thesuperbad234

Okay guys, stupid question for you.. I've been doing some reading on re-gearing, and hearing a lot of people say that with their 3:55's they're running around 1800 to 2000 rpms at 65mph. And that regearing to 4:10's should carry them up to around 2500 or so, anyways.. My truck (according to the tag) has 3:55 one legger rear end. Yet, my rpms at 65 are 2500 on the nose. It's a 1995 f150 4x4 with the 5 speed mazda transmission. Do these trucks with the mazda trannys just run this high or what? I'd like to confirm it just to be sure before I buy a new set of 410s. Judging by my climbing/pulling power and acceleration I don't think that I have any lower, but these RPMs seem just a little high don't you think? Sorry about the long post. Any advice is appreciated!

Mista_Breeze

It all varies by motor too, what motor do you have?

And tire size can affect it as well.

Chris_1

iTrader: (1)

The gear ratio is how many times the driveshaft goes around to make the wheel go around once. So you can jack it up, make a mark on your tire and your driveshaft with chalk.
Turn the tire one complete revolution and watch how many times the driveshaft turns.
3 and a tiny bit would be 3;08, 3 1/2 would be be 3;55, 3 3/4 - 3;73, 4 and a bit - 4;10, 4 1/2 - 4;56

Chris_1

iTrader: (1)

Then the bigger the tires, the lower the revs on the highway - but more of a grunt pulling away from a stopsign, and vice versa for smaller tires. Depends what you want to use the truck for most.

Ford 4x4

I have the 302 and mazda with the 3.55's and at 60-65 I run about 2100 to 2300

unit505

Google "gear calculator". You can enter all pertinent information and then some. The rear end gearing is not the final drive gear ratio. The differential gearing is only one part of that. Tire size calculator, final gear calculator and speedometer calculators are all essential for determining where your at and where you want to be. If you have any plans to change tire size, you need to plug in those changes to your calculations prior to making any moves. Also, if you don't have jack stands or a safe way to measure the differential gear, you can use one of the above calculators and input your transmission ratios and engine rpm at a given speed and determine your final drive ratio without jacking the truck up. You just need to Google your trans and get the gear ratios. The rest takes about 5 minutes of driving to figure out.

thesuperbad234

I've got the 5.0. It's been running those rpms since I still had my 235 tires on it. Now I went a little bigger with 32s and it's still the same

unit505

235 is not your tire height. What was the profile of your 235's?

Mista_Breeze

I know. And they were the factory size: 235/75r15. The 235 is the width in mm, and the 75 is the aspect ratio (percentage of the width, which is the height of the sidewall) and when I swapped them they stood at about 28 inches, my new 31s are approximately 31.8

unit505

That's nearly 4" of diameter on the final gear ratio. That should have changed your speedometer and your rpm. Of course the speed won't be noticed until you use a gps.

If your speedometer reads 55mph, your actually doing 62mph. If you had a 3.55 ratio with the 28's, you now have a 4.03. You would be doing 55mph with a speedo reading of 49.5mph. The difference in rpm is between that 49 and 55mph. Won't be much, probably 400 rpm or so. The big difference is the rpm between 55mph and 62mph. That's where you'll find a big rpm change.
Edit....not being a dick, honestly, but you cannot add a 4" larger gear and not significantly change engine rpm. You should get considerably better mpg on the highway, but suffer in town stop and go. You added gearing with a loss of torque.

http://www.onlineconversion.com/bigger_tires.htm