I am looking for a permanent solution instead of calculating my odometer reading every time I fill up with gas. A young guy I know wants to buy my truck but needs to convince his parents to help with the finances but they are scared of bad MPG. They know next to nothing about anything automotive so I am trying to correct the odometer to make things simple for them. Before anyone suggests it, I have searched these sites with no luck on a permanent solution: R-F.com, TRS.com, and the Google machine.

I have a 2003 4.0 FX4 with the factory 8.8 28 spline axle. Originally had 4.10s and 245/70-16 aka 29.5 inch tires. I also have the X-Cal2

Since 2006 I have had 33s on the truck MT Baja MTZs and recently Good Year Duratracs.
I recently re-geared my truck to 4.56s.
In both cases the X-Cal2 corrected the speedometer but left the odometer untouched.

How do I know?

I am a stickler for checking MPG every time I fill up and after the larger tires MPG when down the same % tire size went up. Same with the gear swap, MPG went down the same % that gear ratio changed. This must to be due to the odometer turning slower in each case.

How do I correct my odometer to read actual miles?

 

double check your odometer reading by driving 60 MPH between mile markers on the highway. the speedometer and odometer should not be programmable independently, whether with the xcal or with a ford calibration. i think your gas mileage went down the same amount due to the added unsprung weight of the new tires and wheels, and added load on the vehicle.

 

odometer is based off speedo.

my guess is the math it off. also your odometer should go up as the drive shaft turns faster. the truck has no idea how fast the axle is turning only the drive shaft.

double check your math.

 

Ya so I am changing my mind about the gears throwing off my odometer. My first tank of gas after the gear swap had my MPGs about 2-3 less than normal I guess because of break in or something. Then my second tank was even worse when I filled up at Sam's Club. I never fill up there but my father in law offered to let me use his Club card to get the inexpensive gas: about 45 cents per gallon less than everywhere else. It seemed awfully weird that my MPGs for that tank were even worse, I think Sam's Club must be tweaking the pumps to make up for the discount or something!

Anyway I filled up yesterday at my regular station and same pump I always use and my MPGs were similar to what they were before the gear swap. Which is way better than Sam's Club. But still weird that my MPGs have not gone up post gear swap?

Today I checked my odometer with the mile marker signs like Toreador4x4 suggested. Over 30 miles my odometer registered 28.6 which works out to about 5% under. Still weird I think. I have another long trip tomorrow I think I will double check with the GPS