Choosing gears

[ksq2]

The rev. thread got me thinking. I was planning on using Richmond Gear's calculator to help me decide on what gears to go in our '78 Cherokee. The main concern I have is pulling a 26' camper trailer with it and not taxing the engine too much. We're going to run 33" tires and I'm planning on going with gearing somewhere between 4:10 and 4:56. Am I on the right track? Richmond gear's calculater puts 33" tires with 4:56 gears at 65 mph at 3018 rpm.

[Gambler68]

Plug in all the right ratios for your tcase and trans? I pull 3k at 65 with 3.54s, a QT, TH400 and 33 inch tires.

[ksq2]

I'm assuming a th400 runs at a 1.0 ratio in third gear, right? Gambler, I'll have the same set up as you th400, QT...

[ksq2]

Just used Randy's ring and pinion calculator and came up with this: 4:10 r&p, 33" tires, 65mph, 1.0 transmission ratio = 2713 rpm.

[Gambler68]

I think in the 'real world' the TH400 can vary in how much power its sucking up..I think your rpms would actually be a bit higher. Just my limited experience running 2 different TH400s, and two different QTs (one FT, one 1:1 PT) in the same 79 Chero with the same tires.

While I hit 3k pretty quick around 60-65 (with stock speedo gears) it can go up to 85-100 before hitting 4k..pretty wide range.

[joe]

You're in the right area. Richmonds calc uses as sold tire size which is close enough but for accuraccy use actual tire size since rarely (if ever?) is actual size same as what's printed on the tire. Most 33's are really 32.5 depending on manufacturer.

Also trannies unless an OD trans have a high gear of 1:1 so it left out of the equation as is tcase ratio since most high range ratios are 1:1 Zuk's used to have a 1.5:high ratio though. For most rigs with 1:1 high trans/tcase ratios it's not needed. for hwy rpm's. Try punching in 4.27 gears and see what ya think?

rpm = [mph x axle ratio x 336] / tire dia.

For a manual trans a different constant is used but I'm not sure what it is (possibly 320 but don't really know).
If figuring anything other than high gear or OD gear ya need to multiply whats in the brackets by that gear ratio first, then solve the formula.

[ksq2]

Joe, using your formula with a 4.27 gear, and substituting 32.5" for 33", I came up with 2869 rpm. Sounds pretty good. Gambler still has me wondering though, are these calculators/formulas "real world" accurate?

[kwai]

The formulas are "real world" accurate. The calculation of speed based on engine rpm/driveline ratios is pure mathematics. The size/hp of the motor is irrelevant.

[Gambler68]

One way to know for sure is to put in your CURRENT setup and see if it's on the money..

[Dirt Wag]

I like this calculator >>>> http://www.grimmjeeper.com/gears.html It lets you plug in YOUR transmission, YOUR t-case etc. You don't need to know the ratios if they are stock.

Auto trannys will rev a little higher than manuals because of the torque converter but you probably knew that already.

[Wagonforever]

Have you gone to the bigger tires yet? If not I would stay with the stock 235/75/15C's in an LT. This will make your truck much more stable while pulling a Travel Trailer as large as your's as the side walls will be stiffer and the center of gravity of the truck closer to the ground. I pull a 24ft Travel Trailer with my 86GW and with stock tires and the stock 3.31 gears it has pretty good power. I plan on putting money in the engine to pull abit more power out of it when I rebuild it as I think the drivability with 3.31's is just about right without an over drive trans. Also I generally tow at 55mph, which keeps the RPMs down, also keeps the rig much more stable, plus trailer tired are only rated to 65mph, and last above 55mph my fuel milage drops CONSIDERABLY the faster I go. At 55mph I get about an average (depending on wind) 9mpg towing, at 60 7mpg, at 65 I could watch the needle drop!

[68glad]

[quote=Dirt Wag]I like this calculator >>>> http://www.grimmjeeper.com/gears.html It lets you plug in YOUR transmission, YOUR t-case etc. You don't need to know the ratios if they are stock.quote]

Agreed. By far the best i've seen.

[ksq2]

That grimmjeeper calculator is pretty cool. I love the given crawl speed calculation! Thanks!

[Wagonforever]

Running the numbers on that calculator I'm even more convinced 3.31's are the ideal ratio for stock tire size.

With the 3.31's at 55mph I'm turning 2,238rpm, at 70mph 2,848

Even just going up to 3.73's really gets this engine cranking at 55mph 2,522rpm and at 70mph 3,210. IMO 3,200rpm is pretty fast to consistantly turn an engine.


Edit: I just put in the numbers with your proposed combination of 4.56 gears and 33 inch tires and got almost 3,500rpm at 70mph.

[addicted]

IMO 3,200rpm is pretty fast to consistantly turn an engine.

X2
Especially an old design V8 or I-6. That and at that RPM you are nearing the end of it's power band, unless you've done some work to it.
My old '80 J10 had a 360/727/219 with 4.09 gears and 32x11.50-15's and it would turn 3200 RPM at 70. I wasn't particularly fond of the screaming engine running down the highway.