Hello all, just purchased a used 08 jku with 35" bfg km2 with a j-spec lift I don't know if it has stock gearing it's an auto. My problem that I don't understand is why my speedo says I'm going 70mph but by GPS I'm only going 64 65 mph? If I remember correctly when I put larger tires on the speedo would be off but it would say like 60mph but I would actually be going 70 or close to it. Any ideas on how to figure out what's going on.
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Verify the gear ratio. (pull a diff cover and count teeth. -or- jack up one corner, mark the tire/ground and the driveshaft/housing, spin the tire two revolutions while counting the driveshaft revolutions. If you lift both tires on the axle, spin the tire once and count driveshaft revs.)

If you have access to the previous owner, ask if they used a programmer/tuner to set a new gear ratio or tire size in the computer. If not, I don't know how to tell what is currently set in there. Maybe just go to a dealership and have them flash it back to stock, then get your own programmer.

 

Bigger tires will make your speedo read slower than actual speed.

 

I have the same issue and it gets progressively worse the faster I go. Tried the previous owners I they had larger tires on before they sold it. I was told it was a jks 3.5 lift but but I don't see the pieces parts that come with the kit on my '13 jk

 

My wife's uncle had the same problem on his Jeep. When I plugged the tuner back in I found the previous owner had entered his measured tire size as 35in. No tire is 100% true to size, especially under the weight of the vehicle. The tires measured 33.75in under the weight of the Jeep. I corrected the measurement entered into the programmer, re-ran the adjustment, and now his speedo is on to within 1mph at 70.

This is most likely your problem.

Happy Jeeping.

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I'll check the axle ratio using that method. I'm thinking previous owner reprogrammed the computer...could I essentialy buy a programmer and reflash to the proper size even tho it's probably not on stock settings.

 

If it was programmed withe the wrong gear ratio you would be in limp mode. My guess if speedo is that far off it was not programmed for the tires. Get a programmer and set it for current tire size.

 

If it has been re-programmed and you no longer have the programmer used, it will likely need to be reverted back to stock before a new programmer will work. I'm not 100% certain on that, but that is what I have heard. Some dealerships or shops will be able to reprogram for the correct tire size for you, or at least set it back to stock so that you can program it as you please.

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The previous owner probably programmed the tire size incorrectly. 35" tires are never exactly 35". From my calculation, using the numbers you give, if the computer is now programmed with tire size = 35, it should actually be programmed with 32.5

I suspect that your numbers for speed are slightly closer than what you have stated here or the computer is programmed for 37" tires. A few decimal points on the speed readings matter when doing this calculation. You might think about using an OBD2 tool to get the exact numbers. I use a program called Torque on my Android phone with a Bluetooth OBD2 connection.



https://www.amazon.com/dp/B00WPW6BAE?psc=1

Note: Don't leave the OBD2 tool connected to the port under the dash all the time. It may run your battery down.

 

Bingo