993 gearing/5th gear-5300-MPH?

[dad911]

Can anyone tell me how fast(mph) 5300rpm in 5 gear would be with a stock 993 6 speed transmission?

[Bill Verburg]

There were 6 different 5^th gears in 94/95(the only years I have info on) all have 3.444 (9:31) final drive

• G50/21 1.027 (37:38) or 1.024 (41:42)
• G50/20 0.921 (38:35) or 0.928 (42:39)
• G50/31 1.024 (41:42)
• G50/32 1.031 (32:33)
• G50/33 0.973 (37:36)

so it depends on the trans and rear tire rolling radius;
the formula Bruce Anderson provides in his books is

MPH = (rpm * rolling radius * .00595)/(trans ratio * Differential ratio)

for a 265/40x17 Bridgestone S0-3 slr =~11.7" with a G50/21 1.027 this works out to a theoretical 133.8 mph in 5^th at 6800rpm

the rolling radius may also be calculated by measuring how far the car rolls for an intger # of rear wheel revolutions

[AES]

Ok, I understand everything but the constant. Where does that .00595 come from?

[Bill Verburg]

The rpm units need to be changed to rph so a *60, the slr units need to be changed from " to mi so a /(12*5280) is required.

(2*pi*60)/(12*5280) = .005949986 ~=.0059

[dad911]

So that puts my speed at about 112 mph (12.5" radius) I think I was going faster, based on previous speeds at this racetrack (Watkins Glen, end of back straight) and conversations with other drivers that I run with(& passed!). Can my Tachometer be off from the 3.6 conversion? Am I doing the math wrong?

5300*12.5*.00595/1.027*3.444=111.44 mph.

I don't suppose tires can 'grow' that much at speed? (I need a 16" radius to be doing 140!)

[Bill Verburg]

I checked BA's work(not that I don't trust him) by deriving the equation and constant from first principles. The math doesn't lie. You can also check against Steve Timmons gear charts which agree with the formula.
Gear charts

The tires do grow a small amount, but that won't change the result much. I don't know how to check the accuracy of the tach, I've always just sort of believed what I see in that regard.

You going to the Glen for the club races? I'd love to see your car.

[dad911]

Haven't raced this one yet, I think I'd be trounced on in gt2 with a stock 3.6. But I will be there for 48 hours/zone1 the end of this month, and at NNJR Pocono in early July.

And I believe the charts/math. Guess I'll just have to enter the esses without the lift....