Relatively new to me 1999 R/T 5.9-- Had it on the freeway yesterday and noticed at 70 MPH I was turning 2,500 RPM. As far as I know the transmission is stock, rear end should be 3.92 if I'm reading things correctly, and tire size is currently 245/65R17. Stock tire size may have been 255/55R17 according to the Build Sheet. Came with some spare parts including ring gear bolts, so not sure if someone's been in there or not. Anyone have a good calculator to help me figure out what gears I may or may not have? Thanks!

 

What rpm are you expecting? What makes you think it is not currently accurate?

 

Not sure what I was expecting, but at that speed and rpm and fuel mileage I'm not likely to make too many road trips. It's been a long time since I had a 4sp auto with a tach but for some reason I expected lower RPM at cruising speed. According to a tire circumference calculator, I should be needing about 4% fewer revolutions to cover the same distance as compared to the stock tires. In reality, what am I going to do or can I do to lower rpms, not much. :-)

 

Change the gears in the rear end. Knock it down to 3.55. That will lower your cruising RPM. With the 3.92's in it now, RPM's look right.

 

Should be 2200 RPM if all those numbers are accurate, have you checked speed with GPS?

 

Now I'm really confused. I jacked up one rear tire, put it in neutral, tagged the driveshaft, and counted revolutions. I got TWO revolutions of the driveshaft for every one of the tire. That's like a 2.0 rear end! Surely that can't be correct! If I'm supposed to double the result because one tire was on the ground or because I have the "Anti-Spin Diff", then we're real close to 3.92 as expected.

 

Nope. You have 3.92 gears. With one tire on the ground, the other tire will spin twice as fast.