How can you tell for sure what gear ratio you have? I remember jacking up my old Mustang many years ago, marking the rear tire, and counting the number of revolutions. But I don't remember what else I did. Top gear at 70mph on my auto 03 QC 4.7L is at 2K rpm, I think I have 3:55 but am not positive.
Also, how much will 3:92 gears affect gas mileage versus the 3:55? What rpm are you guys at with 3:92 gears at 70mph?

 

"How can you tell for sure what gear ratio you have? I remember jacking up my old Mustang many years ago, marking the rear tire, and counting the number of revolutions. But I don't remember what else I did. "

There should be a tag on the diff housing saying what gears you have or pull your VIN and call the dealership.



"Also, how much will 3:92 gears affect gas mileage versus the 3:55? What rpm are you guys at with 3:92 gears at 70mph?"

I'm at about 2200 rpm's but the truck pulls much better making it fell more responsive. Mileage if you drive like an old lady is in the 18 range. Making her growl through the tailpipe will get me 15-16.

 

There should be a tag on the inside of your glove box telling you everything about your model. With 3.92's just as stated above 70mph is 2200-2300 rpm. On my last long trip I averaged 18.2 mpg with the cruise on 70. You wont regret upgrading, makes a HUGE difference. Where are you located? I know a guy that is installing 3.92's with the limited slip for cheap b/c he has them laying around his shop, ask anyone that got the switch they love the new gears.

 

I live in the NW corner of Arkansas. I know a guy that can install them, or I can try it myself. I rebuilt my Talon's block last year, do all my own work on my cars, so I feel comfortable with installing gears.

 

talking about the gear upgrades where can I get a set and what price range are we talking. and what would be the best set to go with if I am running stock 17's.

Also, what would be the best 2 out of these 3 to do first
1) throttle body 70mm upgrade
2) underdrive pulley
3) rear diff gears

I already have a CAI, and aftermarket flomaster 40 series. I am looking for the best upgrades to add to these.

 

so how much do 3.92 gears cost

 

I will agree that the 4.7 is no track burner but that was not it's purpose. I traded an 01 F150 SuperCrew with the 5.4 Triton and yeah, I miss the acceleration some but I didn't buy it for speed. I bought it for the looks, the drive, the ride, etc. My F150 would absolutely smoke my 05 QC 1500 4.7 up to 101mph when the governor kicked in but oddly enough, because of the difference in the torque curve between the 2 I can get my Ram sideways just about anytime but the F150 wouldn't do it without some coaxing.

 

I have a 3.55 limited slip now. If I change out the gears for something like 3.92, do I loose the "limited slip" fuctionality? Sorry, I'm not very transmission-wise.

 

The 4.7 with the 5 speed stick has plenty of power... I'm not saying it would trade paint with a Hemi but my 4.7 stick can boogie.

 

Yep! There sure is a tag in the glove box. Finally got around to looking in there, and sure enough I've got 3.92s . But I'm at 2000 rpm at 70mph. I've got the factory 20s, would that explain the difference? I'm assuming your truck has 17s?