73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 78longbed4x4 on February 23, 2015, 09:07:01 pm
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Ok guys I'm lookin for a little advice. I just bought an 86 k10, stock 350, sm465 tranny with 208 transfer case and I THINK 3.73 gears with 31/10.5/15's. It's just goin to be a daily driver, nothin special. I drive about 60 miles a day to work and back up and down the good ole Kansas turnpike where the speed limit is 75. I don't really wanna have to turn my engine 3500 RPMs or so to keep up that speed. My question is, would it be worth it to change gears to somethin like 3.08s ? I'm sure bigger tires would help a little too, but it came with brand new 31's on it and I'd hate to put new tires on it already. Would higher gears help or should I just drive her about 65-70 and stay outta the way? Any input would help. Thanks guys
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New tires are a lot cheaper than changing two sets of axle gears
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i went from 31s to 33s last year and it only made a few hundred rpm difference at cruising speeds.
you are going to be sucking alot more gas once you get up to those speeds anyway. if i was you i would stay around 60-65 in the slow lane. you are going to be saving alot of gas money rather than spending alot of money just to get somewhere quicker.
these trucks are like barn doors in the aerodynamics department
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Where in kansas do you live?? I live in Lawrence.
I'll trade you axles. I have a 3.42 gear ratio. 10 bolt GM rear axle and 10 bolt font axle. I drove from Lawrence to Wichita a few weeks ago and was cruzing at 65-70 no problem.
The reason I want to new axles is I need a higher gear ratio because I have 35" tires. Let me know if this is an option you would want to explore.
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I wouldn't be thinking I'd be looking at your gear ratio and seeing what you have.
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I wouldn't be thinking I'd be looking at your gear ratio and seeing what you have.
You can calculate the ratio by turning the wheel and watching how many times the axle spins.
- Chalk the front wheels and make the sure you're doing this on a flat surface. This is very important because the parking brake to the rear won't stop the front from rolling.
- Put it in neutral and disengage the parking break.
- Lift the rear end up with a floor jack on the diff pumpkin. If you have an open differential, you must keep one wheel on the ground. You can know it's an open diff because, with both wheels in the air, if you turn one wheel the other wheel will turn in the opposite direction.
- If you do have an open differential, you'll have to put one wheel back onto the ground.
- Place your floorjacks under the axle for safety. Don't forget to chalk the front tires.
- Rotate the wheel 360 degrees and watch very closely how many times the axle rotates.
Here are the calculations:
Wheels Spin in the SAME direction.
A hair over 2 1/2 times or 2.5 times, then safe to say ratio is 2.56:1
A hair under 2 3/4 times or 2.75 times, then safe to say ratio is 2.73:1
A bit over 3 times, then safe to say ratio is 3.08 or 3.07:1
A hair under 3 1/2 or 3.5 times, = 3.42:1
A hair over 3 1/2 or 3.5 times, = 3.54 or 3.55:1
A hair under 3 3/4 or 3.75 times = 3.73:1
A bit over 4 times = 4.10 or 4.11:1
A hair over 4 1/2 or 4.5 times = 4.56 or 4.57:1
A bit under 5 times = 4.88:1
A bit over 5 times = 5.13:1
Wheels Spin in the OPPOSITE direction.
A hair over 1 1/4 times or 1.25 times, x2 then safe to say ratio is 2.56:1
A bit over 1 1/3 times or 1.33 times, x2 then safe to say ratio is 2.73:1
A bit over 1 1/2 or 1.5 times, x2 the safe to say ratio is 3.08 or 3.07:1
A bit under 1 3/4 or 1.75 times, x2 = 3.42:1
A hair over 1 3/4 or 1.75 times, x2 = 3.54 or 3.55:1
A heavy bit under 2 times x2 times = 3.73:1
A bit over 2 times x2= 4.10 or 4.11:1
A hair over 2 1/4 or 2.25 times x2= 4.56 or 4.57:1
A bit under 2 1/2 or 2.5 times x2 = 4.88:1
A bit over 2 1/2 or 2.5 times x2 = 5.13:1
I copy & pasted some of this info from gmsquarebody.com - full instructions here: http://www.gmsquarebody.com/forum/showthread.php?t=3860
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Ok I'll do that this weekend and see exactly what I have
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So I checked into what gears I have and it looks like they're 3.73s....I bought the truck from a guy I work with and he said he has a set of 2.73s that he'll give to me. Also I've decided to go to 33" tires so that should put me somewhere around 2000-2300 rpm at 70 or so, so that should be plenty good for highway drivin. Thanks for the help guys
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2.73 and 33"s is gonna be a slow start off. But with a 465 you're not going any where in a hurry no matter what gear or Tires you have
We have 3.73 and 35"s and it's just about right
Wish they were 4.11
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Yeah I fugured the granny low would finally come in handy....I'll let y'all know how it works out....probably gonna do the gear swap after work this week sometime
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33" tires with 2.73 gears? Hope you're in the flat part of Kansas!
Also, I don't think the granny gear is syncronized. It's not meant to be used as the 1st gear. Correct me if I'm wrong.
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With your TH350, 3.73:1 Rear Gears and 31" tires you should see about 3,100 RPMs at 75 MPH.
With 33" tires and 2.73:1 gears you would see about 2,100 RPMs at 75 MPH. That is about what I pull in OD with 4.10:1 rear gears and 35" tall tires. It is awfully comfy at 75!
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With your TH350, 3.73:1 Rear Gears and 31" tires you should see about 3,100 RPMs at 75 MPH.
With 33" tires and 2.73:1 gears you would see about 2,100 RPMs at 75 MPH. That is about what I pull in OD with 4.10:1 rear gears and 35" tall tires. It is awfully comfy at 75!
I bet it is, but you have 3 or 4 gears to get to that OD.
I have a sm365, 35" tires and 3.42 gears. And let me tell you, driving around town SUCKS! It's a dog up hill, and there's a lot of little hills where I live, lol. I'm working on the engine performance, but I need smaller tires or bigger gears lol
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With your TH350, 3.73:1 Rear Gears and 31" tires you should see about 3,100 RPMs at 75 MPH.
With 33" tires and 2.73:1 gears you would see about 2,100 RPMs at 75 MPH. That is about what I pull in OD with 4.10:1 rear gears and 35" tall tires. It is awfully comfy at 75!
I bet it is, but you have 3 or 4 gears to get to that OD.
I have a sm365, 35" tires and 3.42 gears. And let me tell you, driving around town SUCKS! It's a dog up hill, and there's a lot of little hills where I live, lol. I'm working on the engine performance, but I need smaller tires or bigger gears lol
It's the transmission
We have 3.73. 35" tires and just swapped in a NV 3500 5 speed and it's light years better than the 465 even though around town you don't get to use the OD 5th
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Go to 4low.com it has a calculator to help you figure out the best setup for what you want.
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If you were going to consider changing gears I would probably go with a 3.42:1. That would get your RPMs down to about 2,800 RPMs @ 75 MPH with your 31" tires. However, you have a 4WD so that is 2 sets of gear changes! Just the gear sets alone are going to be $600! Then you probably need to pay someone to do the work. That will be at least double I am guessing...so something like $1,200?! - This gear change will be decent on the highway. It won't be super-low RPMs but it will be okay to drive like that for long trips. My '69 Chevelle turns about 2,600 RPM at 75 and it is decent. You would be better with something like a 3.36 gear if you can find it.
If you wanted to simply change tires you would need something like a 35" tall tire to make that difference, but then you are going to have to lift the truck to fit them under and you are looking at a 6" lift and new tires. Tires will be close to the cost of the gear change, but add the cost of a lift!
The other option is changing the transmission or adding a Gear Vendors overdrive. Neither of those options is cheap.
The least expensive option is slowing down and taking more time to drive your commute!
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Just went on 4lo.com. using your current gears and a set of 285/75r16 tires you would be a little under 2700rpm at 70mph. Ejortan's right about needing 35s to get down lower. There's no real easy cheep fix.
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Well I went ahead and did the gear swap on just the rear diff for now, I'll change the front diff this weekend hopefully. But with 2.73's and 31's at 70 mph it runs about 2100 or so with plenty of power to pass if I need to or if I run into a steep hill. I think I'm still gonna move up to 33" tires here pretty quick. I'll never tow or haul anything with this truck, it's just a daily driver, so I think it'll be just fine with this setup. Feels pretty comfortable to drive and might not be too terrible on fuel