LOST JEEPS :: View topic - Lots of talk on tires. Can PCM be flashed to correct speedo?

Many daily drivers and wives driving the Libbys. I haven't seen any posts re. speedometer error correction. Really curious and interested. What about power/performance differences with larger tires?

I'm not the most internet/ forum savvy, what about a poll showing lifts, shocks, exhaust, tires, chips, mpg......I dunno, as I am posting this it looks like a lot of info. Maybe I'll just start my own log book as I read through the posts.

footnote: Thanks for all of you self volunteered 'guinea pigs' for posting such detailed findings. Great stuff!!

GMsKJ wrote:

No reprogaming to change speedo reading on these, you either us a tire that is same overall height or do the math at what ever speed(s) and keep it handy. Don't know if you can change speedo gears in this rig, in older Jeeps they had all kinds of speedo gears available. I seem to remember our '87 Cherokee had 3 or 4 optional and I used two different ones over the years.

oldnavy wrote:

GMsKJ wrote:

Mine came with 225/75-16 I put on 245/75-16. I had my speedo calibrated at the dealership and they said the closest they could get was 235/70-16 calibration. I looked at my internet tire calculator and the 225/75-16 is larger in diameter than 235/70-16 so it's worse now. http://www.miata.net/garage/tirecalc.html

Not for sure on the Jeep lineup, but with Dodge RAM HD 2500/3500 pickups it's possible to go to Anti-lock brakes, Set Pinion factor.....There is a selection for 10.5-11.5 diff with UNKNOWN tires. It then lets you set the tire size in rev's/mile from 636-690.

Greg

Author:	GMsKJ [ Mon Apr 24, 2006 6:51 am ]
Post subject:	Lots of talk on tires. Can PCM be flashed to correct speedo?
Many daily drivers and wives driving the Libbys. I haven't seen any posts re. speedometer error correction. Really curious and interested. What about power/performance differences with larger tires? I'm not the most internet/ forum savvy, what about a poll showing lifts, shocks, exhaust, tires, chips, mpg......I dunno, as I am posting this it looks like a lot of info. Maybe I'll just start my own log book as I read through the posts. footnote: Thanks for all of you self volunteered 'guinea pigs' for posting such detailed findings. Great stuff!!

Author:	oldnavy [ Mon Apr 24, 2006 7:47 am ]
Post subject:	Re: Lots of talk on tires. Can PCM be flashed to correct spe
GMsKJ wrote: Many daily drivers and wives driving the Libbys. I haven't seen any posts re. speedometer error correction. Really curious and interested. What about power/performance differences with larger tires? I'm not the most internet/ forum savvy, what about a poll showing lifts, shocks, exhaust, tires, chips, mpg......I dunno, as I am posting this it looks like a lot of info. Maybe I'll just start my own log book as I read through the posts. footnote: Thanks for all of you self volunteered 'guinea pigs' for posting such detailed findings. Great stuff!! No reprogaming to change speedo reading on these, you either us a tire that is same overall height or do the math at what ever speed(s) and keep it handy. Don't know if you can change speedo gears in this rig, in older Jeeps they had all kinds of speedo gears available. I seem to remember our '87 Cherokee had 3 or 4 optional and I used two different ones over the years.

Author:	pipefactory [ Mon Apr 24, 2006 8:23 am ]
Post subject:	Re: Lots of talk on tires. Can PCM be flashed to correct spe
oldnavy wrote: GMsKJ wrote: Many daily drivers and wives driving the Libbys. I haven't seen any posts re. speedometer error correction. Really curious and interested. What about power/performance differences with larger tires? I'm not the most internet/ forum savvy, what about a poll showing lifts, shocks, exhaust, tires, chips, mpg......I dunno, as I am posting this it looks like a lot of info. Maybe I'll just start my own log book as I read through the posts. footnote: Thanks for all of you self volunteered 'guinea pigs' for posting such detailed findings. Great stuff!! No reprogaming to change speedo reading on these, you either us a tire that is same overall height or do the math at what ever speed(s) and keep it handy. Don't know if you can change speedo gears in this rig, in older Jeeps they had all kinds of speedo gears available. I seem to remember our '87 Cherokee had 3 or 4 optional and I used two different ones over the years. Mine came with 225/75-16 I put on 245/75-16. I had my speedo calibrated at the dealership and they said the closest they could get was 235/70-16 calibration. I looked at my internet tire calculator and the 225/75-16 is larger in diameter than 235/70-16 so it's worse now. http://www.miata.net/garage/tirecalc.html

Author:	oldnavy [ Mon Apr 24, 2006 9:02 am ]
Post subject:	Re: Lots of talk on tires. Can PCM be flashed to correct spe
pipefactory wrote: oldnavy wrote: GMsKJ wrote: Many daily drivers and wives driving the Libbys. I haven't seen any posts re. speedometer error correction. Really curious and interested. What about power/performance differences with larger tires? I'm not the most internet/ forum savvy, what about a poll showing lifts, shocks, exhaust, tires, chips, mpg......I dunno, as I am posting this it looks like a lot of info. Maybe I'll just start my own log book as I read through the posts. footnote: Thanks for all of you self volunteered 'guinea pigs' for posting such detailed findings. Great stuff!! No reprogaming to change speedo reading on these, you either us a tire that is same overall height or do the math at what ever speed(s) and keep it handy. Don't know if you can change speedo gears in this rig, in older Jeeps they had all kinds of speedo gears available. I seem to remember our '87 Cherokee had 3 or 4 optional and I used two different ones over the years. Mine came with 225/75-16 I put on 245/75-16. I had my speedo calibrated at the dealership and they said the closest they could get was 235/70-16 calibration. I looked at my internet tire calculator and the 225/75-16 is larger in diameter than 235/70-16 so it's worse now. http://www.miata.net/garage/tirecalc.html I assume they were talking about cal by changing gears, unless they do have the option for the two different tire sizees that are standard on the vehicle in the OBDII or can use the StarScan tool. That is how it is done in the StarScan on the LX vehicles, but the Jeep doesn't use the Starscan tool for all computer work. That is another thing that is screwy on these Libertys.

Author:	Cowcatcher [ Mon Apr 24, 2006 10:13 am ]
Post subject:
You can always use GPS too.

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Lots of talk on tires. Can PCM be flashed to correct speedo? http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=7904	Page 1 of 1

Author:	gsbrockman [ Mon Apr 24, 2006 2:33 pm ]
Post subject:
Not for sure on the Jeep lineup, but with Dodge RAM HD 2500/3500 pickups it's possible to go to Anti-lock brakes, Set Pinion factor.....There is a selection for 10.5-11.5 diff with UNKNOWN tires. It then lets you set the tire size in rev's/mile from 636-690. Greg

Author:	oldnavy [ Mon Apr 24, 2006 3:06 pm ]
Post subject:
gsbrockman wrote: Not for sure on the Jeep lineup, but with Dodge RAM HD 2500/3500 pickups it's possible to go to Anti-lock brakes, Set Pinion factor.....There is a selection for 10.5-11.5 diff with UNKNOWN tires. It then lets you set the tire size in rev's/mile from 636-690. Greg Humm, I will have to remember that little tidbit of info. The problem in the Liberty is I believe there in only one differential used and just 2 tire sizes.

Author:	GMsKJ [ Mon May 01, 2006 9:28 pm ]
Post subject:	Speedo error with 235/75 (factory) tires
I had passed a mobile radar/speed vascar thing and noticed a 2-3 mph difference from what my speedo was posting. I then tested that finding with my Garmin 330c Street Pilot and confirmed the same. It would seem that the 235/75's are smaller than the speedo calibration is accounting for. I'll bet that the 245/70's are what the speedo wants.....and eventually will get. I might even go larger and calculate for a slower reading.

Author:	Stan Wright [ Tue May 02, 2006 12:40 am ]
Post subject:
I went on a 1000 mile trip this weekend and had the GPS in the Jeep. At highway speeds my speedometer is ~3 mph fast. This is with the stock 225/75-16 tires.

Author:	oldnavy [ Tue May 02, 2006 7:32 am ]
Post subject:
Ours was the same 3 mph fast when I checked it after we first got the CRD. I didn't tell the wife, so as she would always have a cushion as far as speeding goes.

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