Tire dimension and transmission problems

[trazman]

Hello guys, will i have some transmission problems if I put on 245/75/16 tires?
I have the 02 Liberty 3.7 automatic

 

[LibertyOrDeath]

If you don't lift it first then the excessive rubbing will cause undue stress on all your drive-train components. Also make sure you don't put hydrogen in your tires.

 

[trazman]

yes i know that, but i thought about problems with gear box...
will big tires affect the gear box.

I have already noticed the difference in gear changin between 225/75 16 and 245/70 16, the gearbox works better with 245/70... i dont know why

 

[LibertyOrDeath]

No, I didn't notice any change. You have the 45RFE though which is geared a bit better than the 42RLE.

 

[trazman]

Ok thanks.

 

[trazman]

ok, thanks

 

[LibertyFever]

I'm surprised nobody has yet mentioned the change in the programing within the BCM. I believe the BCM also controls the shift points within the auto transmission. The change in tires size will affect the speedometer readout and of course the shift points.

My 04 Liberty Sport came stock with 205 tires so when I upgraded to 245 tires there was a big difference. By the way, even though I have had my BCM programed for the 245 tires my speedometer is still inaccurate but the transmission shifts well.

Visit a dealership and have them program your BCM with the new tire size.

 

[KYLiberty]

Mine has the 45RFE and I've run 235/70R16, 245/70R16, 245/65R17, and 245/75R16 tires. Increasing the diameter will cause you to lose a little power. The extra weight also costs a little. The shift points will also change some. As long as you reset the computer and get the speedo corrected, you can run any of these sizes. The best way I've found to set the speedo requires the use of a GPS. Get out on the highway and get to an easy to maintain speed of something like 50 mph. Note the speed on your GPS. Let's say it's 53 mph. Go to the dealership and have them read the tire revs per mile. Let's say it's 721 RPM. Now a little quick math.

(Speedo Speed / GPS Speed) X PCM RPM = New PCM RPM

So,

(50/53) X 721 = 680.1886.....

In this case, tell them to set the tire revs per mile to 680. This is the method I used and my speedo is right on according to the GPS and radar speed signs.

 

[Ry' N Jen]

After Jen and I installed the Frankenlift, then mounted the 245-75-16
Goodyear M/T R Kevlar tires, we took the KJ to the local Chrysler dealership
and had the brain reprogrammed for the larger tires size.
Now our Jeep shifts when it is supposed too!

Oh!

Did I mention that the reprogramming didn't cost us anything?

 

[jafstl38]

So what do we do if the gps reads slower than my speedo? On the highway of my speedo is at 75, GPS says 72. Do I still use the same conversion? I have alot of weight on mine ARB, winch, lockers 245:75s, ex.

 

[LibertyOrDeath]

So what do we do if the gps reads slower than my speedo? On the highway of my speedo is at 75, GPS says 72. Do I still use the same conversion? I have alot of weight on mine ARB, winch, lockers 245:75s, ex.

Ignore it. You are going slower than your speedo says so it isn't a big deal. I preferred it so if I was going a few over and was radar-ed I was good to go. If it bugs you a dealer can fix it for you for a fee.

 

[jafstl38]

I would just ignore it but my shift points are all ******** too, it's never been set for my 245s. It also has a major hill problem, I can lose almost 20mph on a hill!?

 

[LibertyOrDeath]

Weird, I never had an issue with that. I just gave it more gas if I lost speed going up a hill. I think a dealer is your only option since they don't make a programmer of an kind.