When I'm not whacking on a misbehaving server, I sometimes actually
think about autocrossing. There are certainly a number of factors
involved in choosing which gears to use. In Street Prepared, one is
limited to ratios available through the update/backdate rule, you may
not be able to pick the "perfect" final drive ratio.
In my FSP Spitfire, I'm running a 4.1 final drive. With the short Hoosiers
and a 6,000 rpm rev limit, I usually run out of first gear somewhere between
the launch line and the clocks. Fun! But with the Spitfire gearboxes, one
has a fairly big jump between second and third gears, going with a 3.89 or 3.6
would likely give me a broader range of speed in any one gear, but I'd bog big
time after a 2 - 3 upshift. The car may only weigh 1500 pounds, but on a good
day I'm likely getting maybe 75 bhp at the flywheel. Acceleration is not the
car's strong suit. Included below is the output of my little spreadsheet to
figure speeds in gears. You may want to play around with something like that.
mjb.
----
Section Width (mm) >> 225
Aspect Ratio >> 45
Wheel Diameter (inches) >> 13.00
Wheel Width (inches) >> 8.00
Section Width (inches) 8.86
Section Height (inches) 3.99
Overall Diameter (inches) 20.97
Rear end ratio >> 4.11
Transmission ratios 1st 2nd 3rd 4th
>> 3.50 2.16 1.39 1.00
RPM 1st 2nd 3rd 4th
1500.00 6.51 10.54 16.38 22.77
2000.00 8.67 14.06 21.84 30.36
2500.00 10.84 17.57 27.30 37.95
3000.00 13.01 21.08 32.76 45.54
3500.00 15.18 24.60 38.23 53.13
4000.00 17.35 28.11 43.69 60.72
4500.00 19.52 31.63 49.15 68.31
5000.00 21.69 35.14 54.61 75.91
5500.00 23.86 38.66 60.07 83.50
6000.00 26.02 42.17 65.53 91.09
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