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Topic: Replace 255-80R-22.5 tires with 295-75R-22.5

Posted By: oldchief7155 on 09/28/11 10:32am

Has anybody gone from 255-80R-22.5 tires to 295-75-R-22.5 tires on a 2005 Itasca Meridian 36G?

Michelin going to go up again up to 8% on October 1st.


Posted By: racerchaser on 09/28/11 10:50am

Do not remember seeing any one do that but many folks have gone up to 275 on Winnebago diesels.


Fox


Posted By: wolfe10 on 09/28/11 02:36pm

Michelin 255/80R22.5 XRV diameter= 38.2". Revolutions per mile= 541. Load capacity single= 5,205 pounds.

Michelin 275/70R22.5 XZA2 diameter= 38.0". Revolutions per mile= 545. Load capacity single= 6,940 pounds.

Can't imagine even considering a 295 to replace a 255.


Brett Wolfe
Ex: 2003 Alpine 38'FDDS
Ex: 1997 Safari 35'
Ex: 1993 Foretravel U240


FMCA Forum: www.community.fmca.com/index

Diesel RV Club:http://www.dieselrvclub.org/


Posted By: randco on 09/28/11 02:59pm

Toyo 265/75-22.5 Diameter 38.15" Load capacity single - 5205 load range G


Posted By: keepingthelightson on 09/28/11 02:01pm

295x75x22.5 are the same diameter as the 275x80x22.5 and are a very good replacement tire size. If you have 255x80x22.5 chances are your wheels may only be 7 or 7 1/2" wide. I believe the 295x75x22.5 require a minimum of an 8" wide rim. Check with the tire manufacture that you decide to buy from. Also, you probably don't need a 295 for the amount of weight that your MH is rated for. [emoticon]


05 Tiffin Phaeton 40 QDH w/4 slides
CAT C7 350+ HP MP-8, Aero Muffler, AFE Filter
06 HHR LT Toad
Ready Brute Elite


Posted By: Doug and Cassi Glass on 09/28/11 02:24pm

Thanks guys.

That's me that asked the original question. I had computer problems and had to start as a new member.

What I'm trying to do is go to a 255-75R-22.5 since the only thing available is a Michelin in the 255-80R-22.5.

Anybody know if the "75" will make much difference in circumference from the "80"? I'm pretty sure I have enough wheelwell clearance but I want to make sure the sidewalls don't "kiss" on the rear axle.


Posted By: rvarner on 09/28/11 06:56pm

Replaced my 255/80R/22.5s with 275/70R/22.5 (G670's) with no problem and gained a weight rating (From G to H). Works for me.
Richard


2004 Itasca Horizon 40AD/2012 K1500 Silverado/DEMCO Xcali-bar/Ready Brake/VMSPC/FMCA 394912/SKPs 115085


Posted By: UltraKen on 09/28/11 07:33pm

You need to consider tire width as well as diameter/revs per mile.

Here's what I put together from the web and some knowledgeable people.

Disclaimer: I'm no expert. Check your choice with a truck tire expert.

-----------------
Selecting Replacement Tires

Some general points to consider:

1. Be sure to get all steel radials; both tread & sidewall. With a high profile,
the coach will walk around too much on fabric sidewall tires.

2. Do not go overboard on ply rating or you will have a tire that is hard as a
rock and will ride like a rock. Remember, tires also have a spring rate and
contribute to the quality of the ride.

3. It’s very important to keep in mind that when going to a higher load rating tire it doesn't increase the load rating of the axle, nor the maximum pressure rating of the
rims. Thus, you usually can't improve an overloaded situation by just getting
higher load rated tires.



First step:

1. Do a 4 corners weighing. You need to find a truck scale that has enough room around it to position the coach so that each corner is the only one being weighed. The scale will be a flat platform, level with the ground.

Try searching Yellow Pages for Public Scales, Moving companies, and the one that works best for me -- Grain Elevators. My local elevator did it for free since I didn't need any paperwork, they just wrote down the four numbers and waved me on. Nice folks.


The two critical issues are tire diameter (revolutions per mile) and tire width. Tire width is important for steering clearance and for proper dual spacing at the rear. You need a minimum spacing, or larger, to get air flow between the duals for cooling. It’s a critical measurement.

For tire width steering clearances you get a good picture of the situation by turning your fronts to each extreme, slide under the coach (engine off, brakes set) with a steel tape rule and eyeball all around for clearances, and visually and tape measure see what additional width will do at the turn extremes.

For the rears you can use a simple method. Engine off, brakes set, slide under the coach at one dual position and look between the tires front to back (or back to front) to measure the closest distance between the tires.

The critical place to look is at the "bulges" near the bottom of the tires, and
this must be done with full load on the tires (not jacked).

If the new tires are 1" wider each, use just 1" decrease in the spacing estimated decrease in distance between tires. You only use 1/2 the tire width because half the increase is on each side of the tire. If the new spacing looks reasonable (on the order of around 3" +/- 1/4" take the coach and your measurements and the tire size you are considering to your truck tire dealer and get his opinion.

You do not have to have exactly the same size tires as came with your coach, you do have to have tires that will give you full clearance in turns and between the duals and are within a few percent of the revolutions per mile of your current tires.

If you can’t get closer than 1 or 2% on the Revolutions per mile you can still get the tires if the clearances are good. The pulse count sensor (pulses per mile) that signals the engine can be
reset by a dealer to give accurate readings with your new tires and the speedometer can be corrected via the dip switches on the rear of the gauge.

BTW, going to a higher load rating can give you a softer ride! You don't have to inflate to as high a pressure as you would with a lower load tire. That's straight off the tire load/pressure tables if you look for it comparatively.

2. Here's how I found a replacement size for my tires. I had 9R22.5s as the OEM tire. They are becoming harder to find as most tire makers have moved towards metric sizing.

Toyo has both standard sizes (9R22.5, 10R... etc) and metric (270/70R22.5). You need to look at each Toyo tire model for sizes to find comparables.

Basically the metric numbers are tire width/aspect ratio (sidewall height as a proportion of tire width) Radial 22.5, or 20.. (wheel
size).

Once you locate a Toyo tire model (medium duty truck - all position) with comparable measurements you need to check the tire specs. There is a link on the manufacturer's website that takes you to a table that includes a variety of special information.

I was able to find a metric substitute for my 9R22.5 OEM tires with a Toyo 275/65R22.5. The metrics are 1" wider, but no taller, have the same revolutions per mile (no need to have speedometer re-calibrated) and were a grade higher (from F to G).

3a. The critical issue is tire clearances if you go to a wider tire. For the fronts I turned the steering wheel to extreme left and extreme right and measured clearances between tires and any nearby structure, checked to make sure that the additional width wouldn't hit anything.

The backs get a little more technical. You need to be sure that the wider tires in a dual configuration are far enough apart to allow ventilation between the tires. The specification tables usually give minimum dual spacing.

Simple method. Measure the spacing between the side-walls of the duals at the "fattest" part of the tires. On my coach that was around 3-3/4". If you go with a 1" wider tire, as I did, you have to realize that 1/2 of that inch increase is on the right side of the tire and the other 1/2" is on the left side of the tire.

Two tires each 1" wider, means the gap between is decreased by 1" (1/2 from tire A and 1/2 from tire B. So my dual spacing was reduced from almost 4" to almost 3". Three inches should give plenty of ventilation.

3b. The technical (and a little more difficult) method is to look up your Wheel Offset provided by the wheel manufacturer. You need the wheel model number from the rim and then apply the following formula:

Min. dual spacing = 2x(offset) - width of one tire

My offset was 6.44". I wanted to use a 10" wide tire.

Mds = 2(6.44) - 10 = 2.88.

Two point eighty-eight was virtually the same number I got from the simple method.


4. I made up a spreadsheet to keep track of all the tires I looked at and a simple calculator for the Mds computation. You can see it here:

Tire comparisons: http://home.roadrunner.com/~kwildman/tire.jpg


5. Also, you need to pay attention to Revolutions per Mile since large differences between old and new tires will show up on the speedometer and odometer.

Warning:

When I started this search I really knew nothing about the issues. I did a lot of reading on the internet and now I know just enough to be dangerous.

You need to look at the tables, talk to your tire supplier, and make your own decision.

Ken Wildman, with assistance from SafariFriends members Jim Mexler and Bill Halberstadt.
--------------------------------------------------------------------------------


UltraKen
Ex - 1968 Ultra Van
NOW - 1996 Safari Sahara 35' - Model 3530 - Cat 300
Pictures of my Coach
A $93 Rear Camera System
My take on replacing tires
Used MH Checklist
Toad: Dodge Grand Caravan on Tow Dolly or 1981 Vespa on utility trailer



Posted By: fcooper on 09/28/11 07:38pm

In 2009 I replaced my xrv 255x80x22.5 with Sumitomo 727 tires in the 275/70R22.5 size. The speedometer error is so small that I can't detect a difference.

The load range H vs the G's I removed gives more load carrying ability and/or the ability to run less tire pressure for better ride. Pleased so far with the tires, ride, and handling.

Check your rim width...preferred is 8.25 with 7.50 as minimum (at least for the Sumitomo's). Also check your minimum dual spacing.

Fred


Fred & Vicki
St. Augustine, Florida


Posted By: wolfe10 on 09/28/11 03:02pm

randco wrote:

Toyo 265/75-22.5 Diameter 38.15" Load capacity single - 5205 load range G

Yup, another choice, but doesn't offer an increased load capacity which I was assuming he was interested in.


Posted By: oldchief7155 on 09/28/11 03:48pm

Yes, a load increase was part of it and the 295 was a typo. Looking for a good tire at a better price than Michelin.


Posted By: keepingthelightson on 09/28/11 05:52pm

oldchief7155 wrote:

Yes, a load increase was part of it and the 295 was a typo. Looking for a good tire at a better price than Michelin.


Yokohama, Toyo, Hankook are all good replacement tires that you should consider.


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