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Discussion Starter · #1 ·
Have not had the jeep for that long maybe 8 months I'll I have done was put bigger tires on "285-70-17" and some 1.75 inch wheel spacers and some pro comp rims. Got the flash cal for it and reset the tire size on it but it is still so off most noticeably between 40 and 50 mph. Rpm drop to basically idle and I feel like the transmission is confused and doesn't know what gear it should be in. If it's in overdrive it drives and shifts perfect. Any suggestions on what it is
Thanks
 

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Welcome to the Forum.

Did you use the nominal diameter to the tire for the Flashcal or did you measure it? Many tires are smaller than the nominal size; few are larger.
 

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Discussion Starter · #3 ·
Welcome to the Forum. Did you use the nominal diameter to the tire for the Flashcal or did you measure it? Many tires are smaller than the nominal size; few are larger.
I measure it I'm pretty sure I did it rite it was 32.
Do you think the wheel spacers with the rim could throw of gearing?
 

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Wheel spacing isn't an issue until you turn but going straight sounds like the problem.
Differential gears sounds like the issue. I have never re-flashed a JK computer but if you had to I wonder if you selected the correct gear ratio for what you really have making the computer think the wheel speed should be faster or slower than it actually is.
 

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i have 315 size tires on mine = 35, but for my speedo to be right i have to program it as a 33" tire, and the tranny shifts better too, 13 jku 6speed auto
 

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Discussion Starter · #6 ·
i have 315 size tires on mine = 35, but for my speedo to be right i have to program it as a 33" tire, and the tranny shifts better too, 13 jku 6speed auto

At this point I'm clue less trying to avoid bringing it to the shop how would I know if I need to set the tire different on the flash cal
 

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Did you check it against gps on your phone? Might have to adjust down a tad....
 

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I just did this the other day. Use a gps to go 65 or 70 mph and then read what the speedo says. Then use the formula (current tire size setting * speedo mph) / gps speed. For example mine with 33"s was (31.5 * 69) / 70 = 31. By adjusting from 31.5 to 31 fixed the problems I was having the trans waiting too long to shift gears and feeling underpowered.
 

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Discussion Starter · #9 ·
I just did this the other day. Use a gps to go 65 or 70 mph and then read what the speedo says. Then use the formula (current tire size setting * speedo mph) / gps speed. For example mine with 33"s was (31.5 * 69) / 70 = 31. By adjusting from 31.5 to 31 fixed the problems I was having the trans waiting too long to shift gears and feeling underpowered.
I'll give that a shot thanks a lot
 
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