How fast am I going?
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
How fast am I going?
I was at the track (Firebird Main in Phoenix) over the weekend and I have a tach but no speedo. This is a pretty fast track since the dragstrip is used as a straightaway. Is this how I calculate speed from RPM?
Tach says 6000 RPM in 5th gear
5th gear ratio in a ZF-6 is .75
Rear diff is a 4.10
Tire circumference is 81" (315/35/17)
Distance covered for one driveshaft revolution is:
81"/4.10=19.75 inches
Working out the numbers towards the engine, a .75 5th gear ratio must mean that it takes 1.25 driveshaft revolutions to equal 1 crankshaft revolution in 5th gear, so:
6000 RPM x 1.25 = 7500 driveshaft RPM
7500 driveshaft RPM x 19.75 inches = 148,125 inches per minute or 12,343.75 feet per minute
12,343.75 feet per minute x 60 = 740,625 feet per hour
740,625/5,280 feet in a mile = 140 miles per hour.
Is this correct?
Tach says 6000 RPM in 5th gear
5th gear ratio in a ZF-6 is .75
Rear diff is a 4.10
Tire circumference is 81" (315/35/17)
Distance covered for one driveshaft revolution is:
81"/4.10=19.75 inches
Working out the numbers towards the engine, a .75 5th gear ratio must mean that it takes 1.25 driveshaft revolutions to equal 1 crankshaft revolution in 5th gear, so:
6000 RPM x 1.25 = 7500 driveshaft RPM
7500 driveshaft RPM x 19.75 inches = 148,125 inches per minute or 12,343.75 feet per minute
12,343.75 feet per minute x 60 = 740,625 feet per hour
740,625/5,280 feet in a mile = 140 miles per hour.
Is this correct?
Last edited by vetracer; Jan 8, 2006 at 06:43 PM.
