I've finally have my car running pretty good and now ready to start tuning. Ran my first tune this weekend; nothing too serious just making small adjustments here and there (lowered fan activation temps, turned off VATS, raised MPH and made small adjustments to lean out idle and BLM locker for WOT). My biggest issue is getting the speedo to calibrate properly. I've tried playing with the gear ratio. I've changed the tire size. Nothing seems to work. The closest I got was within 2-3 mph. I'm using my Scanmaster LT1 as the reference point to what's showing on my mechanical speedometer.

Here's my specs for calculation:

Tires: Nitto 555 285/35r18 which is 25.94" per Nitto site but with that tire size comes in at 25.1 on TunerCat

Gears: 3.42

Transmission: Manual (17 driveshaft revolutions)

I've done a search on this subject on this site and others. Any advice would be greatly appreciated.

 

Are you sure you didn't enter 258 instead of 285? The formula for calculating tire diameter would result in 25.1 if you made that mistake.

 

I'm sure. Even if I put the tire diameter at 25.9" the speedo Is off

 

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I figured out how to fix this. Basically put the tire size as 26" (25.94" is what Nitto says the 555 tire is in my size) which would equate to 807 revolutions per mile. This made my speedo off 3 mph at 70mph. So, I took 70 divided by 73 which gave me .9589. I multiplied this by my gear ratio (3.42) which gave me a new gear ratio to input in TunerCats of 3.28. Looks like problem solved.

 

Sounds good.