RV Tire Safety
with RV tire expert Roger Marble
Question from RVer on a forum
“I need six new tires for a 2004 38-foot Dutch Star on a Spartan chassis. Michelin tire models and sizing has me totally confused. Current Michelin size is 275/80R22.5. Dealers claim this Michelin size is interchangeable to 295/75R22.5 from brands that don’t offer the original size. They tell me there will be no ill effects on the drive line or instrumentation.”
I was concerned with this possibly incorrect information. In this case the two sizes have the same load capacities, so that was not my concern.
Some folks replied to the original question on the forum by suggesting he use this tire size website or similar websites to learn about tire sizes. I did have a little problem of using such a “tire size” site as the information is aimed at car application and does not include special information regarding dual tire application. Also the resulting numbers may or may not match published industry standards.
I pointed out my concern about the Minimum Dual Spacing (MDS) which is a published dimension in industry standards and has been covered previously in this blog.
Some folks reported that they have changed tire sizes and had no problems, implying that any change might be OK.
I provided this graphic to help people understand what this dimension is:
As you can see, the wheel offset and even wheel thickness can have an effect on this spacing. I also pointed out this example: 275s have an MDS of 12.24 and 295s have an MDS of 13.19.
Let’s assume that with your wheels and with the 275s you have a physical 1.00″ clearance down at the bulge in the sidewall near the road. So if you don’t change wheels and put wider 295s on the new, clearance will be 1.00 – ( 13.19 – 12.24) or 1.00-0.95 or a final clearance of 0.05″, which is clearly too small.
I hope that those considering changing tire size consider not only the load capacity, which is very important, but also the dimensions.
Read more from Roger Marble on his blog at RVtiresafety.net.