11-22.5 rpm sizes

[opus]

Generally, what options do I have as far as re-gearing using tire sizes?  Currently they are 497 and I need to go lower rpms.

[HB of CJ]

Sometimes they have what are called in the industry as "Take Offs" that are either tires, wheels, or both that have been removed from heavy trucks and trailers that don't need them any more.

At one time the taller "take off" 11Rx24.5 tires and wheels were plentiful since most trucks were going to a shorter tire to increase cargo space.  About 476 revs new, slightly less with some tread wear.

Now I hear they are becoming more rare as all the converting has about been done and new equipment comes that way from the factory.  However, sometimes a deal can be made.

About 5 years ago, (if this helps any) I could hav gotten a nice set of ten (10) 11R24.5 Michelins on Alcoa aluminumuimumm wheels for $2000 bucks.  I turned it down because the tires were 5 years old.

Anyhow, lots of times it's a good idea to make yourself familiar with your local, friendly, professional, inexpensive ($cheap$) heavy truck wrecking yards.  Lots of bargins to be found sometimes.  HB of CJ (old coot)

[bevans6]

If you want to keep the same wheels, you probably need to look at 12R-22.5's to actually make a difference.  My tire chart (Firestone FS 560+) shows 11R-22.5 at 503 revs per mile, 12R22.5 at 487 revs per mile and if you want to go to new wheels, 11R24.5 at 480 revs per miles.

Brian

[happycamperbrat]

I changed from the 12r22.5 to 11r22.5 last summer. My bus is still stripped though so weight is not an issue right now. I plan on rebuilding it light but I will be watching the weight as I go. I did it for a combination of reasons, mainly because if we are stuck somewhere the size is a lot easier to get and because of cost. Since that time I have made a 3000 mile trip, a 600 mile trip, and several around town. For right now anyway, Im glad I did it.

[TomC]

I went with 11R-24.5 (476rpm) since they are the tallest tires since I have a transit that are lower.  If you have a V730 with .88 overdrive, using the 476rpm tires and the 4.1 rear ratio will give you 1717 @ 60, 1860 @ 65, 2003 @ 70, 2146 @ 75.  Compared to the 11R-22.5 497rpm tires with the same 4.1 ratio will give you 1793 @ 60, 1942 @ 65, 2092 @ 70, 2241 @ 75.  Personally, not enough difference to warrant the cost of changing.  This is one of the problems with running the V730, it is essentially a HT740, but without 1st gear.  This is probably the reason the 4.1 ratio is the highest available for the V drive-otherwise you'd loose startability.  Good Luck, TomC

[opus]

I have an 8.3 with a 643.  63 mph-ish gets me around 2000rpm and 9mpg.  I need to change that just for the rpm sake.  I would like to run a lower rpm more than go faster.  I cant afford to go too much though, too many mountains.

[TomC]

Opus-you got to realize that the Cummins 8.3 liter is a medium duty engine.  While the heavy duty engines are rated at 2000-2100rpm, your engine is rated at 2600rpm.  Cummins marine military 8.3 is rated at 600hp at 3,000rpm, so you're right at where you should be.  On the newer engines in trucks, Cummins suggests gearing for 1800rpm at 65mph.  But, I assume yours is a mechanically injected engine, of which it is probably set at 2,600rpm.  Test it after it is warmed up by flooring the pedal and see where the governor is set at no load.  2800rpm would not be surprising.
For the amount of fuel savings you might see by regearing, it just isn't worth it.  You could go all day running at 2,600rpm with this engine, which would be at 88mph with your present gearing.  Besides, lowering the rpm will put the engine out of its' power band-remember it is only 8.3 liters.  Good Luck, TomC

[RJ]

Opus -

The "tallest" tire I've found over the last 5 -7 years is an 11R24.5 "drive" tire made by Bridgestone.  It turns 470 revs/mile.

Most all of the 11R22.5 tires turn somewhere around 500 revs/mile, +/- 5 rpm.  None of these would give you the results you're looking for.

FWIW & HTH. . .

[opus]

Opus-you got to realize that the Cummins 8.3 liter is a medium duty engine.  While the heavy duty engines are rated at 2000-2100rpm, your engine is rated at 2600rpm.  Cummins marine military 8.3 is rated at 600hp at 3,000rpm, so you're right at where you should be.  On the newer engines in trucks, Cummins suggests gearing for 1800rpm at 65mph.  But, I assume yours is a mechanically injected engine, of which it is probably set at 2,600rpm.  Test it after it is warmed up by flooring the pedal and see where the governor is set at no load.  2800rpm would not be surprising.
For the amount of fuel savings you might see by regearing, it just isn't worth it.  You could go all day running at 2,600rpm with this engine, which would be at 88mph with your present gearing.  Besides, lowering the rpm will put the engine out of its' power band-remember it is only 8.3 liters.  Good Luck, TomC

Tom,

Good to know.  I have a ton of heavy Cummins experience.  I dont know much about the 8.3, was just assuming the normal 1900-2100 rpm limits.  It is mechanical and I will go pin it sometime and see where it takes me.  Wonder what rpm is the power band on this?

[jackhartjr]

Just got in from Miami, no way I could do the math right now;
With the 8V71NA and 4 speed spicer, how much difference fuel mileage wise would the 12R22.5 be over the 11.R22.5 be?
Thanks in advance!
Jack

[happycamperbrat]

fuel savings? hahahahahaha!! I switched and noticed nothing even worth talkin about. I forget exactly what it was, maybe a tenth of a mile or something like that.

[TomC]

If you want the best fuel mileage possible, switch your injectors to C60's (Clean tips); allow your bus to slow down on small hills (keeping the gas pedal off the floor), and cruise no faster than 55mph.  Good Luck, TomC