Originally Posted by dlevi67' post='346162' date='Oct 18 2006, 06:02 AM

BTW - for those who are interested: the formula is:

V = rpm / (60 * g * f) * pi * (2 * Ts/1000 * Ta + 0.0254 * R) * 3600/(1000 * 1.609)

where:

V = theoretical speed in mph
rpm = engine speed
g = gear ratio
f = final ratio
pi = 3.14
Ts = tyre size in mm (i.e. the 225 in 225/45 R 17)
Ta = tyre aspect in % (i.e. the 45 in 225/45 R 17)
R = rim diameter in inches (i.e. the 17 in 225/45 R 17)

The numbers are just converting units in other units:

60: seconds in a minute
1000: millimetres in a metre
0.0254 = metres in an inch
3600 = seconds in an hour
1000 = metres in a kilometre
1.609 = kilometres in a mile

If you want a slightly shorter formula, then

V = rpm / (g * f) * pi * (2 * Ts/1000 * Ta + 0.0254 * R) * 0.03729

simply substitutes many of the unit conversion factors for a single one.

Originally Posted by dlevi67' post='346162' date='Oct 18 2006, 05:02 AM

Why should the engine go ka-boom? It's doing a speed (rpm) for which it is designed... and 179 is at 6,000! 6,500 redline is 194!!!

Originally Posted by SilberGrauE60' post='347461' date='Oct 21 2006, 07:58 PM

I worked out the formula provided by dlevi67 for my configuration and arrived at a theoretical top speed at redline in 4th gear of 141.86 mph. The relevant parameters were

rpm = 6500
g = 1.19 (4th gear)
f = 2.93 (final drive ratio)
Ts = 275 (Tire size 275/35R18)
Ta = 35
R = 18