I just changed my gears from 4.10s to 5.13s and I need to know how much my odometer is off. The computer is programmed for the 32" tire 4.10 gears combo, and my current setup is 35" tires and 5.13s. I plan to get the procal... BUT right now I can't (full time college student on a super tight budget), and I need to know my true mileage so that I can keep up my preventative maintenance. Thanks for the help!!!

 

Your odometer is going to be about 12-15% light depending on the actual measurements of your 35s.

 

The speed error is determined by tire size. My Toyo 35's (actually 34.8") show a road speed of 50 MPH (speedometer) and 54 MPH (GPS). So it's @ 10% low.

 

He also swapped to 5.13 gears. I would say your speedometer will read about 15% faster than you are actually going. If your speedometer says 75, you are probably going about 65.

EDIT: I may be wrong about this. Now that I think about it, I'm not sure the gear swap will effect the speedo. Best to follow Robar's suggestion below. Or get a buddy to follow you down the road and tell you how fast you are actually going.

 

If you know any one with a GPS try to borrow it and document your speed with the GPS speed. This should give you some fairly accurate calculations to work with until you can adjust you computer.

 

I have a GPS, but I guess what I need is some formula to calculate the distance I really travel versus the distance the odometer saysI have traveled. I think that my odometer is clocking miles faster than I am putting them on.

In 4th gear I calculate at 65 my jeep should be going 1931 RPM, but instead it is going 2209 RPM. Flip of that... at 2000 RPM I am now going 59 MPH instead of the 67 I should be. So at that speed I would be going 8 miles less than the Jeep thinks I am. So can I divide 59 by 67 and get 88%? So I am going 88% of what I should be? So that would mean that at odometer reading 2640 miles, I have gone 3000 miles and it is time for an oil change? Or would it be that 3409 on the clock was really 3000 miles? Or am I looking at this completely wrong everywhere??? I know there are some engineers on here. Help me.

 

If it's your maintenance schedule your worried about I wouldn't sweat it. You're spinning your motor harder with the gears and nothing really cares how many miles you're vehicle rolled it's just how much service you've gotten out of the fluids.

That said, I would just take the maintenance intervals off the odometer as it stands. As you're running shy but spinning the motor more it works out about the same.

 

If your speedometer is reading slower than you are actually going, then you are putting more miles on the Jeep than the odometer says.

 

ya I kinda thought that too. And 400 miles either way just does not make that much difference either. That is exactly what I thought about it being early but the motor going more rpm. Then again, the engine gets the jeep moving with less work now. I dunno. I am not trying to get too crazy with it. I am putting less miles on my jeep than it shows though right? Isn't that the direction I am? When I had 4.10s and 35s it was traveling further than it said, and now goes less. I think that is right. Thanks for the input. I hope to get a procal soon. Unless someone has one I can use once lol.

 

The final gear ratio has NOTHING to do with whether your speedo is correct or not. The speed sensors at each wheel feed info to the computer as the axles rotate. The computer, "knowing" that the tires are 32" diameter calculates the speed per tire rotations. 1 rotation of a 32" tire covers 100.528". 1 rotation of a 35" tire covers 109.53". So the computer is giving the wrong speedometer speed 'cause it "knows" how far a 32" tire travels in 1 rotation but the 35" is going further. Your actual speed is higher than the speedometer says and your traveling further than the odometer says. In this example that difference is 9.375%.