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what are the best rear end gearing for city and street driving for a sbc stroker in a 81 trans am auto.Currently have stock2.56 gearing non possi.
 

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If it sees 90% urban/10% highway aim for 3000 rpm at hwy cruise. Over 50% highway, go 2300 or lower cruise - assuming your engine is smooth at low rpm. All stoplight to stoplight who cares? set your gearing to suit your trap speed.
Tyre height is as important as rear end gearing when calculating cruise rpm.Much like prop dia or pitch in a boat.
Try this, loaded radius x 5.92 divided by overall gearing = mph per 1000rpm in direct gear. Assume a 30 inch slick then measure from the centre of the wheel to the ground- should be around 14 inches with a vehicle weight of 3400 - 60/40 distribution, then go 4spd direct top or auto with lock up converter. Multiply 14 x 5.92 = 82.88 then divide by final drive ratio say 4.10 (diff gears x top gear ratio), should be 20.21 mph.
Yep, 3 grand equals 60.63 mph at cruise.
With an auto make sure your cruise rpm is higher than your stall speed. With manuals make sure it doesn't 'pigroot' (surge on the cam) at cruise rpm,years ago - young n dumb - I ran a package that would pigroot at 55mph in top and smooth out at 60. Pull it back 2 gears and it would run like a dog shot in the rse - 93 in 2nd,129 in 3rd and big mph in top, but it was far from toey down low.
 
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