i am lifting my jeep and i want to know exactly what i will need to make my speedometer and odometer accurate

 

you could get a AEV procal you can adjust for tire size and many other useful things.

http://www.quadratec.com/products/17619_1000.htm

 

New to the whole Jeep thing, but I imagine the lift shouldn't cause your speedo to be off. Its the tires and/or gears that do that. If you go with a 32" tire, a dealer should be able to make that adjustment since the Rubicons come with 32" tires. If you go larger than that, you can use a Procal, Flashpaq, and I'm sure there are others available.

 

In the old days you had to change a speedometer gear or just live with it. I used to always calculate my speed when looking at my speedometer in my Bronco with 38" tires. I knew it read 7mph slow. Today is the computer age when you just have to flash your computer with the new tire size and/or gear ratio and it magically makes it all better.

I have a Superchips Flashpaq and it easily reset my speedo and with the automatic just making this change made a big performance enhancement since it adjusted the shift points.

 

Please educate me.

As far as I ever knew the transmission shifts according to the RPM of the engine, load on the engine (Vacuum) and position of the gas pedal.

When I had my stock 32's the tranny would shift at 2-3000 rpm under normal driving and close to the redline under full throttle. Now that I have 35's it shifts at 2-3000 rpm under normal driving and close to the redline under full throttle.

How does the speedo effect the shifting? If the speedo breaks will the tranny stop shifting?

 

hey if i put on 285/70R17 how off will the speed be off

 

The rim and tire size calculator at this site will give you the answer to your question. Just delete the asterisk.

ht*tp://www.rimsntires.com/specs.jsp

 

The onboard computer is programed for a specific stock tire size. The computer checks the engines rpm's and knowing the diameter of the tires and the gear ratio calculates the speed which is then displayed on the analog speedometer.

The computer controls the auto tranny's shift points. After looking at various inputs, including the tire diameter the computer tells the transmission to shift. If the programed tire diameter is no longer valid the shift points are off.

At least that is how I think it works. We are just there to mash the throttle, steer, brake and say hold on I'm gonna try it. Just my two centavos YMMV.

 

my stock sahara was 255/75/18 and that was right on with a military gps with the stock tuning....

my aftermarket tires are the 33x12.5/17 and i kept the stock tune on the superchips flashpaq and it was right on too... i tried inputing 31.5(what it actually measured) and it was off...

however, the superchips does wonders to shifting points and gas mileage iv been noticing

 

Are you certain of this? It's more conventional to either mount the speed sensor at the transmission output shaft, the transfer case, or a wheel (front preferred, since that is less likely to introduce speedometer error caused by tire slippage). I've never heard (not that I'm fount of all things mechanical. LOL) of using engine RPMs to calculate speed--seems rather prone to errors. And, as far as gear ratio: There are plenty of folks on this board who swear that after regearing, their speedometer did not need recalibration. I've never seen that resolved, since there are also folks who argue the opposite (and I do know that my Hypertech has a place to program gear ratio).