Speedo accuracy

pipeliner · **Posted:** Sun Nov 23, 2008 6:07 pm

My speedo always used to overread - when it said 100 I was doing about 96, and when it said 110 it was actually about 104/105 (checked on the GPS). On Saturday I fitted a set of Mickey T 245/75-R16s and now the speedo is correct (within 0.5kph) at all speeds from 80 up - it reads about 2kph low at 60.

Anyone else noticed this? Maybe 245/75-R16 is the size of tyre the KJ was designed for!

MY CRD is a manual so I don't have to reflash the computer for the different tyre size to get the transmission working properly. So unless I get a problem elsewhere I plan to leave the computer as is. Does anyone know whether this will cause problems elsewhere e.g. ABS, ESP, etc?

G'ME MUD · **Joined:** Tue Jul 17, 2007 7:55 pm **Posts:** 370

Mine is about 4% slow with 245/75/16 ATZ's. Don't know what it was B4 as I didn't have GPS B4 fitting the tyres.

Glend · **Posted:** Sun Nov 23, 2008 8:22 pm

My speedo reads about 104-105kph when my GPS says exactly 100kph, and I reckon that they all come from the factory set to read a little bit low (probably seen as a safety feature or a not getting booked feature). BTW my sons JK Unlimited CRD is the same (4% high), and he is on bigger stock tyres than my stock 235/70s. It must be 'a Jeep thing'.

corwyyn · **Posted:** Sun Nov 23, 2008 10:06 pm

Mine read accurately with both the 215/85s and the 235/70s (only about 6mm difference in diameter) verified by GPS and good ol' fashioned timing. With my 245/75s it is about 6% under. I tend to just leave the GPS on the dash and reference it instead of the speedo, and when I don't have the GPS onboard I'll just drive under the limit and not worry about it, especially since AZ DPS (state police) have been installing a bunch of new speed cameras on the freeways around the valley :roll:

KJ Renegade · **Posted:** Thu Nov 27, 2008 11:40 pm

Glend wrote:

My speedo reads about 104-105kph when my GPS says exactly 100kph, and I reckon that they all come from the factory set to read a little bit low (probably seen as a safety feature or a not getting booked feature). BTW my sons JK Unlimited CRD is the same (4% high), and he is on bigger stock tyres than my stock 235/70s. It must be 'a Jeep thing'.

I had the same issue, then I got the Revs per mile changed on my KJ which has 31"/10.5/15's to 677 revs, now the speedo and gps are in sync, in order to do this two settings need to changed and saved the first is in the ECU and the second in the ABS module if I'm not mistaken.

Outback · **Posted:** Mon Dec 01, 2008 5:49 am

I have a Ltd and with the highest diff ratio setting (they call it a pinion reset) it still reads 5% low against the GPS.
Has anyone heard of any other ECU mods that will get around the issue as the auto trans runs crazy unless I disable the overdrive.
P.S I am now running an OBDPRO dongle (bluetooth) into my PDA so I get a read back on most of the ECU sensors real time.

cmohr · **Posted:** Mon Dec 01, 2008 7:40 pm

I think I saw a few post from some of the guys that said in their models there is also a setting in the ABS module. And that theirs didn't take until it was set in both. Mines an 04 and there is just the one setting.

cmohr · **Posted:** Fri Dec 05, 2008 1:01 am

Brian... have a look at this... Top of the page has a module for altering the speed sensor... May be the solve your looking for..

http://www.jetchip.com/Shop/shopdisplay ... 10&catID=1

EDIT... Sorry http link does not hold seach preferances, just type in Jeep Liberty in the seach box, then yr model engine etc etc

http://www.jetchip.com/Shop.asp?catID=1&Redir=PCU_Plus

Outback · **Posted:** Thu Dec 11, 2008 5:09 am

Thanks cmor,
I had a look and it would do the trick at $200US, may work oK if I can save $20 a week on fuel!
Also like the idea of a chip mod but will it deliver with 91 octane and standard exhaust?

Speedo accuracy

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