What is the difference between 4.10 gears and 4.56 gears on the blazer? I know I've got 3.42 right now, and I've read some posts that putting on 31-32' tires will result in a bit more sluggishness due to the larger tires...and these same posts mention upgrading gears to fix the problem.

I understand the general idea of gearing, but how much difference between 4.10-4.56? It's only 10 bucks between the two on Synergy Offroad. Also, would there be any negative effects from putting on the higher ratio gears with stock size tires?

EDIT: through further research it seems the front diff is a sealed unit? Meaning no touching the gears up front, thus requiring an axle swap to up the gearing?

 

if you put higher gears in it now, you will lose low end grunt some i dont know how much or if you will really feel the difference

 

Going from 3.xx ratio to 4.xx ratio is not going to higher ratio, it's going to a lower ratio which would increase low end power at the expense of top end speed. I personally would not go lower than 4.10 ratio in a Blazer. I don't have 4wd, but I think you need to match the front axle ratio to the rear axle when changing gearing.

 

Ah ok that makes sense..Any idea how much top end is being decreased? I'm assuming its not a straight 'cut' but more like at 70 mph instead of 2000 rpm with 3.42, with 4.10 gears id be pushing 2500 rpm or so?

 

You have to take into account the change in tire diameter as well. If you have 235/75R15 (29") tires stock with 3.42:1 gears, you are going 26.6" for every revolution of the driveshaft. If you are going to 31" tires with 4.10:1 gears, you will go 23.1" for every revolution of the driveshaft. Shorter distance means lower gear ratio. So for 32" tires with 4.10:1, 24.4" per rev.

Now, what impact will this have on your engine speed at 60mph...? To make this calculation easier, I'm going to give you 3rd gear RPM with the torque converter lock because 3rd gear is 1:1. Stock at 60mph (5280 ft/min) in 3rd gear, you should be a bit lower than 2400RPM. With 31" tires and 4.10 gears, 2750RPM. 32"/4.10, 2600RPM. For the closest to stock, 31" tires with 3.73 gears will result in a bit higher than 2400RPM. If you made the jump all the way to 4.56 gears with 32" tires, at 60MPH in 3rd gear you would be turning almost 2900RPM!!! To figure your OD RPM, simply multiply these numbers by .696 which is the ratio for OD in the 4L60E.

I wouldn't worry about your top end... With 32" tires, 4.56 gears, and running at 5500RPM in 3rd gear (again for simplicity), you will be travelling at 114MPH. Way faster than you should be going!!

BTW, math is fun! LOL

RPM * tire circumference[FT] / ( trans ratio * axle ratio * 88 [FPM/MPH] ) == MPH

FPM == feet per minute
RPM == revolutions per minute
MPH == miles per hour
Tire Circumference[FT] = 3.14159 * Tire Diameter [in] / 12 [inch/foot]

 

So if I went with bigger tires...say 31's

I would have to go with 4.10 gears? or would stock gears work?

 

That all depends on what your stock gears are. For the OP with 3.42, to get close to stock performance, he would need to go to 3.73 gears with 31" tires. But I made an assumption in my previous post... That he had 235/75R15 tires which should cover a majority of Blazer's. With 235/70R15 tires, you would reduce the diameter by 1" or multiply the RPM listed for the stock performance by ~1.04.

 

So to retain essentially stock driving characteristics (engine wise at least), I would only need to go to 4.10 with 33' tires, which would put RPMs back down around 2400 if Im understanding this right. (rough estimations, Ima bust out the calculator tomorrow).

btw, thanks for all the info Kyle!

Now what about the front end..is it sealed or not sealed?

 

What do you mean by sealed?? You will have to remove the front axle to swap gears.

33" tire with 4.10 gears will run at 2500RPM at 60MPH in 3rd gear. 34" tires & 4.10 gears would be closer to 2400RPM at 60MPH in 3rd.

Now, this is just a comparison of stock to different combinations. There is nothing wrong with going to something different. My Bravada had 235/70R15 tires with 3.73 gears for a 3rd gear 60mph cruise at ~2700RPM. You can have your PCM programmed for the different tires and gears so correct your speedometer and shift points, something I would recommend even if you were to find a combination that was close to stock. By dropping the gear ratio, you will have better mechanical advantage, meaning better acceleration. Something that would be good when trying to move big, meaty tires!

 

I knew swapping the front gears would require opening the diff/dropping axle..whatever is needed to get at the diff..but one of the other posts I read seemed to allude to the fact that the front diff's gears were not able to be changed. Hrm...that doesn't really make sense the more I think about it..then it wouldn't really be service-able would it..