Tire size - speedometer

[tbuckley]

Yes, yes... I'm a newbie... first posting... blah, blah, blah... but I do have a question. Installing the 4" Rancho lift and moving from the stock 35"s to 37"s... curious as to just how much my speedometer will be lagging with the larger tire size?

Thanks, boys!

[MarineHawk]

I can't tell you for sure that my odometer and speedo were properly calibrated with the original tires because I didn't check it then, but with my 37" Cooper STTs at 40 psi, my odometer now reads almost exact 7.0% low. When I had them at 35 psi, the odo read 6.7% low. This from using numerous milemarker comparisons. Always comes out the same. I.e., on many Virginia highways, the have milemarkers every 1/10th of a mile. When I zero mile trip-o at mm 100.0, my odo reads 100.0 as I am very close to mm 207.0.

[tbuckley]

Well done and many thanks for the information, MarineHawk and Phil!

[Fubar]

I may be wrong but can't one of those hand held programmer thingys correct for the difference?

[RIC-H0]

I wouldn't worry about the speedo being off too much, just keep it below 100, and you'll be fine!
At 80mph, I was about 4-5mph off.
My problem wasn't with the speedo, it was with the trail magnet that was built into the auto pilot!:D

[shhrtbs]

Yes, a handheld can compensate for the different tire size. Would let you play with shift points, pressure, etc also. I didn't do it when I went to 37's, but did with 38's. In retrospect i would have done it for 37's also.

i adjusted the programming for my 37's, mileage is back to where it was with the original tires as well as having an accurate speedo. the tighter shifts and performance improvement make it even better .

[tbuckley]

Alright... stupid question, but what exactly is "one of those hand held programmer thingys"... and is that the way it reads on the box? Because in Latin, "programmer thingys" means, well... something else all together.

[tbuckley]

SuperChips, Hypertech, etc all make handheld programmers. I ahev a SuperChips and have been happy with it, but I think they are all pretty similar.

http://www.*******club.com/tech/superchips.html

Phil,
As usual, many thanks and great help you are! I'm no mechanic, but could certainly find my way out if I crawled up under the hood... is the Superchip easy to operate? And the 'diagnostic connector'... easy to find and hook up too?

[HummBob]

Phil, What would you suggested as the better programmer to get:confused:

[tbuckley]

They are all very easy to use. You simply plug them into the ODBCII port (by e-brake release) and follow the on screen prompts.

If you can use a calculator you use a programmer.

Lift / tires are installed tomorrow... afterwhich I'll purchase the Superchip and give 'er a go. Thank you!

[HummBob]

I really don't think there is a lot of difference between them. I think any of them will do the job just fine.

Thanks Phil:o