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Old 01-07-2004, 11:36 PM
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Rear End Change

I have a 1966 Impala. The summer of 2003 I built a SBC with 530 HP. Motor runs great. But my rear end is keeping the car from getting to its true potential. I figures my rear end gear ratio with a ratio calculator. But I think I did it wrong. It said my rear gear where 3.73. But that was with the tall tires. So actually my gear should be a lower numeric number. So where in the 3.3's I am assuming......

I have 25 3/4 tall tires (235/18/50's all the way around) I am pushing 2900-3000 rpm at 70. Is there a way to find what my rear end gears are now from this info??

And I am setting my car up to be fast from light to light. The car gets up and goes now. But I want to put a posi unit with better axles in this summer and want to go with 3.73, 4.11, or something in that range. I am not really worried about getting on the freeway. So my main focus is be 1/8 - 1/4 mile quick....... Thanks so much......

Dave

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Last edited by black66; 01-07-2004 at 11:41 PM.
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Old 01-08-2004, 05:49 AM
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The taller tires will have some effect on the final drive ratio but they're not going to change the gear ratio of your rear end.

To determine what gears you have, get the entire rear end off the ground and rotate the passengers side tire one complete turn. Count how many times the drive shaft rotates for one turn of the rear tire. For example if the drive shaft rotates 3 and a half times then your ratio is 3.50 if it rotates just a shade over 4 times then its probably a 4.11 etc. Besides actually having the factory tag on the rear indicating the ratio or opening it up and actually counting the teeth, this is the easiest way to figure your gear ratio.

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Old 01-08-2004, 04:35 PM
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thanks

I will try that tonight........thanks....
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Old 01-08-2004, 04:46 PM
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The richmond gear calculater shows

3000rpm
70mph
26 in tire

It says 3.32

You have 3.31 gears

http://richmondgear.com/101032.html
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Old 01-28-2004, 09:21 AM
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For a 235/50/18 tire at 70 MPH I get:

2858 RPM's with 3.31 gears
2953 RPM's with 3.42 gears
3065 RPM's with 3.55 gears
3220 RPM's with 3.73 gears

http://www.geocities.com/z_design_st...ion_z28_6.html
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