Need some Diff./Gearing Help
Hey all, I just found a RX7 speed calculator site and did a little bit of number crunching to decide on what final drive ratio I wanted after I switched to my Miata Gears and 17 inch rims. I am kinda thinking that the site might have been a little bit off though. The first thing was that it said my new 17 inch tires would have a smaller diameter than the 14's that are on it now. the 14's were at 25.1 inches and the 17's were at 24.4 inches. The tires that are on my car now (I am NOT the one that had these put on) are 235/60R14 on my phone dials. My new tires would be 235/40R17. I will post all the combos that I put in and maybe someone out there could tell me if they look right.

Possible Gearing
Tires

NOW - 235/60R14 25.1 inches
PLAN - 235/40R17 24.4 inches

Trans Raitos
1 2 3 4 5
S4 N/T - 3.475 2.002 1.366 1 0.711
Miata - 3.136 1.888 1.330 1 0.814

Max Speed 4.10 w/ 235/60R14 at 7500 RPM
1 2 3 4 5
S4 N/T - 39 68 100 137 192
Miata - 44 72 103 137 168

Max Speed 4.33 w/ 235/60R14 at 7500 RPM
1 2 3 4 5
S4 N/T - 37 65 95 129 182
Miata - 41 69 97 129 159

Max Speed 4.10 w/ 235/40R17 at 7500 RPM
1 2 3 4 5
S4 N/T - 38 66 97 133 187
Miata - 42 70 100 133 163

Max Speed 4.33 w/ 235/40R17 at 7500 RPM
1 2 3 4 5
S4 N/T - 36 63 92 126 177
Miata - 40 671 95 126 154

Max Speed 4.44 w/ 235/40R17 at 7500 RPM
1 2 3 4 5
S4 N/T - 35 61 90 123 172
Miata - 39 65 92 123 151

 

what type of car? and where are you getting the 4.44 R&P?

 

1987 n/a, not sure yet, been kinda hard to find and I might just have to stick with a 4.33, the only way I have been able to find 4.44 R&P is for a miata, I thought that might work since the 1994 Miata diff is based on 1986-1991 n/a diff.
But tim is it correct that a 235/40R17 would be smaller than a 235/60R14?

 

for the math drop outs. the theoretical(tire sizes are not very exact no matter what the numbers on the tire) diameters are:
235/40/17=24.4 in.
235/60/14=25.0 in.

so for practical purposes they are the same. different tires maybe a little smaller or larger. the 17 in rim is bound to be heavier, so I would check weights of tire/rim combo.

 

I was looking at the enkei RPF1 17x8 which weighs 15.5lbs, about 3.5lbs heavier than phone dials right? But I was thinking that totaly rim and tire weight wouldn't be that much different since there woudl be more alloy in the diameter than tire, and rubber is heavier than aluminum right?

 

tires have different weights, so I would check. I would also verify that wheel weight. is that a DOT legal wheel or strictly a race/track only wheel? if it is DOT legal it sounds a little light.

 

Its a road/track wheel, and it might be a little heavier cuz that is the weight listed on enkei's sigth, I wouldn't doubt that they made it seem lighter than it really is.

 

do you have a link to that sight? if the weight is right I might look at one of their other 15" wheels.

 

http://www.enkei.com/highperformance...erformance.htm They are the ones in the upper right.