Ideal Highway RPM's and Gearing For 37s

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boy&hisdogs

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I was going to suggest the 4.88s. I've got a CCLB SRW 5.7 (K3500) and a 6.0 (Suburban 2500 2wd) both 4L80e/4.10s and run about 2300-2400 @ 70 MPH. Initially I thought RPMs were high because I came from a C1500 4.3/3.42, but I really like the 4.10s now. In fact the C1500 is now a project truck, and I'm going to swap to at least 3.73s, along with an engine/tranny swap. Would of preferred 4.10s, but the AXN axle I got has 3.73s, so that's what I'll run. All my trucks run LT265/75/16 tires.

Last week I drove out of town on the suburban through a toll that goes around Austin area. Speed limit on that toll is 80MPH and a short segment up to 85 MPH! RPMs were in the 2600-2800 RPM in the 80-85 MPH, and still felt like there was power left and had acceleration. My K3500 is in rough shape, at 75 MPH it starts to feel like the dash is going to fall off! So that one stays for local duties. LOL. But both feel like they are in a sweet spot for engine power on the highway, to me at least.

Yeah I think with vehicles as heavy as a CCLB and a 2500 suburban that the lower gears are much better.
 

letitsnow

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For real world driving in both my k2500 (5.7/4.10 gears) and s10 zr2 (4.3/3.73 gears), running at low rpms doesn't help mpg much. If you are driving only on level ground, going 55 mph, no wind - then low rpms might help. But... That is rarely what we seem to end up doing. It is always windy, we usually have to drive 65+ mph to stay out of the way, and hills are there.

Maybe a 7.4 would have enough tq to benefit from low rpms, but for your 5.7 - I'd pick 4.88 or even 5.13.
 

Supercharged111

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My dually hits 2000 RPM at 52 mph (4.10 and 29" tires) while my 1500 hits 2000 at 57 mph (4.10 and 33" tires). They sing on the highway and there's nothing wrong with that.
 

boy&hisdogs

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Today I finished my first tank with the new gears. This truck has had 3 gear ratios and 4 tire sizes in the time that I've owned it, and here's what mpg I was getting (tank avg) for each setup:

265/75/16 + 3.42 = 15 mpg (Stock)
285/75/16 + 3.42 = 13-14 mpg (Steelies and AT tires)
305/70/16 + 3.42 = 12 mpg (Steelies and MT tires)
37x12.50r17 + 3.73 = 9-10 mpg (6" Lift, Alloys and Toyo RT)
37x12.50r17 + 4.88 = 11 mpg (6" Lift, Alloys and Toyo RT)

11 isn't so bad for what it is, but the real lesson here is to just get a bigger engine in my next truck. Two of my uncles have gas 2500s (99 Chevy 6.0 and 20 Dodge 6.4) that pull 10 mpg, uphill, downhill, towing, big tires, doesn't seem to matter. I'll gladly trade that one extra mpg for the 100+ extra hp. Or get a diesel...
 
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stutaeng

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Looks like the MPG improved as the final overall drive ration became numerically higher? In both the lifted and unlifted case?
 

Pinger

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Looks like the MPG improved as the final overall drive ration became numerically higher? In both the lifted and unlifted case?
My Suburban runs a 4.1 and stock sized tyres wrt diameter and when I arrived on the forum and saw all the higher diff ratios (numerically lower) available felt a bit cheated and thought I was losing a load of mpg.
Started reading more about the real life experiences with different ratios and cottoned on that higher diffs (numerically lower) tend to force more down shifts into 3rd on hilly terrain to the point that the lower diff (numerically higher) ends up delivering better mileage spending more time in OD as it does.
And that's how it seems to be panning out with my truck. It will do everything asked of it from 40mph onwards (and hold 4th at 30mph) in OD (with the TC locked) despite this being a very hilly part of Scotland. I like it!

My combination works out at around 29-30 mph/1000 rpm in OD4th. 3rd is 20 mph/1000 rpm and if that was in OD4th it would need a 6.15:1 diff on mine. Even with a 3.42 it would be 5.13. So spending all day in OD4th on a 4.1 does possibly make more sense than spending some time with a 3.42 and quite a bit of it at 5.13 (ie, 3rd) - if that makes sense. If it was a 50:50 split the aggregate of 3.42 and 5.13 is 4.275 - pretty close to 4.1. And that's before factoring in unlocking the TC and having it churning away fuel.
 

boy&hisdogs

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Looks like the MPG improved as the final overall drive ration became numerically higher? In both the lifted and unlifted case?

Every gear ratio has it's ideal tire size and vice versa. This is determined by what engine/trans your vehicle has and how you drive it. For our trucks, we have a low revving, torquey V8 or big V6 with a final transmission gear of .7-ish depending on exactly what trans you have.

These trucks came with either 3.42 or 3.73 gears, and you could get the 2500s/3500s with 4.10. The general rule of thumb for these trucks (for general purpose daily driving) is:

3.42/3.73 for stock size tires (31"-32")
4.10 for 33"s
4.56 for 35"s
4.88 for 37-38"
5.13+ for anything bigger (At this point you are getting into serious custom stuff and probably have a bunch of other mods that affect how the truck drives)

Now, if you have a specific use for the truck in mind you can go up or down a gear ratio to tailor it to your specific use. Are you a grandpa just cruising around, going to church and going fishing? You would want "taller" gears to keep your cruising RPMs lower on the highway, keep the truck quiet and save some gas. This hypothetical grandpa probably got 3.42 gears and kept his stock all-season tires. Or maybe you plan to use it as a work truck, load it down with racks and toolboxes and tow a trailer around town. You would want "lower" gears like 3.73 or 4.10 (still assuming stock tires) to help get all that weight rolling. This also applies to oversize tires. With 37s, 4.56 gears would have been closer to stock by the numbers, but those tires are wayyyy heavier than stock and I'm pushing a lot more air resistance being lifted so thats why people end up recommending 4.88s.

Taller (lower numerical) gears give you lower cruising RPMs and a higher theoretical top speed but worse acceleration, towing and hill climbing. Lower (higher numerical) gears give you the opposite: stronger acceleration/towing/climbing but higher RPMs on the highway. Too far in either direction kills your MPG and makes the truck drive bad.

Too tall (like me with 3.42s and 33s/37s) and truck STRUGGLES to maintain speed on the highway and is constantly shifting between 3 and 4 to try and find some power to drag itself up the next hill. This is one way that transmissions die early. Ask me how I know. :3811797817_8d685371 It also sucks off-road because it's hard to smoothly deliver power. Imagine you are trying to ease yourself up a big rock. You slowly start to push on the gas... nothing... so you push a little more... still nothing... nothing... OOPS too much now you're just spinning and digging holes.

Too low and it's like driving a tractor. Have you ever tried to drive a tractor on the road? You mash the pedal to the floor, the engine is screaming... and you going all of about 12 miles an hour, and getting "gallons per mile". :Big Laugh: But the good news is that your power delivery at low speeds, with heavy loads and up hills is smooth and strong.
But... around here it's usually a broke kid with his first truck, can't afford a lift or a gear change... Just trim and throw on some used 33s and cool wheels with your 3.73s. :rockit:You can usually get away with being one gear ratio or tire size off and be fine. Any more than that and it'll suck driving and waste gas. Even for me 3.42 and 33s was fine, until I started to tow more, add racks, boxes, steel wheels, etc. Then I started thinking I needed lower gears... then that turned into "If I'm going to pay for gears anyway, I might as well lift it and get even bigger tires... new ball joints and tie rod ends and wheel bearings while I'm in there... then I need a bigger axle and brakes to handle those tires... and lighter wheels to help save weight... and stiffer springs... and might as well get upgrade shocks instead of the cheapo ones the kit comes with... gotta re-do my exhaust so I gotta buy a welder for that..." on and on until all my money was gone! o_O

Keep in mind that a gear change is probably the single most expensive part of the average lifted vehicle. A lift kit is $1000, you can get decent used tires for $100 a pop, wheels for about the same, but a gear change is $2000-$3000 plus the price of a locker/posi if you get one. I sprung for all Yukon parts so mine really was 3k out the door. It was worth it to make the jump from 3.42 to 4.88 but just going up or down one ratio would be a waste.

This stuff isn't cheap, be careful not to dig yourself into a hole you can't afford. I burn $20+ a day in gas just driving to and from work and spent a down payment on a new truck's worth of money to do it. You have to ask yourself: Are you ok with the idea of working the first hour or so of your shift basically for free to cover the cost of gas in your lifted truck? :shrug:
 
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boy&hisdogs

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Or do full exhaust and a PCM tune on this truck if you haven't yet.

Thats next. Weld up all the leaks in the exhaust and send off for a blackbear tune. Probably new spark plugs too, I have a little over 120k miles on my current ones.
 
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