Download a GPS based speed app on your phone, take the car out and compare indicated speed with the phone app at 30, 60 and 90 mph, do the math to calculate how much of a tooth decrease /increase you need.
Be sure to take 3 or more readings from slow to fast. and pick a range that is closest to where you drive most.
ie {@ 30mph indicated, GPS shows 25mph = 1.2
1.2 x what you have in there now = adjusted tooth count count.
For me, Freeway speeds can be hardest to judge consistently when the car has overdrive.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)
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