New tire and wheel size selection?

[bevans6]

Related to the other current tire topic, but didn't want to steal that thread.  Up here in Canada I can still get new 12R-22.5's, but every advice I get is that they are scarce on the road.  I want to buy new wheels anyway, so I have an opportunity to replace things out.  I see  almost no trucks running anything but 11R-22.5's up here, unless you count the fronts of the big gravel trucks.  I see almost no 24.5's on anything.  with my bus I could go to 11R-22.5's on 8.25" wheels and be in amoungst the common crowd, - what I see as the drawback is a probably harsher ride, with a smaller tire and higher pressures, and loss of speed.  I am on the governor at 72 mph as it is, this would drop me to around 69 mph. 

Another way to go might be 315/80 -22.5 metric tires, Firestone makes an FS400 bus specific tire in that size, recommends 9" rims (they suggest 9" rims for their 12R-22.5 as well), basically the same size tire as the current tires I have, but in load range J, which seems overkill for a MC-5C.  295/80 tires are too small, I think, they drop to 515 revs per mile which is 5.5% higher than my current 487 revs per mile.  Firestone (what my local dealer sells, although I guess he can get anything) doesn't make a suitable 305/85 metric tire, which makes me think it might be hard to get in the boonies as well, and so not a great choice.

My instinct is to look at the 11R-22.5" size.  I expect they will be cheaper but not  by much, they can run on the 8.25" rims that I can get anywhere, and they should be common enough on the road so that i wouldn't get stuck with no tires deep in the US somewhere.  What do you guys think?  Would I get a noticeable reduction in ride quality?  the speed loss I could care less about, I may gain some hill climbing capability with the higher overall gear ratio, and I can always turn up the governor to 2300 if I really wanted to.

thanks for your thoughts, I appreciate the input.

Brian

[TomC]

I have 11R-24.5 on mine since I wanted a bit higher tire (I have a transit) and a bit better top speed.  If you switch from a 12R-22.5 (485rpm) to a 11R-22.5 (500rpm) your top speed will go from 72 to just under 70.  The 11R-24.5 (476rpm) will get you 73.3.  My advice is for you to go to a truck stop and see what they have in stock.  I'll bet they will have a couple of 11R-24.5's, where you won't find the 12R-22.5.  Course the 11R-22.5 is one of the most common sizes.  Size of tire doesn't really determine ride quality-the most important factor to ride quality is proper tire pressure.  Good Luck, TomC

[txjeff]

One other factor to consider is the weight capacity of the different sizes. 12R22.5 have about 1000 lb heavier load capacity than the 11R22.5. May not be a issue for you.

jeff

[JohnEd]

Well the load range "should" be an issue.  While Tom is correct  (normal) about high air pressure giving you a harsher ride you can get FUBAR by getting a higher load range than whats called for.  If you widen the rim and straighten up the sidewall a little your ride will deteriorate.  Steel plys will hurt unless they are Mich.  Rim, load range, air pressure and tire design ALL have to be taken into consideration.  Another theory of mine, personally, is that the narrow tire gets lots better MPG and rolls further...Comments?  Dallas, where are you?

John

[Sean]

315/80R22.5 are far less common, more expensive, and harder to find than 12R22.5.  So if your motivation to change sizes is to go to a tire that is easier to find on the road, moving to 315s is the wrong direction.

Also, note many 315's are speed-limited to 65 and some even 55 mph.  Only a small handful are rated to 75 mph.

Moving down to 11R22.5 or its metric counterparts will reduce your top speed, lower your ground clearance, and require higher air pressures for the same loads.  It will also significantly reduce traction, increasing your braking distance on many surfaces, reducing maximum safe cornering speed, and increasing the chance of getting stuck in soft conditions like dirt, gravel, mud, etc..  It is also only possible if your axle weights are within the load limits of those tires, generally lower than the 12R22.5 size.

FWIW, we have 12R22.5 tires and have never had trouble finding one.  We briefly ran 315/80R22.5s, but gave up on them -- they were hard to get, cost a ton more money, made steering harder, and increased our turning radius.  For the small improvement in traction we decided it was not worth it.  (Our coach was designed for 12R22.5, so we have all the traction we ought to already.)

-Sean
http://OurOdyssey.BlogSpot.com