Changing to larger Tyres

[SteveC]

On my WG I'm currently running standard size At's (245/65/17) and wish to add mud tyres so might as well have a set a bit larger, (hopefully 265/70/17) I know that's not huge change but I'm worried about the affect this will have on the components such as drive train and stress on other bits and pieces.
My Q is Do I need to be worried or just put them on a drive?. What needs to be considered? Thanks guys

[glend]

I'm certainly no WG expert, but it does have ABS. Most ABS systems and electronci speedos rely on tyre size information you may need to get a 'pinion reset' done in the computer system (ECU). Your larger (and higher profile) tyres will have a different diametre and circumference than the stock tyres.

[casey]

if its anything like the JK, a computer update with the new tyre diameter will all you need to do to keep the ABS, ESP, speedo, odometer etc happy. Such a small increase in size shouldnt cause too much stress on the drivetrain.

[SteveC]

Thank you for that. I was thinking I needed more serious consideration. By the way - how do you alter the ECU?

[casey]

i dont know much about WG's, does a scangauge do it?
http://www.scangauge.com/

[glend]

Can't be done with a scan gauge, you need the DRB tool that Jeep Service guys use. If its anything like the KJ software there are two places that would have to be changed to the new tyre size: the BCM (Body COntrol Module) and the ABS. They both have menu systems to guide you to the tyre size change options. Suggest you do a search in AJOR on the Grand forum and you'll probably find someone who has had it done.

[Cpage66]

IF I remember correctly,(it's been awhile since I last looked into this for Grands) there is a couple of reprogramming tools available from the States that would do it...and even in metric for the hockey pucks north of the US border. Gotta' see if I can find those again.

[SteveC]

A bit of a problem if you change between on-road and offroad tyres of differing size, I prefer to keep the ats around town and use muddies for trips etc. I suppose the obvious thing is same size for both but can't afford $3200++ for a set of 5 each that really is too much for me...The change is 29" to 32" so not a big diff I suppose. I have much to think about guys...

[glend]

A change from 29" to 32" is a big diff when it comes to revs per mile (circumference) and speedo impacts (as well as ABS functionality). A 29" tyre (like a 235/70) will do about 718 revs per mile, where as a 32" tyre (like a 265/75) will do only 657 revs per mile. Your speedo will be 9.3% slow compared to your actual speed (the cops will love you). If you go up to a 285/75 your speedo will be 13.4% slow with only 633 recs per mile.

Suggest you use one of the many tyre calculators available on the net to determine the impact on speedo and importantly performance (bigger tyres sometime mean gearing problems), and the importance of getting your vehicle software set correctly for the tyres your running.

Here is a tyre calculator that you might find useful: http://www.1010tires.com/TireSizeCalculator.asp

[Steve F]

When I went from 29" to 32" (via 30's and 31's) there was a marked drop in performance and increase in fuel usage. 32's are huge compared to 29's even though it doesn't sound like much. I'll be regearing the XJ soon to cope better. Regearing probably isn't such an issue with the V8 though

Cheers
Steve