Choosing the right diff ratios for your engine/gearbox

[Vossie]

Hi all, read a ton of info on the subject but figured this might be a nice way for people to get straight answers pertaining to what they run.

so heres my current setup:

12HT engine => h42 gear box => 37:9 ratio diffs => 31.6" tyres

what would be the best setup for my rig? i am planning on upgrading to a h55 in the near future. so then my rig would be:

12HT engine => h55 gear box => 37:9 ratio diffs => 31.6" tyres

should i change the diffs when i do the gearbox? if so, to what? 4.11s?

i also plan on going to 33' tyres when my current set are done.

 

[shawnfj40]

if you are going to 33 foot tall tires, your gonna need a much taller gears. If your going to 33 inch tall the 4.11 will be fine.

 

[Vossie]

if you are going to 33 foot tall tires, your gonna need a much taller gears. If your going to 33 inch tall the 4.11 will be fine.

so the 4.11 are taller (longer) than 3.9? i thought numerically lower gears are longer/taller gears. so i thought the 3.9 would be over all a better gear for keeping rpm down than the 4.11. would the increase in tyre size by 2" really warrant needing a lower gear set?

 

[Vossie]

now im just getting confused... 37:9 ratio is the same as 4.11, correct?

 

[pb4ugo]

Yes, 39/9= 4:11. I guess you got those #'s either by counting the ring gear and pinion teeth or is stamped on the ring gear itself.
You can find on the web, a RPM calculator if you do a search. It will allow you to input, tire size, trans and rearend gears ratios. This will help you figure out the rpms to be for a certain speed.

 

[Living in the Past]

What you will get is a much lower first gear with the H55. If your happy with the current setup you lose the first gear your use to having. The H42 is better city driving transmission then either the H41 or H55. My my use a H55 with H42 gears in the main gear box and the overdrive gear in back with 4.11 gears would be perfect. Lower gearing for off road done in the transfer case. I gasser and don't have the low end torque of a diesel. With the roughly five to one first gear would going any steeper diff gears then you already have unless you plan on only using first gear for off road.

 

[fjdiesel]

I hate to use the word J**P on here, But... this website has been awsome for me on figuring out ratios and final drive speeds for my build.

http://www.grimmjeeper.com/gears.html

 

[1911]

I like my combination: 2F => H41 => Orion 4:1 transfer case => 3.70 diffs => 33" tires. The lower 1st gear in the H41 is good for getting rolling with the 3.70 diffs and "rubber overdrive" 33" tires. In top cog, I am only pulling about 2,600 rpm to go 65 mph. On the trail, I still have low gears in the Orion.

 

[Vossie]

ok gentleman, after A LOT more reading, have some info to contribute towards this. please feel free to correct me at any point

Gear box Top Gear and Ratios (info from www.marks4wd.com):

3 speed ... 1.1:1
4 speed (h42) 1:1
5 speed (h55) 0.845:1

Theoretical RPM at 36MPH/ 60KpH with 4.11 final drive and 31' tyres
(info from www.csgnetwork.com/multirpmcalc)

3 speed ... 1760 rpm
4 speed (h42) 1604 rpm
5 speed (h55) 1355 rpm

Theoretical RPM at 60MPH/ 100KpH with 4.11 final drive and 31' tyres

3 speed ... 2940 rpm
4 speed (h42) 2673 rpm
5 speed (h55) 2259 rpm

Theoretical RPM at 70MPH/ 120KpH with 4.11 final drive and 31' tyres

3 speed ... 3430 rpm
4 speed (h42) 3118 rpm
5 speed (h55) 2635 rpm

Theoretical RPM at 36MPH/ 60KpH with 3.7 final drive and 31' tyres

3 speed ... 1588 rpm
4 speed (h42) 1444 rpm
5 speed (h55) 1220 rpm

Theoretical RPM at 60MPH/ 100KpH with 3.7 final drive and 31' tyres

3 speed ... 2647 rpm
4 speed (h42) 2406 rpm
5 speed (h55) 2033 rpm

Theoretical RPM at 70MPH/ 120KpH with 3.7 final drive and 31' tyres

3 speed ... 3088 rpm
4 speed (h42) 2807 rpm
5 speed (h55) 2372 rpm


so, from this data, looking at what will suite me and how i drive and where i drive, the 4.11 and h42 will do fine for me for the time being as i do most of my driving around town in 60KpH zones. this is great seeing as one of the reasons i put the 12HT in was to save on fuel.

In the near future i have an h55 swap lined up. this will help even more to save fuel. however, if i put in 3.7's, the propensity to save fuel big time is there, but i may find that the rpm is starting to get a little low for prolonged periods of cruising. or will a turbo diesel be able to maintain 1220 rpm for extended periods of time?

hope this info is helpful to some

 

[iptman]

Nice work! Might be FAQ worthy right there.

 

[tls3601]

Yes maybe a moderator can put this stuff into FAQ. My brother and I both have same diffs, his is peppier on acceleration with 31's vs my 33's. But I didnt buy mine for drag races.................

 

[Tokamak]

This is a nice calculator. Pull down menu for most transmissions. Even has the H42 and H55. Don't let the heep name put you off. Tire sizes in inch and metric. Calculates speed in different gears at specified RPM.

http://www.grimmjeeper.com/gears.html

Sorry about the duplicate. Just saw fjdiesel's post.

 

[crashn]

I have a 78 with 33" tires 4:11 gears H55 and around town @45MPH in 5th = 1500 RPM. On the highway 60MPH = 2100 RPM.
The five speed works great in town also with the 2F. Off road low range with split case it crawls just fine in the Rockies.
No regrets with the H55

 

[Vossie]

I have a 78 with 33" tires 4:11 gears H55 and around town @45MPH in 5th = 1500 RPM. On the highway 60MPH = 2100 RPM.
The five speed works great in town also with the 2F. Off road low range with split case it crawls just fine in the Rockies.
No regrets with the H55

awesome, so the theoretical calculator is pretty much on the money.

the one i used says for your parameters 45MPH would be 1591 rpm and 60MPH would be 2122. thats a 6% variation for 45MPH and 1% variation at 60MPH.

what that means is that the theory and practice are very close, with accuracy improving with faster speeds to your ratios.

if more people can post up theyre speed vs rpm with diff, tyre and gear box parameters, then we can build a better base line for others, like me without tachometers to base their expectations on

 

[crashn]

Vossie,
I think some of the difference might be actual size of the tire with different air pressures and actual size compared to what the tire company say they are. My experence with the tires are when the tire company says it is 33" tall and when mounted on a 10" wide rim turns out to be 32" or less. So there are a lot of varables to consider.

 

[Vossie]

Vossie,
I think some of the difference might be actual size of the tire with different air pressures and actual size compared to what the tire company say they are. My experence with the tires are when the tire company says it is 33" tall and when mounted on a 10" wide rim turns out to be 32" or less. So there are a lot of varables to consider.

true. tyre size is probably the main reason for a higher variation at low speeds and smaller variation at high speeds. as you speed up, inertia on the tyre increases, forcing the tyre outwards and effectively "growing". this might be a very very small amount, but it will be enough to change the figures.

ultimately, the figures i posted are a rough guide. they should be able to help some one know how hard their engine is working to maintain a certain speed. its helped me big time already.

 

[Vossie]

alright, after a lot of work on excel ive come up with a pretty good RPM calculator that can be adjusted as far as final drive ratios, gear box gear ratios and tyre size.

for any one interested, the formula is as follows:

(Axle Ratio x Vehicle Speed MPH x Transmission Ratio x 336.13) / Tire Diameter INCHES
EG
(4.11 x 60 MPH x 0.845 x 336.13)/ 31 = 2259 rpm

how might you use this info? well, if your speedo is working and you have this info, then you know, to within +- 6% what your engine RPM is. knowing your rpm is a great way to get the most torque/ power and also drive efficiently.

so, for those who might be able to use this info, here is RPM's at various speeds for the H42 4 speed gear box with either 4.11/3.9 diffs and either 31/33/35 inch tyres:

 

[Vossie]

and bc im not allowed to upload too many pics to one post:

im still working on the H55 5 speed box, so its coming