I swapped my 3.90 rear for a stronger complete unit with 31 spline axles and detroit 31 spline locker plus it had 4.10 gears. I can feel the slight difference stepping up to 4.10s. I went with 3.90 after everyone saying go 3.90 with a power adder, when i shouldve went 4.10s all along.
My 97 mustang has a a mechanical to computer speedo. So i still needed a Speed Cal, even thouh i have a SCT 4 position chip. Sucks i had to buy the Speed Cal, I did think the SCT would do it.
The speed cal was easy to hook up it says an hour time, but only took 15 minutes to do. Plus you can go on their site and put in your tire size and Axle ratio, and really fine tune it. Your tire size affects it too. So you cant just set it by changing the dipswitch 3.27 to 4.10s. Nice Feature. It lets you calibrate in increments of .2% steps!!
Get at least 3.90s if you have a Mod motor and get a tune to remove the Rev limitor if you cant go fast enough or worried about running out of steam at the end of the quarter. After going 3.90 to 4.10s, no way id be happy with 3.73, If I had one of the newer mustangs with the .62 Overdrive instead of my .68 id be all over the 4.30s, since my car was mostly built for the track and not commuting.
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1997GT SOHC, PI Heads,intake, SCT 4 bank, UPR CAI
Zex Nitrous system 125 hp shot,Steeda Tri-AX
March Underdrive pullies,2 1/2 O/R Pipe, Magnaflow Catbacks,Mac Upper and lower Control arms,5 speed
4.10 Gears,31 spline, Nitto 555 drag radials Spinning
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