[ZTWsquared]

cornflakes,Mar 23 2009, 01:33 PM Wrote:Just an odd question i have but something i've been watching carefully over the years.

When you are cruising at a steady speed on the highway and your RPM needle is steady at around the 2.9 - 3.0 rpm, what speed are you travelling at in km/h?

Usually when the needle is at 3 (3,000 rpm) my speed is at 120 km/h or just slightly under it.


But sometimes i noticed when the needle is at the 3,000 rpm, my speed is at a tad lower to about 117km/h now.
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Funny you should ask this question today - I was out this morning answering the exact question for my new Fusion in preparation for doing the XCal math to accomodate the new tires and rims. At 3000 RPM the Fusion runs exactly 112 kph in 5th gear - confirmed by both speedo and GPS.

Unless your clutch / torque converter is slipping, your speedo should read the exact same speed at the same rpm assuming you're on the same tire / rim combo. Also: if it's an auto it could be the difference between a locked and an unlocked torque converter - which sometimes happens IIRC when you let the car decelerate and then get back on the gas ... the revs go up for a bit as the t.c. unlocks when you let off the gas, and then remains unlocked momentarily when you get back on the gas.

Wind and other external factors should have no impact whatsoever on the RPM (perhaps the throttle position to maintain that RPM, but not the revs themselves.

Underinflated tires will have the same effect as reducing the rolling circumference of your tire, but that would have the effect of increasing your indicated speed at the same RPM, as the wheel has to turn more times to travel the same distance compared to a wheel with a larger circumference - the more rotations in a given distance, the greater the indicated speed.

My bet is on the locking and unlocking of the torque converter.