Highway gears

[Nomadicwildflowers]

Is it recommended to put highway gears on a skoolie? We have a thomas built diesel pusher with a Allison MD3000. Any thoughts?

 

[kromboy]

I think it's an attractive upgrade for most cases, but with an MD3060 myself, I have the option to unlock 6th gear or change the ratio, or both?

Personally, I'm gonna go with the 6th gear unlock and not mess with the rear diff.

I'd say it also depends on what your ratio is currently.

 

[s2mikon]

6th gear is 13 percent IIRC. If you need 25 percent a rear axle re-gear is needed. I drove one that was a 5 speed with a 63 mph top speed so unlocking 6th would have given me a top end of 72??? I passed on it because it had a Cat.

 

[cadillackid]

on an MD3060 5th gear is a 0.75 ratio and 6th is a 0.65 ratio.. its an incremental gear... when paired with the right rear diff gears its a nice couple hundred drop in RPM for cruising on flats and then a nice little drop to spool the turnbo a bit on minor grades..



while you can use it to increase your top speed a few MPH, it may not be the best option to keep your engine RPM in the nice cruising range...


this is all theoretical as "thomas pusher with MD3060" is not enough to know what your situation is.. every bus could be ordered with different tire sizes (ie busses with wheel chair doors and no interior wheel wells had lower profile rear tires)... and gear ratios were options as well.. ive seen overdirve busses with MD3060s built with everything from 5.13 rear all the way down to 7.17 rear gears.. withiut knowing your bus build and what your actual desires are its hard to say whether 6th gear will get you there or not...


lets play a little math though..


https://spicerparts.com/calculators/transmission-ratio-rpm-calculator


that page is what I use for messing the numbers..


so lets say you have a bus with 11R22.5 tires (41.5" tall generally)
6.51 rear gears
MD 3060. 1.0 4th gear, 0.75 5th gear, and 0.65 6th gear



CAT C7 with max RPM of 2500. the C7 likes to highway cruise in the 2000-2200 range.. anything above that it loses MPG and makes heat.


if I plug the numbers in for max speed without 6th gear i get 63 MPH top speed at 2500 RPM. if you want to cruise at 65, you are out of luck.


unlock 6th gear and now you can cruise at 65 at about 2250 RPM not absolutely perfect but def a nice upgrade...



want to cruise at 70? now you are at 2400 RPM. you can do it but def not ideal..
75? you are out of luck this bus is out of steam at 74 with 6th unlocked..



thats just an example of where 6th can get you a nice 65 MPH cruiser and barely a 70 MPH cruiser..



unlock 6th gear and regear to 5.57 and now you cruise at 70 at 2050 RPM and can go 75 at 2200.. if you needed to climb a small grade you could downshift to 5th and max out about 73-74 MPH ...


play with the numbers plug in what your bus actually has for physicals and then see what you can gain from a regear or 6th gear etc..


I did both a transmission upgrade and a re-gear , as well as engine soup-up on one of my busses and now its the ultimate highway cruiser..



sometimes you get luclky with thomas and they print the rear gear ratio on one of the stickers inside the bus.. otherwise you have to crawl under and get it off of the rear diff tags