Best GPH upgrade ever

[Rickb1b]

I just made the best upgrade to my MPG ever. What I did was get a GPS and calculate a very acurate MPG. I discovered my odometer is reading about 25% low. So with an accurate calculation I’m getting 13 mpg.

So here’s my real question. My speedometer is reading about 5% fast, close enough in my book. However, how is it my speedometer is reading fast and my odometer is reading low. Seems to me, if my speedometer is reading fast, I should be getting more miles covered on my odometer, not less miles covered.

Any thoughts. I actually would like to get my odometer to read correctly, but don’t know if I can do it without affecting my speedometer.

Edit: I guess I fly airplanes too much. I meant for the title to read MPG.

 

[Crush]

bigger tires make your speedo read slower and the odometer reads less miles covered than actual. conversely, smaller tires make the speedo read faster and more miles covered than actual. you can correct this a few ways. A new speedo gear on the transfer case, or have the speedo re-calibrated for actual, or re-gear the front and rear ends. if you say the speedo reads fast and the odo shows less miles covered than actual then you have a problem in the speedo cluster need to have it rebuilt by Kevin at Classi Chassi. He does great work

Chuck

 

[Broncobowsher]

You have an issue in the speedometer head. The odometer is a pure revolution counter, nothing else, not time dependent. Pure issue of gearing.

If you fix the odometer the speedometer will be way off. My guess is someone altered the speedometer and adjusted the speedometer drive gear to get it right. Or the speedometer just went bad and it was regeared to get it right.

What I would do at this point, pull the speedometer cable out and count the teeth on both the cable and in the transfer case (that one is a little tougher but can be done by looking through the holes and putting a paint pen/sharpie mark on one tooth and spin until you get back to that tooth). Take those two numbers and look at Viperwolf's speedometer gearing chart and see if you are on it.

 

[blubuckaroo]

Like Broncobowsher said, the odometer is a positive, gear driven, indicator. The speedometer, however, uses a spinning drum/magnet affair, that allows the drum to spin and drag the magnet and pointer along. The amount of slip depends on the gap between the magnet and the drum.
The most important place to start is to get the odometer right by using the correct speedo gear. Then the speedometer has the correct input. If the speedo is now incorrect, the problem is with the drum/magnet that drives the pointer.