http://www.differentials.com/calc.html


The link above calculates under 2200 rpm for an auto with 4.56 gears @ 70 mph and over 2400 rpm at the same speed with a manual so as tani asked what do you have auto or manual trans. From what i know i would definitely go to 4.56 with an auto and I might think about 4.10's with a manual, I usually only drive 65 mph max (no interstates in Canada) so I'd go with 4.56's either way. I'm running 31's with 3.55 and can't imagine going with bigger tires without changing gears.

Tani I wish. I've just got my truck to the point it where it pulls reasonably close to where it should. Up to about a week ago I'd say my '99 Malibu (150 hp) would kick my truck's ***, but thanks to mods that I've mostly picked up from this site it would be good a race. I've got no sticker, but man I so must've gotten the death flash.

ddog737374
I'm planning on putting 33's on and then upgrading to 4.10's, but if I had planned on running 35's I'd definitely be putting in 4.56's.

Big_N_Bad.
I know it doesn't sound right but I know at least 5 people (most are forum members and that's in a town of 4000) that are running lower gear ratios than me and they all get as good or better mileage than me. Bringing the truck up in rpm's closer to the power band seems to do a lot for mileage as long as you don't go to far. There's been a debate or two on this issue and I don't believe anything has truly been resolved. I think the bringing the rpm's helps, but then again my truck sounds awesome pegged so I'm not the one to ask. The last time I checked I got 10 mpg in town, but half the time I run in low 2 (auto trans.) just to **** off the nieghbours.

 

It's not just about RPM, alot of it is about keeping the engine in the most efficient powermaking range.

 

maybe or i just got one hell of a motor oh yea i forgot to mention mines a manual though

 

so how hard is it to change out ur gears any one every done it? also with 4.56 what would it do to my higway speeds?

 

I hear you on that one. My truck takes forever to get up hills from a stoplight. I constantly see people goin around me because I don't feel like redlining it just to get up a hill :/

I've been thinking about gears/tires/lifts for awhile now and I got a question. How much should I be looking to spend for 4.10's (next up from 3.55 should be fine with 33s right?) and a 3 inch suspension lift? (susp > body right?). I've been thinking about getting smaller tires when I need to but don't really want to, but I'm crazy how there's very little room in the wheel well as of now. To me it makes my truck look very low. So I was thinking possibly 33's with 4.10 and a 3" body lift. The 3" should clear my of rubbing I am getting now at full wheel lock to the left (weird only happens when I make left turns). How much should I be lookin to spend? And from the lift and gears should I be getting better or worse milage, I would think better from the gears, I don't think 3 more inches would create that much more resistance, but would that extra rougly 200-400 more rpm in overdrive on the highway be bad?

 

stock 245's with 3.55's @ 70 mph should be running 1916 rpm
changing to 35's and 4.56's @ 70 mph should end up running 2145 rpm.
A difference of 229. Should be good on the highway, close to stock rpm's.

 

Here's a handy Tire/ gear/speed chart....note RPMs in O.d. may be about 7 to 10 percent less than shown.

I hope this helps...


http://www.enjoythedrive.com/content/?id=25828




BTW, I made a trip up to Prescott from Phoenix today. With an unusual headwind, I could not run my O.D in the mountains. I ended up running about 2500 rpm at about 65, the wierd part is that my mileage round trip either stayed the same...OR IMPROVED. I'll let you know when I run the tank out.

Also..here's the accepted formula for figuring your RPM ...or gear ratio by the RPM you desire.
Experience tells me the torgue/hp curve meets at about 2500 rpm on the average stock or lightly modified 5.9.

MPH x Gear Ratio
--------------------- x 336 = Engine RPM
Tire Diameter

 

ok well im still stuck 4.10s i think would be better for 33s and 4.56 would be good for my 35s the only thing is never go faster then 80 mph on the highway but even at 80 mph id be pushing 3700 RPM is that good or bad? what do u guys think 4.10s or 4.56s i do about 80% city and 20% highway and i still want to be able to get my truck to 80 mph

Thanks Dan

 

What transmision do you have (auto or manual).

I'm not sure I see how you'll be running 3700 RPM with 35's and 4.56's @ 80 MPH.

What RPM are you running @ 80 MPH now with your 3.55 gears.

I was thinking you'd only see an increase of 800 RPM +/-, which means your turning something like 2900 RPM @ 80 MPH right now. I don't turn that much and I'm only running 31's with 3.55's.

Someone correct me if I'm wrong but 3700 RPM seems high for 4.56's with 35's @ 80 MPH

I was having trouble deciding on 4.10's or 4.56's and I was only going to run 33's. I was going to tow and I only run about 65 MPH max so I figured the RPM's wouldn't climb that high????

 

Your numbers do not make sense-- its like your figuring them without counting the tire size as gear altering.

With 35's you should be great with 4:10s...I would not go to 4.56 because you would be unable to tow at a decent rpm.....most guys running 37's or more may push the 4:56s but not with 35's

My rec is 4:10's with 35's and you'll be pushing about 2500 rpm (tops) at 68 to 72-- pretty good range to be in!!!!!!!!!!!!!!!