Mixed Brand And Size Tires On Drive Axle

[Briarhopper]

Have a FCCC XC-R chassis that we acquired a few months ago. Discovered during a routine ant hunt a few days ago that the inside tires on the rear axle are Michelin XRV 255/80R22.5 (M hereafter), whereas the outside and steer tires are Bridgestone R250 265/75R22.5 (B hereafter). Studying the specs for both tires they match up fairly close: the load/inflation tables are identical; max inflation 110, speed rating 75mph, rim size 7.5, load range G all the same. The only significant difference I see spec wise is the loaded radius; M is 17.9” and B is 18”. Using the loaded radius, original tread depth and existing tread depths, I calculate the M’s are 3.5 - 4.5 32nds smaller than the B’s on the drive axle. The Coach spec was 255/80's.

I’d be more concerned except we have driven the coach 3,300 miles before I discovered the M’s and the only issue I had picked up during my rest stop walk-a-rounds is slightly elevated temperature of the left outside drive B. When I noticed it, I checked all tires with a spot radiometer (IR thermometer) and the three other (2 M’s and right B ) were close in temp and a few degrees cooler than the left B. The left B read 104lbs and the other three read 105lbs on the gauge. The left B also shows very slight more wear than the right B; less than 1/32 average using a tread depth gauge. The right drive B and steer B’s in all grooves show 1/32 or less wear from spec.

I had previously weighed the axles, but not the wheel positions. Axle weights are fine and inflation of 105lbs is right on for the rear axle load and a little high for the front.

Though the M’s are older (5204), the coach has been warehouse kept when not on the road for the last 5-6 years and they appear in good condition with no sidewall cracks, etc.

I am aware the ideal scenario of only running identical tires on the same axle, but realize that ideal and reality seldom coincide. So, I called a truck tire dealer back home (the kind of place that will tell you they are not the cheapest). They did not seem concerned with the mix and also said their rule of thumb on size difference was 4/32s which we are at (sort of).

I figure I should be diligent on the daily pressure check and hourly rest stop temp checks. Maybe run a few 32nds off the M's or replace them sooner than later, or buy 6 tires equal to the OEM spec, or something I haven't thought of yet. Appreciate any worthy input or comments.

[Icefixr66]

Stuart, If I had 7 year old, almost 8, tires on my coach I wouldn't be concerned with the tire being 2 different sizes. I would spec the correct size and replace them all! Regardless of what they LOOK like, They are old and should be replaced! Safety is the MAIN concern here. How much do you value the people that ride with you? The price of new tires is pittance compared to that! If you've ever had a blowout at highway speeds, it's a real wakeup call! Been There,done that and glad to be able to talk about it!

[Guest BillAdams]

Both tires on the passenger side drive axle and both tires on the drivers side drive axle should be identical tires. Even if one blows out you should replace both and move the remaining tire to a new location. The 2 tires need to have the same size and shape and tread depth to wear evenly and provide the maximum safety. If the 2 tires on the passenger and drivers side are slightly different, it's not as big of a concern as having 2 different sized tires on the same dual setup.

[Briarhopper]

Thanks for the input.

Icefixr66, I should have mentioned the Bridgestones are (4909) and have been on the coach maybe a year +/-. Bridgestone and Michelin both use 10 years as the age when a tire should be retired regardless of condition. So, age alone is not a disqualifier. I do realize though that I don't really know these tires since I have a only had them for about 4 months and 3,300 miles. If I'd raised 'em from pups, I'd feel better. Now that I think of it, them being unknown to me is proabaly more of a disqualifier than the age.

Bill, Makes sense. Seems though that there's always going to be some difference except when a set of tires is brand new, especially if you had one wear for whatever reason. Manuals talk about rotating if needed because of uneven wear, which means its likely you end up with two tires that won't be identical. I suppose the question is, how much is allowable wheel to wheel and dual to dual???

A couple other bits of info I'll throw in to alleviate the family's (Forum folk) concern for our overall safety. We generally avoid higher speeds, wet pavement, heavy traffic and/or darkness while driving for the sole purpose of increasing the margin of safety. Doesn't mean I won't drive in traffic or on a wet road, but if I have to, I ease up more on the throttle. I'm thinking the fact I discovered the two older tires will reduce my top speed by 5-10 MPH, which may also dictate operating when there is less traffic.

Thanks again both for your thoughts.

[hermanmullins]

As stated before, tires that old should be replaced. As to the size, ALL, ALL tires on the drive axle need to be the same size, age and same mileage (wear). The two outside tires being slightly large then the inside tires puts more weight on the outside hence forth the temp. diff. Never, I say never run different size tires on one side and a different size on the other. Tires with different Circumference will fight each other on every revolution causing tremendous strain on your differential.

I hope I didn't sound too harsh.

Herman

[Briarhopper]

Herman: Not too harsh. I've always enjoyed a good discussion and if you feel the need to raise the tone just a little to make a point, works for me.

For the sake of good discussion:

I wouldn't run smaller tires on one side, but assume tires on both duals are identical circumference: On the average road in North America what percentage of time do both duals turn at the exact same RPM?

If you find one tire on a dual worn 1/32 more than the other, would you replace both to get back to identical tires? What about 2/32?, 3/32?, etc. There has to be a tolerance range.

If the manufacturer’s service life limit on a tire is 10 years, why remove it from service at a lesser age if it is in good physical condition??

I do appreciate the thoughts.

[Briarhopper]

Found the dual matching answer in Michelins truck tire warranty: "Tires mounted in duals must be matched so that the maximum difference between the diameters of the tires does not exceed 1⁄4 inch.", which also agrees roughly with Goodyears RV guide which allows a circumference difference of 1.5" between duals. If they are willing to cover a 1/4" under warranty, then I imagine my 5 or 6 32nds is not an issue.

I also found reference to the idea that outer duals usually wear faster than inner, which may explain why they installed the worn tires to the inside.

Planning to do a more thorough inspection and if I can't find any problems, see if I can find a tire professional to inspect, and go from there.

[hermanmullins]

Briarhopper,

Valid points. I have replaced all eight of my tires in the last two years. Five last year (all were 7 years old) and three this year (6 years old). Four on the drive axle and one on the tag and then this year I replaced the one tag and both fronts. Both times the tires had no, none, nada, cracking on the side walls. They had great tread and looked almost new. However in both cases when the tires were off and I got a look between the treads. There were cracks 360 degrees around every tire. On two I even saw some steel cord showing.

My advice to all is, if your tires are older then five years old, to inspect them both on the side walls and down between the treads.

I know people understand Safety and I will always lean on the side of Safety and I want all RVers to do the same so we can meet and swap war stories and not road problems.

Happy and safe RVing to all.

Herman

[tireman9]

Spec is 1/4" but in real life few have the tools to accurately measure the Diameter. Once the tires are off the ground you can more easily measure the circumference and check that they are within 1/4".

Note dimensions published are new tire and all tires grow a bit but all at different rates so you really can't compare published dimensions. Especially across two companies.

More info of the special considerations for duals HERE.

[Briarhopper]

Was hoping The Tireman would show up at some point. Appreciate your input. Since my last post on this topic, I have gained some more info that I can throw in.

I did a more thorough inspection of my own and have found no visible signs of any problems with any tires, new or old. Did discover the 2nd older tire is 2 months newer than the 1st (effectively same age). Spoke with two more (total of three) professionals, who had no issue with the configuration.

Also spoke to the previous owner. They had bought the coach in 2006 and replaced the 2 front tires at that time. Last year they hit an exposed corner of concrete in a construction zone and blew one of the inner duals. So, moved the newer front tires to the inner dual positions and bought four new tires. Would have been nice if they had bought four new XRV's as I think they are better suited to our driving.

I have always kept an eye on my tires, and especially when on the road traveling and have caught a failing tire in a rest stop walk around (trailer we bought so unknown tires). So, my plan is to run the tires as is, keep a closer watch on all tires, do a thorough inspection at every multi-day stop and a tire pro inspection once a year. I’ll monitor the wear and if the inner dual Michs wear faster than the outer dual Bridges, I’ll likely move the fronts to the duals and buy 2 new front tires.

Somewhere I read the idea of knowing your tires. I don’t think I’ll name ‘em, but I do plan to get to know ‘em better.