Hi all, I just purchase a 1982 truck with a I6 and the tranny np435 and 35 size tires, it has a 2.75 axle ratio so I want to change it, I read the best fit could be a 4.56 what you recommend?.

 

Welcome to FTE.

What model truck? Sounds like an F150 with that gear.

Are you going to stay with that tire size?

What are you going to use the truck for?

Look at your rear axle, and see if it looks like this:

http://www.cometeastcarclub.org/images/75753-500-0.jpg

Right now, you should be running about 1584 rpms at 60. Just a little low. Does it pull 60 in 4th ok? 38 mph per 1000 rpms.

With a 4.56, you will run 2627 rpm at 60, or 23 mph per 1000 rpms.

So the "correct" ratio depends on what you want to do with the truck and where most of your driving will be.

 

thanks for your reply, its a F150 2wd and I will keep the same tire size, I use the truck off road and highway, my problem is the gas millage is really low 11 or less MPG.

the Differential its the same as your picture a 9 inch 31 spinline.

 

4.56 seems like too low of a rearend to me. The 300 is a low revving torque motor, so you'd probably be better off thinking of it as a gas powered diesel motor. I have a 5sp with 4.11's, and I'd much rather have something higher, somewhere between 3.50-3.75. Just my 2 cents. BTW, 11 mpg seems pretty low too. That's what I was getting with my 250 CC, and found a plethora of vacuum leaks, 9 bent pushrods, and a clapped out cam. I think I'd poke around your motor first to see why its getting such bad mileage.

 

You can't possibly be getting accurate MPG ratio's. Unless someone has changed the speedo gear, your MPH are way off. If someone put 35's on and didn't change the diff gears, then I would suspect the speedo gear wasn't changed either as it's a much easier job to do.

Now as far as gears go, I wouldn't go any lower than 4.11. I ran 4.11's on 38's with an I6 and NP435 and it was perfect. If you use 4.11's and run the 35's you will be a bit low on the highway. It should scream around town though. Personaly I would try and find a stock 9 inch drop out from a 4x4 that has the factory locker and 3.55's in it. It would be an easy bolt in swap with no real technical ability needed. No need with having someone set up your gears and everything.

 

I'm confused....you're not happy with your gas mileage now, yet getting ready to make it worse with the new gears?

 

To put it another way, your current set up is not ideal for MPG's, but it is almost the "best" set up for MPGs with a 2wd truck. The big tires are not good for mileage.

As per prior posts, check your speedo and odo. The combination you have would require at least a change in the driven speedo gear to be close to accurate.

Check your odo with a GPS if you have one.

Also, check your engine for other issues.

Going to a 4.56 would be worse for gas in almost all circumstances, all else being the same or equal.

 

"The big tires are not good for mileage"

"...check your speedo and odo."

"...check your engine for other issues." Especially the possible vacuum issue.

You have some work to do before changing the rear-end ratio.

 

Agreed with the others. Gears are $$$$. The amount of miles you'd have to drive to recoup would be a lot.

Also, as others have said, be sure your speedometer is correct before anything else. Changing out the gear to correct your speedometer is inexpensive and easy. The hardest part is probably figuring out which one you need since you don't always know what the internal ring gear is (it's either 7, 8, or 9 tooth if I remember right).

There are two gears inside the transmission. One is internal, is about $30, and takes removing some of the housing to access.

The other is on the end of the speedometer gear that goes into the tail housing of the transmission. It's held in with one bolt. A little tug and it'll come out. Pull off the C clamp, swap it, replace C clamp, reinsert, replace bolt, done. 2 - 3 minutes. That one is about $5 - $8 depending on where you get it.

The way to calculate which one you need is:

(Tire RPM * gear ratio * internal ring gear) / 1000


Either way, back to gears. 4.56 gears and 35" tires is usually V8 guys talking. The 300 likes its RPMs a little lower.

Use this formula:

Final Drive = (RPM * Tire Diameter) / (Speed * Transmission Gear Ratio * 336)

The NP435 has a top gear ratio of 1:1. If you wanted to cruise at around 2100 RPMs (which would probably be ideal for 35" tires)

Final Drive = (2100 * 35) / (60 * 1 * 336)
Final Drive = 73500 / 20160
Final Drive = 3.64

Closest would be 3.73s. 4.11s if you want it a little more aggressive.
Remember, you have that insanely low granny gear to help get you moving if you ever need it.