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Discussion Starter #1
Hey all, so I have a 15 JKU with the 3.21 ratio. (Which by the way does perfectly ok with the new tire size, I was actually astounded how well it does). I originally put on 33s fearing it would be a dog with 35s, but 3 months later upgraded to 35s and love it) but I digress...


Anyway, I installed a set of Duratrac 315/75r16 tires which come in to just under 35s.

However, when I adjusted to 35" size tire with the superchips flashcal, the speed against a gps unit was way off...

I was able to get the speed closest at a setting of 33.5" tire using the flashcal programmer. Now anything up to about 40mph is spot on, then is jeep show speed faster by one mile at about 60mph and 2 miles at 80mph.

What am I doing wrong or could it be an issue with the programmer itself?

I know what tires I have on, and my settings for gear ratio set with the programmer is 3.21.

Any help would be appreciated.

Thanks!
 

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Started at 36.5 and the speed was off, went way down to 33 and the speed was opposite so next I went with 35 and my gps matches my evic speedo from 0-70 mph
 

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Discussion Starter #4
This forum is awesome with the quick responses. So it seems that one just has to play with the sizing on the programmer against a gps unit to get it just right.

Weird. Shifts seem to be on point and no issues finding gears or weird shifts.

Thanks for confirming.
 

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I'm having the same issue. Put on 35s and Flashcal at 35" is way off. I'm down to 34.25 and that is a bit closer on the speedometer vx gps. However I reset my odometer at a mile marker on the highway. 9 mile markers later, my odometer says 9.4 miles. Check your odometer vs mile markers also, not just speed!

So I will be adjusting to smaller tire size again on the flashcal. Not impressed with this device!
 

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It's not necessarily the device. Tire sizes can run larger or smaller than published size.

A Procal will have the same issues.

For example, BFG KM2's in 35/12.50R17 which BFG states 34.8" but they are actually closer to 33.8" when mounted.
 

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I'm having the same issue. Put on 35s and Flashcal at 35" is way off. I'm down to 34.25 and that is a bit closer on the speedometer vx gps. However I reset my odometer at a mile marker on the highway. 9 mile markers later, my odometer says 9.4 miles. Check your odometer vs mile markers also, not just speed!

So I will be adjusting to smaller tire size again on the flashcal. Not impressed with this device!
You are confusing "diameter" with "weighted rolling diameter". A 35x12.50 tire has a diameter of 35" (or, more likely, about 34.8). Under load - weight - it loses the crown, (the convex surface running across the tire), which is about 1/2" per radius, or 1" for the diameter. So, if you enter 33.75" into your device, your speedometer will be very close. If you actually have 315s instead of 35s, start with 33.5".

And no, you haven't been cheated or lied to. It's how pneumatic tires work.

Mark

Sent from my SM-G935V using Tapatalk
 

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The best way to do this, is to get your tire pressures set close to where you think you are going to run them, park your Jeep on a nice level pad, preferably concrete or blacktop, take a straight edge(I use a level) and lay it on the top of your tire, and take your measuring tape and measure from the ground up to the underside of the level. That will/should get you very close to tire diameter.
As the other members have said, the tires are never equal to advertised dimensions.
 
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