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skigoose
05-04-2011, 07:43 PM
I have a 1968 Charger it had a 383 and the rear is a 8 3/4 but don't know the gears, I am putting in a 6.1 modern hemi and a 5 speed auto trans (modern) I heard I should use 3.91's for the rear, sound a little tall? any ideas..

I am looking to be under 2500 rpm at 70-75mph

High Plains Mopars
05-12-2011, 12:13 PM
Sounds a little tall, but it depends on tire diameter choice and percentage of overdrive.

Go play around here: http://www.wallaceracing.com/gear-speed.php

skigoose
05-16-2011, 02:47 PM
Thanks Tony, but now I am confused, everyone tells me that a 3.55 or 3.91 with the 545rfe will be fine for cruising at 70-75 @ 2200-2400, but all the websites say I will be more like 3400RPM!!!

5th gear is .67, and tire size will be 18 or 19 inches.

Ron.in.SoCal
05-16-2011, 03:05 PM
^ I think you're using 18 or 19 in tire size, which may be your size, but a 19in tire will yield a 26 - 28 total diameter. Plug that into the above calculator and see what you come up with...

High Plains Mopars
05-17-2011, 06:53 AM
x2 on trying 26-28 as the tire size. The calculators are asking for overall tire diameter, not simply the rim size.

skigoose
07-03-2011, 12:13 AM
ok all thanks for the help, I had Anaheim Gear rebuild the rear with a Tru-trac and 3.73 gears, they did it all even installed my rear brakes was sub $1500, so now I am happy... for now