not clear on the above).