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Help Me Choose A Gear Ratio!

Which Gear Ratio?

  • 4.10

    Votes: 0 0.0%

  • 4.56

    Votes: 2 7.7%

  • 4.88

    Votes: 9 34.6%

  • 5.13

    Votes: 13 50.0%

  • Other

    Votes: 2 7.7%
  • Total voters
    26
  • Poll closed .
RoccoK5

RoccoK5

1/2 ton status
Joined
Dec 8, 2019
Posts
148
Reaction score
274
Location
Phoenix, AZ
I'm re-gearing my K5 (along with a bunch of other things, see my build thread) and am having trouble deciding on a gear ratio. I already bought a set of gears but now am having second thoughts based on some feedback in my build thread. I figure I'd throw up a poll and hopefully hit a wider audience here.

Truck is a 1977 K5 Blazer. Will be running 37" tires, 14 bolt FF rear, 10 bolt front. 6.2 Gen IV LS Engine, 6L80 6 speed auto, and NP241 Transfer Case. Realistically this truck will see most of its time on the street with light/medium trail duty. Mainly a camping/overlanding rig, and some trips to Moab from Phoenix where I would like to do Hell's Revenge (bypass the serious obstacles). Also will do some occasional light towing (side-by-side). Really would just like to be able to cruise down the freeway at 70-75 in 6th gear (0.67:1) without a lot of downshifting for slight grades.

The 6.2 makes great power down low (300+ lbft of torque at 1K RPM) and has a really flat torque curve, and the tranny has a super low first gear ratio (4.03:1), so I am hoping I can "get the best of both worlds": a decent crawl ratio and great highway manners, but I understand there is likely to be some tradeoffs. Anyway, here's the poll. Thanks for any input!
 
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As a reference point....what is the original factory rpm at 65 and 75mph for the 6.2/6L80 setup?
 
Grimmjeeper.com/gears.html is my go to.
I'm leaning on 5.13s. 2250 at 70.
 
Ned Kelly said:
As a reference point....what is the original factory rpm at 65 and 75mph for the 6.2/6L80 setup?
Click to expand...

Factory RPM is 1588 at 65mph and 1833 at 75mph. This is based on the 2010 Silverado w/ 3.42s and 31.6" tires that the drivetrain was removed from.
 
i drove a 2500hd crew cab short bed pulling a trailer with a 3/4 ton truck on it for 4 hr's and it SUCKED . . . if you got hills and stuff to drive in or tow a lot the thing was always screaming the rpm's to keep the truck going . so i say if you got o/d or 2x o/d go DEEP ! make the gearing work for you and not against you .

i put 5.13 in one of my projects knowing 35" tires and 4l80-e and 454 bbc will be doing around 2500 rpm at 70mph . but thats the working zone and right in the fat of it so it will not be lugging .
 
RoccoK5 said:
Factory RPM is 1588 at 65mph and 1833 at 75mph. This is based on the 2010 Silverado w/ 3.42s and 31.6" tires that the drivetrain was removed from.
Click to expand...
I voted 4.88 even though I would do 4.56 myself, seeing the first gear is 4.03:1
I put a 700r4 with a 3:1 first gear on a k5 with the same 37" tires, 4.88 gears and a tbi 350.
Does great.
 
First of all, you have to tell us how you're going to get the 6L90 working in your square body.


Secondly, Since nobody has posted it here, these are the awesome numbers:

1st :4.0267
2nd: 2.3635
3rd: 1.5319
4th: 1.1522
5th: 0.8521
6th: 0.6667

So it almost seems hard to pick a bad differential ratio. With super mild 3.73 gears, 1st gear would be similar to a 700R-4 on 5.13s, which is the common recommendation on CK5. Meanwhile you could run "full deep" 5.38 gears on the 6L90 with highway RPM similar to running 5.13s with the 700. If you ran deeper gears, like 5.13, you essentially have highway cruising ability, plus the crawl ratio of a 400/203/205 doubler.
 
4.56 would give the most similar cruise rpm to stock silverado setup. Using 37's that would have you at 1,943 rpm at 70. 4.88's come in at 2,079 at 70 and 5.13's come in at 2,186 at 70. (RPM's caluclated on the Tremec toolbox app) There's no doubt the 6.2 can rev and won't be hurt by it spinning a little faster going down the highway.

The thing that gets overlooked (I know I did with mine) is where the torque comes in at and where the gearing puts the engine in the sweet spot of torque at highway cruise speed. If you go too low numerically on the rear gear the trans will have to make up for the lack of torque at speed by forcing downshifting sooner and possibly with multiple gear downshifts. The 5.3 is more known for that over the 6.2 but I've had many get surprised by the multi-gear downshifts when climbing a steep grade when the 6 speeed transmissions came out at the dealership. I think GM under-geared these combinations in the trucks to maximize fuel economy over max power. This is why they are barely turning 1800 rpm at 70 mph. The lower the engine spins at speed the better fuel economy it's going to get. That's fine for the masses buying 1/2 ton trucks right now and look at fuel mileage numbers more than full blown power and maximum towing ability. But for a truck shaped like a barn door turning 37's you'll find if you gear it to stock values in cruise RPM at 70 mph you'll find the trans is going to be very "active" or quick to dump a gear when the load increases like on a steep hill or a grade. Gear it a little deeper and the engine will stay more in it's sweet spot for torque where the transmission won't have to drop to a lower gear as aggressively or by as many gears.

My gen 3 5.3 with a 700r4 is under-geared for my truck's weight and aerodynamic load. 70mph comes in right around 2,000 rpm with my 4.10 gears. That engine speed is just too low for the engine to maintain speed in OD if the truck starts going up hill. 2,000 rpm is low in the 5.3's torque curve. It just don't have enough cajones to maintain. Now if I kicked it into 3rd at the same speed, the RPM is now around 3,000 and well into the meat of the torque curve and it is much less likely to lose speed as quickly and require a detent downshift to 2nd gear.

What I'm getting to is if you want it to behave like stock (busy downshifting included), stick with the 4.56. If you want a more mechanical advantage to keep the engine in the torque band for more power and less downshifting go for deeper gearing.
 
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I am running a similar set up to you in my truck except I have a Dana-60 in the front. , and I have already bought a set of 5.13 gears for it.
 
obijuank5 said:
Grimmjeeper.com/gears.html is my go to.
I'm leaning on 5.13s. 2250 at 70.
Click to expand...

Thanks for the link! This is a much more detailed calculator than I was using.

sweetk30 said:
since he is going 6.2ls swap i would assume he is doing a matched combo and harness for the 6.2ls/6l90 combo .
Click to expand...

Correct, although tranny is a 6L80, not 6L90. (y)

ZooMad75 said:
What I'm getting to is if you want it to behave like stock (busy downshifting included), stick with the 4.56. If you want a more mechanical advantage to keep the engine in the torque band for more power and less downshifting go for deeper gearing.
Click to expand...

Thanks for your detailed and thoughtful reply! This is exactly what I am shooting for: selecting the right gear ratio for cruising at 70-75 mph that will 1) not be hard on my trans, 2) not result in downshifting at every slight grade, and 3) not waste engine RPMs (and fuel) by going too deep. The last point is the tough one for me, but seems the consensus is that 4.56 might not be deep enough and that it's generally better to err on the side of too deep rather than not deep enough.

1-ton said:
I am running a similar set up to you in my truck except I have a Dana-60 in the front. , and I have already bought a set of 5.13 gears for it.
Click to expand...

Wish I had a Dana 60 up front, but those things seem to be unobtainium (or $$$$) around here.

Looks like 5.13 is getting the most votes so far. This just seems sooo deep to me. :unsure: But looking at this chart from grimmjeeper (thanks, @obijuank5!), the difference in RPMs at 75mph with 4.56 vs 5.13 actually isn't too dramatic (2081 vs 2341):

road-rpm.png

Honestly, if I knew 4.56 would be deep enough to keep me in 6th most of the time on the highway, I'd have made my decision by now. The JK folks at rpmextreme with similar setups (6.2/6L80/37") say that 4.56 works fine (see about halfway down the page), but there is a lot of emphasis on trans tuning (I am definitely going to tune), and I'm not sure whether comparing results with a 4 door JK vs a K5 is valid...
 
As I already recommended in your build thread, 5.13’s all day. I run them with a 3spd and with an extra gear they would be perfect for highway cruising.
 
Dont compare a jk on 37s to your truck or a stock half ton.

It's harder to keep bigger tires rolling so you have to gear for the rolling weight as well as the trans. Adding bigger tires is like pulling a small trailer. It's just extra crap the truck has to deal with and you need to include that in your gear choice, not just a calculator. Look at the entire scope of the project. Here, jk people who travel to the coast alot say they want higher gears to keep their rpm down. Its windy as hell blowing in from the coast so you're fighting it all the way down for 3 hours. In reality their jk on 35s need the most gear advantage to fight the wind resistance and actually use OD.

Gear for the duty of the truck, not necessarily the od ratio.
 
The only thing I don't see mentioned here, is with the 10 bolt front, or any differential, the lower the gear ratio, (higher numerically), the weaker the ring and pinion becomes, due to less tooth engagement. There will be a strength trade off, going for the lower gears. Driving style will tell the tale.
 
500$k5 said:
The only thing I don't see mentioned here, is with the 10 bolt front, or any differential, the lower the gear ratio, (higher numerically), the weaker the ring and pinion becomes, due to less tooth engagement. There will be a strength trade off, going for the lower gears. Driving style will tell the tale.
Click to expand...
That's why I like to stay with 4.56 or less
 
I daily drive a '12 Silverado with the 6.2/6L80/3.42 and 33" tires. The 3.42s and stock tires were an okay compromise combo for both economy and power. Since upgrading to 33"s I do wish I had 3.73s or 4.10s. However to compensate I've raised the shift points and got quite a bit of power back. The stock shift points put it in 6th at 42mph which is just ridiculous. I have it set to shift to 6th at 54mph now. There's a lot to be gained with transmission tuning on these 6L80s.

As far as power goes, the torque really starts coming in at 2000 rpm so I'd shoot to be at or just above that with highway cruising. With worse aerodynamics and much more rolling resistance, I'd shoot for a little higher. Tuning can compensate for lesser gearing but why not start with a more ideal setup?
 
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