96 C-280 high RPM at hwy speed? MPG? - PeachParts Mercedes-Benz Forum

blackmercedes · #1 12-24-2002, 10:22 PM

On W202 four speed 722.4 equipped cars, the fourth gear ratio is a direct drive 1:1.

The 195/65-15 tire is 25.0 inches tall. Now to figure out revs/mile, you can do a little simple trig, or look it up at www.tirerack.com. Here's the math:

C = TT (pi) * D

C = 3.14159 *25

C = 78.54

C = (expressed in feet) 6.55

Revs/mile = 5280/6.55 (5280 feet per mile)

Revs/mile = 806 revs.

Now, I checked Tire Racks notes for a typical 195/65-15, and they list 840 revs/mile. Since the tire makers publish this data, I decided to use that number.

At 60 mph, we are traveling one mile per minute. This simplifies things a little. We start at the final drive and work our way forward.

840 revs * 2.87:1 final drive ratio = 2410. This is the number of rpm that your driveshaft is turning at 60 mph. Now, let's go through the transmission.

2410 revs * 1:1 fourth gear ratio = 2410. This is the number of revs your engine is turning. Pretty simple.

Now, suppose we want to figure out 100mph. We could redo the math, but why bother? 100/60 = 1.6666. 2410 * 1.6666 = 4016rpm at 100mph. Okay, not accurate to any extreme degree, but pretty durned close.