Another dam tyre thread

[Clarky]

Originally Posted by TimM

I'm sure this question isn't new, but how do you deal with speedo variation when fitting non standard tyres? Can the WH be programmed to deal with the new size or do you just take the variation into consideration when driving?

I'm looking at new tyres later this year and am thinking about 265 ATs in lieu of the factory fit that I have at present.

with standard 245/65/17 at 100km (speedo) my GPS said 96 kmh
If you go 265/65/17 the speedo will be close to spot on, if you go 265/70/17 the speedo will be out about 3 or 4km (96 on speedo is really 100)
All tyre diameter's are different ie Cooper A/T3 are 803mm & BF Goodrich All-Terrain T/A are 832mm

http://www.bfgoodrich.com.au/media/2...n_brochure.pdf
http://www.coopertires.com.au/index....res&tyre_id=14

Yep BFG should be 804mm

[fester]

Originally Posted by Clarky

All tyre diameter's are different ie Cooper A/T3 are 803mm & BF Goodrich All-Terrain T/A are 832mm

http://www.bfgoodrich.com.au/media/2...n_brochure.pdf
http://www.coopertires.com.au/index....res&tyre_id=14

I think you've got a misprint there Clarky else you are comparing different size tyres. In 265/70x17 the Cooper is 803mm and the BFG is 804mm.

If the WH can't be reprogrammed for the tyre size you want (there are various tuning tools out there that do cater for reprogramming ECU tyre sizes) there are various devices that can be spliced into the speed signal wires and allow you to adjust the speed signal output in percentage terms. So you can pick your worst variation range and adjust to suit that. most speedo when they are out are not linear so you need to pick the variation at the range you want to adjust the most. There's one done by 12 O'clock labs that comes to mind for about $100 posted from the US.

[Clarky]

Yep, need to put my glasses on
If you have a CRD the diff ratio (I think) is 3.55 so if you get the dealer to change the ratio in the computer to the 3.75(again need to check) of the 5.7 hemi the speedo should read under again. By how much I don't know, there is a formula to work it out but at the moment I cant think of it

[drover]

Now please correct me but,,,, a 245/65/17 to a 265/65/17 shouldn't have any effect on the speedo as only the tyre width has changed not the diameter.

[JamesLaugesen]

Originally Posted by TimM

I'm sure this question isn't new, but how do you deal with speedo variation when fitting non standard tyres? Can the WH be programmed to deal with the new size or do you just take the variation into consideration when driving?

I'm looking at new tyres later this year and am thinking about 265 ATs in lieu of the factory fit that I have at present.

There are a few devices that can recalibrate (speedo, shift points, timing, etc.) for different tyre size (or axle ratio.).
I have the Flashpaq; http://www.superchips.com/Store/JeepFlashpaq.aspx
Not cheap but it's been very handy on long trips.

PS; I'd offer to let you use it (and I'm sure everyone else here would too.), but Superchips don't like us sharing , the device gets 'locked' to a single vehicle.

[JamesLaugesen]

Originally Posted by drover

Now please correct me but,,,, a 245/65/17 to a 265/65/17 shouldn't have any effect on the speedo as only the tyre width has changed not the diameter.

Nope, the diameter has changed.

The "65" is sidewall height as a percentage of width.
So 245/65R17 has a sidewall height (theoretically) of 159.25mm (and 265/65R17 is 172.25mm.).

[drover]

Of course.........been a long day......