For those of you that are trying to decide what gears to get, and what difference it makes in the actual torque applied to your back tires... try this out for size.
So, I have been doing a little homework trying to figure out if I want to get 4.10s or 4.30s. One thing that I did to help me decide was to graph the actual torque to the wheels (dyno WTQ x gear ratio x diff. gear ratio) for a variety of different gears versus my car's speed.
For the dyno WTQ numbers I used this graph that I found for a stock 1999 Cobra (since I haven't dynoed my car...yet):
(I forget where I found this, but it seems kind of typical, and my car is sort of near stock... for now...
)
I then mapped out the torque curve and input the data into a data processing program that I use. Once the data was acquired, I calculated the torque to the wheels (after gears, and differential ratios). For speed I used the info for the stock 245/45R17 tires.
Here's the chart for 3.90 differential gears:
and for 4.10 gears...
and for 4.30 gears...
and for 4.56 gears...
Since actual torque to the wheels is directly proportional to your acceleration, just think of the vertical axis as the kick in the pants that you feel for that gear at that particular speed.
As you can see, from a roll in second gear (say on the 4.10 graph) from anywhere between 20 and 50 mph, you have an extra 500 or so ft-lbs of torque to the back wheels! So, for 4.10 gears, second gear pretty much feels like second gear with 3.27s with a 100 or so shot of nitrous!
These graphs and data are rather useful for other things; for instance, thrust (forward force) is your torque to the wheels divided my the radius of your back tire. Since the stock 245/45R17 tires are close to a foot in radius (12.84") you can approximately think of the vertical axes as the forward thrust of your car. Then divide that number by your car's weight and you have your forward thrust in g's. (Of course this is all assuming that you don't break traction...) Also, where you see the curves for a given set of gears cross is your optimal shift point for maximum acceleration and 1/4 mile times.
My conclusions:
After playing around with the numbers, I am definitely going with 4.30 gears! With sticky tires, first gear will launch you out of the hole like a freaked out cat on carpet. Then for regular city driving on street tires, second and thrid gear would make it feel like I've got another 100 to 130 ft-lbs of torque to pin me back in the seat for sh1ts and giggles!
I had the "fun" of analyzing this data for my own info and gear decision, but I thought the info and graphs might be useful for the rest of the community.
Enjoy...
