Bigger Tires & Lift for the 2.0T 4-Cyl

Foster1

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Earlier you said your mileage went down too. If you didn't adjust for the bigger tires then your speedometer/odometer is going to read low and your displayed and hand calculated MPG is going to be lower regardless of impact of the bigger/heavier tires.
I use google maps for distance traveled, not the odo on the Jeep. I calculate from gas station to gas station on long road trips.
 

Strommen95

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You would get a great deal of the pep back, by correcting the programmed tire size. Right now, your shift points are all over the place because your transmission is confused.
Its not just a simple issue of an inaccurate speedo due to larger tires. The ZF 8-speed is an adaptive unit that learns your driving style and adjusts the shift points, on the fly, to be able to perform optimally. It can't accomplish that unless its fed accurate info.
Adjusting the speedometer will do nothing to help or hurt the transmission. The transmission shifts off output shaft speed which is not impacted by tire size. To put it simply it's like shifting off of engine RPM. 3000 RPM may equal a different speed with a larger tire size but it's still 3000 RPM regardless of tire and gear. A new tire size or greater weight load on a vehicle will affect the shift points but a corrected speedometer will not.

I'd bet you have a Tazer. Radically adjust your tire size in any direction and judge the results. The transmission won't behave any differently.
 

Headbarcode

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Adjusting the speedometer will do nothing to help or hurt the transmission. The transmission shifts off output shaft speed which is not impacted by tire size. To put it simply it's like shifting off of engine RPM. 3000 RPM may equal a different speed with a larger tire size but it's still 3000 RPM regardless of tire and gear. A new tire size or greater weight load on a vehicle will affect the shift points but a corrected speedometer will not.

I'd bet you have a Tazer. Radically adjust your tire size in any direction and judge the results. The transmission won't behave any differently.
I noticed a greater reduction in performance going from 315/70-17 ko2's to 38x13.5's, than when I went from the factory 285's to the 315's. Even under light throttle input and acceleration, it was holding gears longer than it needed, overall acceleration rate was slowed down, and it wouldn't shift beyond 6th on the highway. As soon as I'd adjust the true diameter, it would return to factory drive ability. Rotation per mile is one of a handful of data points used by the transmission to optimize performance and efficiency.

I didn't have the Tazer instructions on my phone until more recently. So, on both the 2nd and 3rd set, I had to drive a ways before getting home to my tablet with the instructions. The up side was getting a firsthand feel of how it drives both before and after changing that one factor.
 

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