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I replaced factory tires with the Firestone Destination ATs. I went to a 255/70R17. The spec from Firestone was 31.1. I bought the hypertech speedometer programmer as well. Here is what I found. I actually did the math (pie*d) and then measured against the GPS. If you install a tire my size, you will immediately be under by about 6% on the speedometer and mileage. It also appears to have some impact on how often the econ mode is used by the jeep. When I programmed it, it had the option of programming 31.0 or 31.25. I used 31.25 and went over by 4.5%. GPS confirmed. I reprogrammed to 31 and it was at approx 99.5% of the speed (very close). I know it is probably in my head, but the shifts, and feel of the engine seam better. My GPS and speedometer are dead on. Mileage should be accurate. I would bet that a local shop (maybe a dealer but certainly not mine) would program it for you cheaply if you didn't want to drop the 180 bucks for the hypertech.
 

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Nope, it's not just in your head. Wheel speed directly effects your shift schedule (learned through use of the scheduling app w/in the SuperChips tuner). Without calibration, Superchips, Hyper, or dealer, my up/down shifts were annoying, at best.
 

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Going with the same setup later this week. I am definitely interested in the hypertech speedo calibrator. Where did you find it for $180 if you dont mind me asking
 

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Sorry for the bad info. I actually had a 25 dollar discount off of 229 at autoanything.com not 25 off of 205. It was 205 after discount. Works flawlessly. I haven't messed w/ gear ratios, however.
 

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Hey Indy, I have the same speedo calibrator and after much tinkering and verifying my speed with a GPS I actually had to change my tire height down to 29.50" in the programmer.

I have 250/70R17s with a diameter of 31.1" and I started with the same settings you did yet my GPS indicated I was traveling 4 mphs slower than what my speedometer indicated, suggesting the tire size was too large. I kept decreasing the tire height in the programmer by .25" every time until I landed on 29.5". Odd, right? OEM tire size is 29.75". Think I might've gotten a bad programmer? I checked the speed with an iPhone speedometer app too and it matched the speed on the GPS. Has anybody else experienced this?

And you're right about the economy mode, before settling on the right tire height the econ mode wasn't kicking in as often. I wouldn't say the shifting feels better, but I think it now feels right.
 

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I'm getting ready to swap to a taller set of tires as well, BF Goodrich 265/70/17 KO's and will prob invest in a hypertech speedometer programmer as well. I think that for the money, its the way to go... rather then having to rely on the dealership to set things up for you.
 

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Check eBay, I got mine for $120 from there.

Regarding the issue I noted above, I messaged Hypertech and they suggested that I measure the tire height instead of using the manufacturer specs. I'm assuming this is because the height changes once they're installed. I haven't had a chance to test this yet but I'm thinking that since my programmer landed on 29.5" for accurate speedometer readings then I'm expecting my tire height to be just that. Just keep that in mind once you program the new tire height.
 

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If you change the tire diameter, it has virtual effects on the Final Drive Ratio. The tire rpm is less for the same speed, just as if you had changed the axle ratios or gear ratios in the transmission.

Taller Tire = Taller FDR, so it will reduce acceleration performance a bit, might improve fuel efficiency a bit.

As already stated, that throws off speedometer and odometer. It can throw off the EPS/Traction Control/ABS as well, that measures speed the same way as the Speedo/Odometer, by wheel rpm.

Greater tire diameter results in slower tire rpm for the same speed, thus meaning slower engine rpm for the same speed. Usually that would get you better fuel economy, but there are so many factors involved in this and the vehicle's top gear is already optimal for fuel economy with the stock tire size, it is very possible that a taller tire "could" keep you from shifting to higher gear as soon, resulting in worse fuel economy. NOT saying it will happen, just that its possible. The bigger tire is also wider and has more mass, that increases rolling friction and energy to accelerate, that can reduce fuel efficiency as well.
 

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Superchips 3874

Now That I have new tires and went to program my tuner, it told me that it was not compatable with windows X.P. as far as the ignition update.

My question is can I use the tire size feature w/o the update?


Swanny
 

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Now That I have new tires and went to program my tuner, it told me that it was not compatable with windows X.P. as far as the ignition update.

My question is can I use the tire size feature w/o the update?


Swanny
Swanny,

You should still be able to use your flashpaq 3874 to adjust your speedo to your new tire size - that's a very basic feature of the tuner and it shouldn't require any updates to function properly.

Now that being said, why on earth are using a computer that is still running Windows XP???

http://www.jeepcommander.com/forums/49-programmers-engine-mods/30697-tuner-programmer-suggestions-3.html
 
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