Converting Tire Sizes From Old to New

This unfortunate roadside occurrence was shared in the Vintage Camper Trailers Facebook groups page.

If it is time to replace the old tires on your vintage trailer, you may have a hard time matching up the numbers with the new tires sold today. We rely on Tire Rack to help us figure it out when we need new rubber. Here are some guidelines you can use to replace your old tires before you are left stranded on the side of the road.

Click on the chart above to download a pdf.

Depending on the sizing system used when the tire was manufactured, there may not be a good rule of thumb or step-by-step process to follow to make the conversion. The chart above has many of the different sizes you may encounter.

​For something like a 6.50R15, the 6.50 indicates the nominal section width of the tire in inches, and the 15 is the wheel diameter, also in inches. Since modern tire sizing typically uses millimeters for the section width, we need to convert 6.50 inches to millimeters, so 6.50 x 25.4 = approximately 165mm. When the aspect ratio is not listed, like in 6.50R15, it is assumed to be 82, meaning the sidewall height is 82% of the tire's section width.

Modern tires use aspect ratios in increments of 5, so the closest current size to a 6.50R15 is 165/80R15. The conversion is very much an approximation, not an exact science. If your travel trailer has a tight fitment without much room for variance in tire size, it would be best to take some measurements and give us a call before pulling the trigger so we can help ensure you get a tire that fits.

In issue #52 of the Vintage Camper Trailers Magazine, we will also cover when to replace your tires and how to read the dates on tires, why you need trailer tires instead of passenger car tires, how to determine load ranges, and whether or not to balance your trailer tires. Don't miss it. Subscribe today.