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Old 03-30-2012, 09:34 PM   #160
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Originally Posted by suss6052 View Post
70 mph should be 3007 rpm with the 5 speed manual and 15" tires on the s model, that works out to 42.96 rev/mph. With the 16" tires that brings the rpm at 70 mph down to 2958 or in other words 42.26 rev/mph. With the 17" wheels this would only theoretically get you down to 2934.5 rpm.

The DCT with its 6th gear is doing 2627 rpm at 70 mph with the 15" wheels or 381 rpm less than the manual with the 15" wheels. With the 16" wheels the car would be doing 2584 rpm at 70 mph or 374 rpm less than the manual car in 5th gear. The 17" and 18" wheels effectively have the same outer diameter and would result in an rpm at 70 mph of 2563 rpm. This is again a reduction in engine speed of 371.5 rpm.

This was calculated using the relationship between the road speed to the engine speed, factoring in the gear ratio, rolling radius of the tire, as well as the axle ratio, (which in the case of the dct there are actually two ratios, but since we were just focusing in on the rpm at 70 in my comparison it made the end math simpler even though I'd already gone and calculated all of the N/V ratios (engine speed divided by road speed using the following equation: 168*Ra*Rt/rr

Where 168 is a conversion factor that allowed me to use the English unit system of measurement with the rolling radius for the tire in inches, I did have a table to look these up based on the size of the oem tires used on the ford focus, and the gear ratio specifications from the ford media page.

From what I've heard this seems to fairly accurately calculate the engine speed at various road speeds if held constantly in one gear. Of course in the real world one would shift out of that gear if they felt the engine laboring too much or lugging too much.
thank you, that is helpful.
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