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General Forums => The Mess Hall => Topic started by: omnisi on February 28, 2007, 02:44:17 PM

Title: Interesting odometer calculation/calibration and mileage adjustment
Post by: omnisi on February 28, 2007, 02:44:17 PM

I went to this link to find out how my oversized tires affected my speed and distance traveled.

http://www.dakota-truck.net/CGI-BIN/TireCalc.cgi

Last fill up I traveled 190 miles according to the odometer and it took 11 gal to fill up.  That comes to 17.27 MPG.  The above calculator states that (using 100 miles for easy math), for every 100 miles traveled, my odometer will only show about 82 miles with the larger tires.  Or, for every 82 miles that show on the odometer, the actual distance traveld is closer to 100 miles. Thats a difference of about 18%.

So, according to that, the 190 miles shown on my odometer I traveled plus the 18% will give a calibrated total of closer to 224 actual miles traveled.  Recalculating my gas mileage with the 224 miles/11 gal then comes to 20.36 MPG....a difference of about 3 MPG.

I tested it (the odometer) on a short trip today that my GPS said was 36 miles.  My odometer showed almost exactly 30 miles.  30 miles plus 18% equals 35.4 miles...almost exactly what the GPS showed.

Dont know if this is interesting to anyone else, or where there may be an error, but I thought I would just pass it along FWIW...

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: oldjeep on February 28, 2007, 03:01:10 PM

Easier to just go here:
http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

Figure out which sppedo gear you need and make it read right ;)

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: omnisi on February 28, 2007, 03:06:15 PM

Thanks

Didnt mention, my speedo gear has already been changed and its about right.  I was just curious as to how much further I was traveling for mileage purposes..

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: oldjeep on February 28, 2007, 03:23:46 PM

Quote from: omnisi on February 28, 2007, 03:06:15 PM

Thanks

Didnt mention, my speedo gear has already been changed and its about right.  I was just curious as to how much further I was traveling for mileage purposes..

If your odometer is still reading wrong, then so is your speedometer.  The odometer runs off the speedo gear

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: omnisi on February 28, 2007, 03:28:53 PM

Right!   LOL, I wasnt thinking about that...Ill check it again...one day...

Thanks

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: rwing1 on February 28, 2007, 03:38:08 PM

even better.....use the chart oldjeep linked and find the AXLE gears you really need..... :biggrin:  but check the tire manufacturers specs to see what the ACTUAL diameter of your tire is! a 31" tire can be anywhere from 30-31.5" depending on tread and manufacturer.  I was puzzled how your ODO and Speedo were different for a second myself!  BTW: that same chart oldjeep linked helps to point out what factory vehicles might have the speedo gear you need..... if you pull it yourself it may be very cheap!

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: Jeffy on February 28, 2007, 05:33:05 PM

http://4bangerjp.com/forums/index.php?topic=731.0   :whistle:

I think this one is more accurate since I've actually tested some of the combos and had to readjust after following other charts advice.  Many are close but most are still not dead on.

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: Chad on February 28, 2007, 06:22:02 PM

Okay so I was looking at the link old jeep posted and noticed that for a jeep with a 4.10 gears (like yours truly) the optimal tooth count is 39 for 29" tires which are the stock ones i think.  And 39 tooth gears are also optimal for 4.56 meaning that my odometer and speedometer will be accurate with new gears?

Title: Re: Interesting odometer calculation/calibration and mileage adjustment
Post by: oldjeep on February 28, 2007, 06:28:10 PM

Yes, for 33" tires and 4.56 the gear should be the same.   Mine should stay 39 tooth also - moving from the stock 4.10 to 4.88 and 35's