Yep, it'll mess up your speedo. Maybe 5 mph or so. You can get a 23 tooth speedo gear for the trans for about $16, but they tend to wear out pretty quick b/c there's so many teeth.
Another option are the new electronic compensators, which also allow you to recalibrate the speedometer for tire changes as well, via dip switches. Granted, it's a little more than the speedo gear (ballpark $175) but also much more versatile and more accurate..
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96 GT: KB subs, March pulleys, Bassani X pipe, 3.73's, alum. drive shaft, 70mm TB, K&N, KB Carbon Kevlar BP, Steeda Tri-ax shifter, Flowmasters, MSD wires, H&R springs, Tokico Illuminas
MM C/C plate & GW RLCA sitting in the garage waiting for a free weekend...
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