Originally Posted by WarBaby
Hmmm....I don't understand the 29" part--why?
Likely because of tolerances. For example, remember the old days when police packages had "certified" speedometers? That is because standard speedometers are allowed an accuracy tolerance - I've have heard up to +/- 3%. Also, 30 inches is what my P275/70R17 tire is "speced" at, per tirerack, but just like when setting up stagger at a dirt track, you really need to measure the circumferance to get a truly accurate number as two different tires could easily measure 2 inches in circumferance different despite the same sidewall specs. When I put the 285/70R17s on my 2500 CTD, which are "speced" at 32.8 inches and I set my programmer to 32.75 inches since it uses .25 inch increments, you would think that would be the most accurate setting...nope. When I passed a radar sign, my speed was still off. I had to use either 33 or 32.5 (I don't remember which, just that it is .25 off). Now when I pass a radar sign, my speedo is dead on.
When I put the P275/60R17s on my 2011, I set the programmer to 30 inches. I passed a radar sign and my speedo still read higher than the sign indicated. I haven't passed a radar sign with the new setting, therefore my previous statement was not 100% correct, but the point is they need some tweaking to be precise. I'm willing to have my speedo err on the slow side to give me a little extra cushion so I don't hit the govenor at the track, so I set it to 29 inches for now. I was running 14.3 @ 98mph. Since then, I now have the shorter tires for more gear, removed the clutch fan, put in a 180 t-stat and removed the spare tire. I'm shooting for 13.9s. Will all that get me up to 105 mph?? If the air is good it might (my Camaro used to run 13.8 at 105 mph stock with drag radials) and if the speedo is off the wrong direction by even 1%, it could hit the govenor. If I really wanted to make sure, I could set it to 24 inches at the track and it would probably read about 70mph when I'm at 105!