RPM check with 4.10's
 

Yesterday I took my car on the highway for the first time since I got the 4.10's, and the rpm's did not seem to add up. I have a 25" tire (BFG KDW II 255/40/17), so I thought that my rpm's would be higher than what I've heard. I thought that I would be at 2700 rpms at 75mph, but I was only at 2500 rpms. I didn't hit 2700rpms until I was at 80 mph. At 45mph I'm at 1500 rpms. At 35mph, 1200 rpms. In 4th gear I'm about 1900 rpms at 35mph. I changed the gear ratio on the Xcal 2, but left the tire ratio stock (245/45/17). Do these rpms sound right to everyone? I've always heard that at 75mph I would be at 2700 rpms with 4.10's on a stock tire, so wouldn't a shorter tire put me over 2700 rpms? I hope Ford didn't mix up and put 3.90's in a 4.10's box.

that sounds about right. cruising at 35-40 mph puts the car in its "sweet spot/drone zone" in 4th gear.

With a 255/40/17 tire, 4.10:1 gear, 0.67:1 5th at 75MPH = 2767 RPM
If you were only seeing 2500 RPM, then you were only going 68 MPH.
I'd say reset the calibrator for the correct tire size, you are putting on excess "miles" on your car with the setting you are currently running.

I wish I could change the gear ratio using my XCal2. Thanks for unlocking the user adjustable parameters American Muscle. :mad:

My car has a .62 5th. Where would that put me at 75mph?

So really, one could use a programmer to adjust the gear ratio too high (in the computer) so you rack up less miles than you actually drive? What is the highest gear ratio setting in the sct programmer?

I ask this because I have 4.10's and have been racking up unnecessary miles on the odometer...

Just because the odometer will read less, does not mean you will have less wear and tear on the motor. To me it really doesn't matter what the odometer says. As long as the motor is running strong i'm happy.

In that case 75 MPH = 2560 RPM

:t:I thought everyone could change gear ratios on the XCals? No?

You can, but AmericanMuscle forgot to unlock his end user adjustability. So he can't adjust anything until he gets it unlocked.