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Ok, so I've been running Cale's 140 tune since May and I've loved it.

But I'd upgraded from 33" tires to 35's and bumped my tire rev's per mile down closer to what they should "mathematically" be...I had never measured my actual tire circumference, until today!

So, I thought my trans was getting worse on the shifting and actually just bought a spare used one for when it goes. I decided to measure out my tire's rolling circumference now that I was working on the truck and had spare time...

I put a piece of masking tape on the ground and a piece on the outside of the tire. I rolled forward watching my side spotter mirror until it was close, got out and checked it and it was one full rotation, marked spot on the pavement. I measured between the 2 peices of tape on the pavement and came up with 104" or 105" depending on which side of the tape I went to...Divided 63,360(thank god for google on the phone) by both and came up with 604.xx and 609.xx. So you can see that 1 inch does make a bit of difference. I went with the 610 on my tuner as thats what others have used with 35's and took it for a spin to the hardware store for a few things.

All I can say is that EVERYONE should do this when custom tuning regardless of stock tires or not, it made it shift 10x better and I was only off by 15 originally(595, then 590 as experiment) and it made if shift a lot better. Now I'm thinking my trans is probably fine after all, no shift flares and it even down shifts in non-tow/haul mode when it should!

According to a circle circumference formula I should be around 580 rev's per mile, but we can see that the measured 610 is much different...

:doh:
 

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I bet the tire manfacturer's number will be off as well. I recently did the same thing and changing the revs to what I actually was driving on made a difference.
 

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Yeah I think my tires are technically 34.9" (BFG's) and I still havent changed the tire revs....Im gonna go do it right now....
 
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