Originally Posted by theterminator93
Verify speed with a GPS first rather than rely on the speedometer or PCM readout since it goes off the VSS. The VSS gear isn't guaranteed to be the right one - in fact, the popular belief that the 21 tooth gear is needed for 3.73s is usually wrong (20 tooth is better in most applications) unless the tire is really short. Determine your tire's revs/mile, then you can determine what gear you have based on the actual vehicle speed and RPM readout.
Already done. GPS speed is dead on accurate even with my 255/40/18 (26.0" outside diameter, vs 26.1" with the 255/50/16 that were on when I got it), and they are shorter than the stock 26.6" tire size.
I have my app set up to read GPS speed/ECU speed/ RPM, Coolant temp, etc. GPS and ECU speed are in lock step, and I had a second phone with GPS using a different speedometer app that is pure GPS and it reads in lock step as well. Also when I pass the "your speed is" signs they always match my speedo.
On the online calcs, I have inputted tire Rev per mile, and come up the same. By all the calculators I can find, the car has 3.90 gears in it. Next thing I'll do is jack it up, put it in neutral, and mark the drive shaft and the tire. Since I have a trackloc, I can spin either tire and count how many times it goes around compared to the driveshaft. For every one revolution of the shaft, the tire should go around either 3 and 3/4 (with 3.73) or just shy of 4 (if 3.90).
I don't want to take the diff cover off, but this will bug me till I find out what's up. I do not know what gear is in the VSS, I just know that on the 255/50/16's it measured up perfectly, (within one or two tenths of a mph) so I used that as a baseline when getting new wheels/tires and matched them up. I kept the width because I liked the big contact patch, and the 18's are only .1" smaller in OD than the 16's were, well within acceptable limitations.
97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com
, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.