Need new tires: Should I upsize?

[wardy]

I have a Spartan MM w/ IFS 2007 (built in 06) that came with Michelin 275/80/22.5 XZA3's. They are now 6 years old and the front show sidewall cracking so I want to replace all of them. Since these tires are right at my maximum front weight I'd like more margin. Has anyone up-sized to 295/80's? How did it effect the ride?

Thanks!

[Dave44]

Before up sizing your tires make sure your front axle is rated for more weight

[Mr_D]

Originally Posted by Dave44

Before up sizing your tires make sure your front axle is rated for more weight

Why?
It won't change the load on the axle. And the amount it can carry won't change unless the tires are the limiting factor.

[Gary RVRoamer]

If you put them on the rear as well as front, the larger tires are going to affect your speedo and odometer - they will have to be recalibrated. Also make sure your rims will still provide adequate dual spacing. I think the 295/80's require a minimum 8.25" wheel.

If you are using them only on the front, you only need to worry about the wheel well opening size and whether the tire will rub at the maximum turn angle. Depending on just what model 275/80 you now have, a 295/80 may be as much as 1.1" larger in diameter as well as being 20 mm (0.8") wider.

[wardy]

Guys: The key issue for me is that the 275/80's are at their max weight now. The tires max weight carry are 7160 lbs ea or 14,320 front axle and my actual weight is 14,160 lbs. So I must run at 120 PSI max/cold pressure always. There is no extra margin of safety, if the tires drop a few PSI I'm over-loaded (for example @ 115 PSI the tires are rated at 13,840lbs). I do not wish to carry more weight even though the axle can handle 14,600. I consider the stock tire size to be under engineered.

[Gary RVRoamer]

I'm running 295/80's on my 14,600 lb axle, so that part is fine. You just need to make sure they fit in your front wheel wells without scraping.

295/80's would be overkill for your drive or tag axles.

[wardy]

I agree with you Gary. Unfortunately I must up-size my rims to 8.25 from my current 7.5's in order to fit the 295/80's. Then I'll need to re-weigh the front & tag since the tire's are a little taller. I may need to adjust the airbag height slightly on the front's or tag. If I'm missing anything else please chime in. Thanks.

[Cruzer]

I have the same coach as you so I'm familiar with your situation. The 2007 42QRP was marginal on front tire sizing. Like you, I'm running close to maximum and need to run at 120 PSI. The axle is rated at 14,600 so that's no problem. The tires are what limit you to 14,320 lbs. If you upsize to 295s you can realize the benefits of the 14,600 lb rating. That will let you drop your tire pressure roughly 7 PSI and still maintain the same load bearing capacity. It's not a big gain, but it is still a gain.

However, the Spartan chassis uses 7.5" wide rims. You have the same rims that I do and 295 tires need wider rims. The 2007 Freightliner chassis uses 8.25" wide rims so with a Freightliner you could simply upsize the steer tires at minimal cost. But, with the Spartan you are going to need a pair of wider rims as well so the cost versus benefit ratio just headed in a downward direction.

Be glad that you have a 2007. In 2008 the tag axle Allegro Bus coaches got even more nose heavy and many coaches were overweight at 14,600 lbs. Eventually Tiffin counter with a 15,600 lb front axle assembly but 2008 was a real headache year.

295s will fit in your wheel wells. You just have to make sure that your tires are straight when you dump the air bags and that you don't drive off and turn them until the suspension is fully aired up or else you'll be causing some fiberglass damage.

I recently replaced my steer tires and stayed with the 275s because I felt it rode good enough and didn't justify the additional cost of new rims. If I were over 14,320 lbs on my front axle I would definitely have upgraded to larger tires and rims. Actually, the difference between the 14,600 lb axle and the 15,600 lb is only in the wheel bearings and spindles. It's not that significant unless you are using it on a daily basis to haul heavy loads over lots of miles. RVs are considered "vocational use" and get cut some slack in the specs, although I would never exceed what a tire was designed to carry.

[wardy]

Cruzer: I appreciate the insight behind your decision to stay with the 275's. All that you wrote is true and it applies to me equally. I was disappointed when I learned I'd need bigger rims at $400 a pop or so. I also would never knowingly exceed a tires max weight designation hence forth this thread. I will be adding a TPMS to the coach.

[Mr_D]

Originally Posted by wardy

I agree with you Gary. Unfortunately I must up-size my rims to 8.25 from my current 7.5's in order to fit the 295/80's. Then I'll need to re-weigh the front & tag since the tire's are a little taller. I may need to adjust the airbag height slightly on the front's or tag. If I'm missing anything else please chime in. Thanks.

I went to Michelin 305/70's on the front of ours since the 275/70's were actually over their rated weight (allowed in writing by Michelin) even at 120 psi. I did have to buy new wheels all the way around. I kept the 275/70's on the rear as they were no where near max.

[ru499]

Why not just get a higher load range tire? If you have load range G, go to an H. Simple.

[wardy]

Originally Posted by ru499

Why not just get a higher load range tire? If you have load range G, go to an H. Simple.

I wish it was that simple.... no one makes the 275/80 in a higher load range.

[ru499]

Continental HSR II 275/80r22.5 LR H


http://www.conti-online.com/generato...rundum_en.html

We use This brand at the truck company I work for. They are great tires. Alot tougher than Michelin.

[wardy]

ru499: Thanks for the recommendation. I did look at this tire but it has the same 7160 pound capacity as the Michelin. Plus it requires an 8.25 rim.