22.5 vs 24.5

[daddyoften]

I have read several things on this board and others about this topic but I'm still up in the air about it.  The bus we are getting has 12-22.5's on it now with a rpm of 476.  It only has 2 aluminum rims on the front and 4 steel on the back.  We are going to get all new tires eventually and I can find a couple of used aluminum for the rear.  I was looking into tire prices and see the 12-22.5's are kinda pricey and not as available as the 11-24.5's are.  The 11-24.5 have a rpm of 465 so not much of a difference but the price of the tires are drastically cheaper and there are a lot more to choose from.  Also my local rig tire shop we use for work has a dozen used bud aluminum 24.5's stacked up out back and I can get 6 of them cheap! One last wrench in the works is the PO, where the bus is still located, hasn't gotten around to swapping tires for me yet.  The bus is sitting on cracked steer tires on the aluminum rims and good tires on steel in the back.  He has good tires on steel rims sitting in his shop that he was going to take the good tires and mount them on the aluminum rims and then I can drive it home.  Since he hasn't gotten to swapping tires and I want to go get it tomorrow, as originally planned, I was thinking if I'm going to 24's anyways then he can just keep the aluminum wheels, install the good tires with the steel on the front, then I can take it home and use the trade of aluminum wheels towards some more rv parts he has.  Thoughts???

One more random question is the PO had changed the oil and put in 15w40 instead of the sae 30.  Would it be a big deal to drive this bus 3 hours with the 15w40 in it?  is that a big deal?  I will eventually put the correct oil in it later.
Thank you very much,
Eric.

BTW it's a 4107 with a 1977 8v71 turboed and Auto dropped in.

[Bill Gerrie]

Compare the load carrying capacity of the 12R22.5 to the 11R24.5. I'm not sure as I don't have the tables here. I blew 11R22.5 years ago because it couldn't carry the weight. I run 12R22.5 ever since and no problems. I would get the 22.5 aluminum rims from him just in case you need them. 

[HB of CJ]

I think the 12Rx22.5 desired or required, (not sure which) a slightly wider rim than the 11R22.5 but I am not sure.  My old Crown Supercoach ex schoolie had a slightly wider 22.5 wheel along with the aforementioned 12Rx22.5 tires.  Finding the proper width aluminum wheel, (if necessary) might be a concern, thus defaulting to the more popular 11Rx24.5 stuff, that plus a little bit more road clearance and reduced cruising RPM.    HB of CJ (old coot)

[luvrbus]

The bus came with 12r 22.5 for reason I guess the GM engineers knew what they were doing and to answer the other question you are not going to damage the engine driving a couple of hundred miles with 15/40 when you get it home change to the good old Delvac 1240

[RJ]

Eric -

I've written about this topic of tires & tire sizes on a GMC coach numerous times, having been around them for over 20 years.  Lots of folk read what I have written, but buy what they want (or get the best deal on) anyway.

If you're really interested in the reliable information I share, send me a PM and I'll respond privately.

FWIW & HTH. . .

[TomC]

Every 12R-22.5 tire I've seen is right at 485rpm and 11R-24.5 is 476rpm-so there is a difference plus about a 1/2 inch higher tire. The 11R-24.5 16ply is just a few hundred pounds less than the 12R-22.5 16ply. I switched my tires from 12R-22.5 steel wheels to 11R-24.5 aluminum wheels and have been pleased. Good Luck, TomC

[gumpy]

I run 11R-24.5 on my MC9. No issues.

Where in WY are you located?  How much are they asking for the used aluminum rims?

If you put aluminum on the rear, you will probably have to change your studs to longer ones if that has not already been done.

[sparkplug188]

gumpy- Another option is only changing the outside dual rim to aluminum.  That is what I did on my conversion.  All you need is longer inner cap nuts.  I got my aluminum wheels off of a wrecked Peterbilt.  The salvage yard gave me the longer inner cap nuts for free with the purchase of the wheels.  I have never pressed out semi truck studs... my gut tells me they aren't going to fall out with a few taps of a ball peen hammer.

[gumpy]

gumpy- Another option is only changing the outside dual rim to aluminum.  That is what I did on my conversion.  All you need is longer inner cap nuts.  I got my aluminum wheels off of a wrecked Peterbilt.  The salvage yard gave me the longer inner cap nuts for free with the purchase of the wheels.  I have never pressed out semi truck studs... my gut tells me they aren't going to fall out with a few taps of a ball peen hammer.

Good point. I forgot. It's been a few years since I did mine.

Getting the studs out is not too difficult. Getting the new one's in is more challenging.

[RoyJ]

I'd recommend 11r24.5s as well.

As to why the 12R22.5s were chosen in the first place, I'm guessing ride quality AND payload. 11r24.5s couldn't offer both, especially back in the day.

315/80/22.5s largely replaced 12R22.5s on modern buses, for the exact same reasons. And when that's not enough, heavy Prevosts moved to 365 fronts and tags...

Personally, my bus is geared on the tall side at 4.1:1. 11r24.5s dropped the cruising rpm ever so slightly to 1939 @ 60 mph (473 rpm drive tires). 12R22.5s were too rare, and 315s too expensive.