Rear End Gearing....what is what?

[Ironbeard]

OK, stupid question time.

I currently have a 6.50 ratio in the rear end of my 54 passenger 30' bus and I'm considering the possibility of changing this gearing out for a different ratio in order to give me a little better top end speed when highway cruising. I talked with the rep at the local International shop and he said that if I wanted a little better highway speeds I might consider changing out the current 6.5 rear end for 3.9 or 4.11, or perhaps 4.56 or 5.17 so I wouldn't loose as much starting and climbing power for hill driving. I would just like to be able to cruise at 65-70 on the flats rather than 55 mph while standing on the pedal. When going cross country I'm a bit of a slug on the highway, holding up traffic cause I can only peddle so fast!

Thoughts, ideas, suggestions, warnings?

 

[Excalibrate]

Les Lampman posted this link once and I think it will help you decide. The lower the number the more speed but at a cost of hill pulling power. You have a very low geared (high number) bus and probably would really benifit from a gear change. My gears are 4.33 and I could probably go 80 if I wanted to (no I don't want to). If I were you I probably wouldn't drop below 4.56 to 4.88. Try the calculator!
http://www.csgnetwork.com/rearendgearcalc.html

 

[frank-id]

Rear axle gears decisions

Just about every one who gets a bus wants to change the rear gears. With a high axle ratio, it is sometimes required to make more horse power.
One really nice result from changing the gears is the engine will cruise on the interstate at a reduced RPM. Lower engine speed yields better fuel economy.
In choosing a rear ratio, it is wise to consider all aspects of bus ownership as gross weight, areas of travel, and tire/wheel size.
In many cases it is cheaper and easier to change the complete rear axle assembly. This is not a difficult task and some money can be saved. I will soon change my rear axle assembly to a unit the will have either a 3:73 or
3:55 ratio. I am also changing to an Allison with a 1 to 1 final drive. My tire/wheels will be 24.5, and hoping to get a 11-13 mpg. Depending on location, complete rear axle asemblies can be bought for as little as $900. Most units are sold complete from drum to drum. Installing can be done for about $600 at most truck shops. An additional cost could be the driveline change if necessary. Buying about any rear axle will modernize my almost 40 year old rear assembly and provide any new parts wanted or needed.
Finding about anything is nearly impossible for my 1968 Crown. Frank

 

[Ironbeard]

AWESOME website Excalibrate. 4.56 - 4.88 looks to be a good match for the cruising speeds I'm looking for. I'll probably look for something in the high 4's or low 5's range. That looks to be the best compromise between pulling and hill power vs freeway cruising speed. The 6.5 is just too low and to keep up at any respectful speed on the freeways and requires me to quite literally stand on the gas pedal....not a good idea and hard on the engine.

THANKS!

 

[Ironbeard]

Again...Excalibrate's suggested website was a lot of help. Ok spoke to my friend the mechanic. It seems that there's an inverse sliding scale when it comes to rear end gearing. Lower range gearing like what I have at 6.5 are great for power pulling at the start and going up and down hills and all, but totally suck at over the road highway speeds. Higher gearing like 3.90 and 4.11 are really good for highways speeds but suck at pulling power at the start and going up and down hills. Lower gears eat more gas at higher speeds, higher gears don't get off the line very well but get higher speeds and better gas mileage...Hmmmm compromise - compromise.

He was telling me that also just swapping out ring and pinion gears may not be all that easy as its a fair split in Spicer units as to whether they are bolted in vice riveted in. The problem comes in getting them set up correctly and getting the "lash" properly set. He mentioned something about swapping out the central section of the rear end but I got a little confused there, anyway it's a method that avoids having to completely tear everything down to individual pieces and then putting them back again.

Also a lot depends on what is actually on hand and readily available out there. So I guess I'm off to search the local bone yards and see what I can find. He also mentioned swapping out with someone looking for higher gears like mine that are in demand in hilly country or pulling heavy loads. I've got a couple of folks locally I need to talk to and I'll fill you in on the great REAR END hunt as things go on.

 

[Excalibrate]

Your mechanic friend is probably right about switching out the "pumpkin". That would probably be easier and you could probably buy that whole assembly for less than the new ring and pinion and the labor might be less time as well. Basically what he is talking about is disconnecting the axles and axle housings and just replacing the center unit (that kinda looks like a pumpkin) and viola, new gear ratio. See if he will look at anything you plan to buy or get him to help search for a good replacement for yours. Also keep in mind the "target" rpm range you want to be in. The higher rpms, the more fuel it will use. If you want to go 65mph then aim for a max speed around 72 to 75 max. That way you wont be running your engine wide open all the time and will get a little better milage than if your max speed was also your target speed. I hope that made sense . . .
Your gearing now would fine if you were in WV and wanted to tow a 10,000# trailer (maybe your monster truck) around short trips or so. I had a bus once similar to yours that would only do 54mph on flat land and 57 down hill. Heck, I doubt it would go faster if dropped it out of a plane. However, I had it heavily loaded and pulling my jeep behind it and never dropped below 44mph on steep hills and it was a non-turbo detroit.

 

[sportyrick]

swapping rearends

Another problem with swapping whole rear axles is the brakes are quite different from one rearend to another. You might have air brakes but what kind of pots do you have, swept area of drums/diameter, wheel bolt pattern, etc. Look at least twice before you leap, you might regret it. A pumpkin swap would make more sense, especially when you try and take those 20 year old U bolts loose from the leaf springs and try and drop the leafs so you can get the rearend out, etc. A higher top end will kill your hill climbing ability. I have 6:83s with a top speed of 65 and still can't go up hills very good, horsepower is the answer. sportyrick