This is the most sensible explanation to date.

 

if inflation increases tire size by half an inch taking the diameter from 19 to 19.5 inches (swag wheel plus tire) this increases the circumference by 1.57 inches or 2.6% (59.69 inches to 61.26 inches). Assuming the final ratio is 3.36:1 (engine revolves 3.36 times for each revolution of the wheel in 5th) an 8.7% difference at 3K RPM is 261 RPM.

Class, your assignment is to measure the increased height of your Fits when changing the pressure from 28 to 34PSI. (it would have to raise the car 1/4" to account for a half inch greater sized tire).


edit: On second thought, never mind. Increasing the pressure may decrease sidewall distortion but it won't change the circumference of the tire. Older tires with cloth belts might expand a bit, but I don't see steel belted tires increasing measurably.

I'll fall back to "it's a mystery."

 

it said thay checked the preasure,even if i was off i dont think even 5 psi would drop the RPMs by 3 or 4 hundred

 

That's my smushed old base GD3, taken out by a Ford Econoline. Fortunately I replaced it with a sport GD3.

 

Maybe its temperature related? it is getting cold.