i recently put larger than stock tires....rather large.....on my truck and a K&N intake kit.

what advantages/disadvantages will changing my rear gears give me?
what differential/rear axle cover replacement would you recommend?

also, is there any truth behind what people say about k&n's worsening gas mileage?

 

If you went with larger tires, then you could go with a different gear ration to bring it back to stock. Going with a higher numerical gear will give you more torque, which you lost when you went with a bigger tire.

As for the diff cover, I have heard that chrome does not dissipate heat and so they should be avoided. I would go with a forged aluminum one if you could find it. They are nice and dissipate heat very well.

 

would bringing back that torque that i lost help any with gas milage?
i noticed a huge drop when i added the tires.

and also, will the truck over rev at highway speeds?

60-70?

 

More than likely not.

Now, with the new tires, did you recalibrate your speedo? If not, and you are computing mileage without a correction factor for the tires, then your fuel calculations will be waaaaaaaayyyy off.

With proper gearing, you won't be overreving. Your simply returning back to the stock "overall effective" gear ratio. This will make your truck accelerate, shift, and cruise like it did before.

 

I disagree with Pocket. I think getting that torque back will likely increase your mileage. You may move your horsepower/torque into a more usable rpm or speed and that may increase your mileage if your truck isn't "lugging" as much. Your truck may become more efficient if the horsepower/torque is at a more usable rpm or speed. It's dependant though, on your tire size, what speed or rpm you drive, etc.

 

If you have a four wheel drive you cant just change rear gears you have to change both front and rear.

 

Yep, final drive makes a hugh difference in mileage. By huge tires, 44" should have a very low gear ratio, 35" normally likes around 4.30-4.88:1, the 4.11 or 3.73's are not much help over 33". My 35" tires and 3.73's suck on my 5.4. I am going to do 4.56 to level it out.

 

Well so far with my own experience, I haven't gained any mileage by going from 3.73's to 4.30's on the same set of tires (325/60r20). I'm still checking mileage, so I have a few more tanks to go before I'm certain. But I just pass on the knowledge that I've experienced first hand so far.

As far as torque in a more useable range..... of course yeah. Finally I can go up mountain passes without smoking out the rest of traffic because before the gear swap I'm going up a 8% grade and only running at 1400 rpm's.

 

gears will definetly help out with your larger tires. you will regain your loss of low end power and with less "strain" you will efectivly restore most of your gas milage...........now keep in mine that there are other factors that will change the power "feel" from the seat, wide tires, narrow tires, differnt mods, different truck configurations ect.... BUT you will be able to re-gain MOST of what you lost!

 

how can i figure out what gear ratio it has from the factory?
and whats the best way to tell what gear ratio i need to even it out?

 

and what brand name can i trust for gearing?

how does the truck measure speed?
will correcting the gearing in the differential fix my speedo?

 

Your gearing is on the tag on the differential. You crawl under there and look at it. It'll say 3.73 or 410 on it.
Most sites you can order from will at least narrow down what you have for axles through a drop down list. You could have a dana 50 or 60 front, a Sterling 10.5 rear. That'll get you ordering the right gears. And you have to know that.
Another thing you can find on those sites are calculators. So you go to one, you say stock I had 3.73, I had 32" tires, I want 37" tires. So it might say stock you're 1650 for rpms at 60mph.
Now you put in the 37" tires and recalculate. Maybe that gets you in the low 1400's. So try out the proposed gear and see what gets you back where you started.
Other calculators will give you the new effective gearing with the tires if you leave the diff alone. And let you solve for effective gearing if you change it.
I just went and looked. Accuautoparts has the drill down list and a calculator. Try that and see where it gets you. ring-pinion . com also has a different calculator.

 

I don't know if you have a gas or diesel truck, but typically you want to err on the side of lower gearing, so say 4.10 gets you almost to the rpm range you want or almost to the effective gearing but 4.30 gets you 100 rpm higher than stock, you typically would want to go with the 4.30 gears. But a diesel and gas engine have their useful rpm/torque curve, etc in different rpm ranges, so just be aware of what your stock rpm usually is for whatever mph you drive and try to get close to that, and possible err on the side of one step up in gearing.

Also, you might call a local driveline shop about what brand and cost for installation. You may get a good deal on gears online, but the shop may charge more for labor since they're not supplying the gears, so it's best to do your homework about local places too. They might give you a good idea of what brand they use, or if they use more than one brand due to price (they may offer a lower quality brand to lower cost as an option).

Another thing to consider, since you're in the differential, is if you ever want to add a limited slip or locker, NOW is the time to do it (or have your stock limited slip rebuilt). You're paying for labor to install the gears, and generally the shop won't charge any more for labor to install the limited slip or locker at the same time, so it's cheaper (labor-wise) to do the work all in one fell swoop.

You can also use your VIN or Axle code (typically on the door jamb) to find your axle ratio and whether you have the stock limited slip. I hope that helps.

Edit: By the way, and this is just my opinion based on my 20 years of offroading experience, gearing and limited slips/lockers are THE best way to increase torque, increase crawl ratio, and make a vehicle a better "offroad" vehicle. It may cost two grand to add gears and lockers to a 4x4, but I think it is the most cost effective upgrade (except maybe a good set of tires).

 

thanks to all!

what would be a good rpm to be running at at say 65 mph on the highway?

 

Sorry if I missed it, but what engine do you have, they all develop their power in different rpm ranges?