my speedo is off by 4-5 mph.. how do i know? when my speedo shows 65 mph my navigation shows 70 mph... is not as bad at slower speeds about 2-3 mph off

on the last fill up i timed my trip odometer thingy and i got 221 miles and when i filled up it took 16.6 gallons so 221 divided by 16.6 = 13.3 mph...

but my speedo is off and i cant figure ou the formula on how to get the real MPG


i am running 30 inch tires and wondering what do i need to do to make my speedo accurate? is there a sensor to replace?

Check this out!!

http://go.jeep-xj.info/HowtoSpeedoGears.htm

Assuming your "navigation" is accurate, your speedometer is about 7.14% off. Multiply 221 * .0771 and you get apx. 237 miles; divide by 16.6 gallons of fuel, and you wind up with apx. 14.28 mpg.

Frankly, 14 mpg stinks for any Jeep. My stock 2001 Sport averaged 23.2 mpg at the last fill. We don't baby it --full throttle, smokey-tire launches off the line happen now and again. And we do a LOT of city driving. And at 152,000 miles, it looks and performs like new.

And I plan to be buried in it in about 30 years.

Another option is to use the county mile markers and compare that to your odometer. I'm not sure how accurate they are so I did a few of "samples" of 15-20 miles and averaged it out. (and samples within those samples). My 235's seem to be 5% over the stock 225's, so I add 5% to my miles before dividing.

5% is 2-1/2 MPH at 50. I cover a mile in a minute if I hold it on about 57, so I think I'm pretty close.