I'm pretty sure (but not positive) it used to stay closer to 3000 RPM, pretty steadily, at 80 mph.
Now from around 75-80 it's at 3500.
I also did try coasting at around 25-30 mph , then put it into 5th gear to see if it would slip, but it didn't. Just accelerated the car slowly, as you'd expect from a higher gear.
Could it still be a slipping clutch?
SCION tC 05,147k miles.
No.
If the clutch were slipping continuously at speed on the highway, there would be a smoke screen behind you and a horrible smell of burned friction surface when you stop.
When you are in-gear and the clutch slips when you step on the gas, you will see the RPM go up a lot while the speedometer doesn't change much. If you then get off the gas, the RPM goes back to where it was.
So either the calibration of the tach or speedo or both has changed, or you're mis-remembering the rpm at a given speed.
If you want to know which calibration has changed (if any) compare your speedometer with the displayed speed on a gps receiver on a long straightaway at constant speed. (Before gps, a stopwatch and measured mile was the way to check.) Once you have your speedometer corrected, measure the circumference of a tire, calculate how many circumferences are in a mile and look up and multiply by the final drive ratio for a chosen gear in your car to calculate what the rpm should be at a given speed in that gear.
Example: Econobox Model X has a final drive ratio of 3.00:1 in 4th gear and tires with a circumference of 78 inches. Divide by 12 and that's 6.5 feet. For one mile, 5280 feet, dividing by 6.5 means the tires turn 812 times. Sixty mph is one mile in one minute, so multiply 812 by the 3.00 final drive ratio to get 2436 rpm at 60 mph.
My apologies to the metric world, we have a stupid system here in the U.S. I'm just glad I didn't have to convert to furlongs per fortnight.
