3.73-4.56 Gear by Tire Size Spread Sheet

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Nazman

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Hey Guys,

I build this helpfull spreadsheet to help you choose the right gear for your application. It has the RPM & MPH by Gear (1-4th), by tire size (26, 27 and 28 inch tire).

Gears are 3.73, 3.90. 4.10, 4.30 and 4.56.

Right Click and Save!

Gear Calculator Spread Sheet

Naz
 

brkntrxn

Inappropriate Motorsports
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Trans-Rear Ratios and MPH Calculations

To add to Naz's information, I want to provide the calculations for everyone so they can make their own spreadsheet to do their own comparisons if needed. Countless times I have searched for gear/tranny/tire/mph calculators and I either cannot find one or it does not do what I need. So, I made my own in a spreadsheet and figured I would share the calculations. If anyone wants the spreadsheet, send me a PM. Or better yet, if someone can host it for me, I will post it up.


To find the effective ratio of a particular transmission gear and rear end gear:
effective ratio = transmission gear ration X ring and pinion ratio

example: 1st gear in a T45 is 3.37. Couple that to a 4.56 rear gear and the effective ratio is 15.37 (3.37*4.56). This is the ratio that your halfshafts see as compared to your engine. The engine makes 15.37 revolutions for every one revolution of the tire.

Effective ratio is important because when coupled with tire size, you can determine the exact ratio that your engine sees in order to rotate your rear tires.

To find the miles per hour at a given engine RPM in a specific transmission gear with a specific rear gear:
mph = Tire Diameter x PI x Engine RPMs / Effective Gear Ratio x 60 minutes per hour / 63360 inches per mile

example: 26" tire X 3.14159265 for PI X 6600 rpms / 15.37 effective ratio X 60 minutes per hour / 63360 inches per mile = 33.21mph
Therefore, a T45 equiped car with a 26" tall tire will go 33.21mph at 6600rpm in 1st gear.


Why is any of this important? How often do you hear the argument "My T56 is awesome! With the 4.30 gear, the overdrive just idles right along!" Or, "Don't fear the gear! I have 4.56s with a 28" tall tire and it rocks!"

Well, let's compare how that all shakes out.

Here are the gear ratios from a stock 99 Cobra T45 with a 4.10 rear gear (effective ratios are listed below the trans ratios):
1st 2nd 3rd 4th 5th
3.37 1.99 1.33 1.00 0.67
13.82 8.16 5.45 4.10 2.75

Here are the gear ratios from a stock 03 Cobra T56 with a 4.30 (effective ratios are listed below the trans ratios):

1st 2nd 3rd 4th 5th 6th
2.66 1.78 1.30 1.00 0.80 0.63
11.44 7.65 5.59 4.30 3.44 2.71

I'd say for the most part, those ratios are all pretty close and the ODs are almost dead on (highlighted in yellow). In fact, you have a better gear ratio in 1st and 2nd with the T45/4.10 combo for getting the car out of the hole. To me, all that T56/4.30 combo got you was more money out of your pocket and an extra gear between 1:1 and overdrive. Oh, and the cool 6 speed shift pattern on the knob.

Now, there are reasons to go with a T56 over a T45. My only point is to think about why you are doing it and make sure you are not spending money that should be spent elsewhere.

Here is another example. Someone says that they have 4.56 gears and 28" slicks at the track. Unless you are looking for tire wrap and a softer launch on a bigger sidewall, this does not give you an advantage over a 4.10 with a 26" tire. (stock T45 ratios in both comparisons)

4.56 with 28" tire (mph at 6600rpm in each gear):
1st 2nd 3rd 4th 5th
35.78 60.59 90.65 120.57 179.95

4.10 with 26" tire (mph at 6600rpm in each gear):
1st 2nd 3rd 4th 5th
36.95 62.57 93.62 124.51 185.84

Again, there are legitimate reasons for going with a taller tire and lower gear ratio. As I stated above, maybe you need the extra sidwall to soften the launch. Just make sure you are going with a big gear, big tire combination for a reason. If you car doesn't have the torque and power to justify it, don't spend the money.


Feel free to correct anything I may have stated incorrectly. I just wanted to post this up so you can make your own INFORMED decisions without a lot of BS.
 
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