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Old 04-22-2012, 05:45 PM   #1
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Gear Options with MT82, Boss IM and Tall Tires

Ok guys ... I've found myself a bit bored and crunching some numbers regarding gearing since I'm running a bit taller of a tire. We all know by now that if you're running a stock tire size you want to stick with 3.73:1 or 3.90:1 gears in the rear since 4.10s will have you shifting right before the trap.

Where I started thinking was with an effective gear calculator to see about where you would end up when throwing a 29" tire on with 3.90 and 4.10 gears. The following is what I found.

29" tire with 4.30 gears = effective ratio of 4.003
29" tire with 4.10 gears = effective ratio of 3.817
29" tire with 3.90 gears = effective ratio of 3.631
29" tire with 3.73 gears = effective ratio of 3.472

This got me thinking that you might be able to run a 4.10 and still be in 4th gear at the trap. I plugged the same numbers into a MPH at redline calculator next and came up with the following for top speeds in 4th gear with a presumed redline of 7600.

4.30 = 114 MPH
4.10 = 120 MPH
3.90 = 126 MPH
3.73 = 132 MPH

I also ran the numbers with a 7800 RPM redline:

4.30 = 117 MPH
4.10 = 123 MPH
3.90 = 129 MPH
3.73 = 135 MPH

So what does all this mean? Well it depends greatly upon your power level. I ran the following ET calcs (very rough) with a weight of 3750lbs to include the driver and came back with the following trap speeds.

400 RWHP = 110 MPH
425 RWHP = 113 MPH
450 RWHP = 115 MPH
475 RWHP = 117 MPH
500 RWHP = 119 MPH

Now, I think these trap times are a bit low based on what I have seen these cars run and trap at, but for most people a 4.10 with the taller tires should be fine to mid-11s unless my math is off.

If any of your gear/et/mph calculation wizards can chime in I would appreciate it ...

Here are all the calculators that I used:
Gear Design Equations Formulas Calculator Effective Gear Ratio
Top Speed Calculator
ET Calculator


Here's a good example of why I think 4.10s would be fine when running 29" tires.



Lethal ran an 11.26@122MPH. I would say that few of us will be deep in the 11s and will be trapping at much lower speeds. 123MPH in fourth should be more than enough to accomplish my goal of high 11s all motor (not very ambitious, I know...). I'll hang on to my 3.55s in case I go with a blower later on.
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Old 04-22-2012, 07:34 PM   #2
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You want the 3.90's.

Make sure it takes tranny gearing into it.
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Old 04-22-2012, 07:44 PM   #3
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Quote:
Originally Posted by wnt2gofst
You want the 3.90's.

Make sure it takes tranny gearing into it.
The only part that does is the top mph per gear calculator, and I entered the mt82 specs from ford there.

Effective hearing with 4.10s and 29" tires is ~3.8:1. That would be the closest to having 3.90s on stock sized tires, right?
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