I have an '05 Road King FLHR and am wondering what was the biggest size rear tire I could install without any problems. thanks,

 

Assuming the existing tire is a stock size you can only increase the width of the tire by 10mm. That allows for 5mm on each side of the tire to total 10mm. If you go any further in width the belt will be drilling and shear off the sidewall in the tire. Of course, you knew that, but others who read this may not know. H-D has a very, very close belt tolerance to the tire. What does this mean to you? It means you just can't go and buy any tire brand to fit your bike and I will tell you why. Tire sizes are not truly standardized across the industry. Example: a 150/80/16 size tire may not be a true 150mm wide despite the tire size saying it is. You got to buy a tire that is "designed" and "certified" for Harley-Davidson tires to get the tire to be the correct size to fit your bike. Otherwise, you could order a 150/80/16 tire that is actually a 160/80/16 when you actually measure its width, add 10mm you'll get a tire that is too wide. Then you can easily see how you can run into trouble hitting the drive belt. If you want tires, make sure they are built specifically for fitment to Harley-Davidson motorcycle. The mfg. will tell you in the sales specifications literature. When you get the right size tire, then the 10mm oversize rule you can use. I know, it is maddening that manufacturers can not standardize their tire sizes. Hint: Try the new Mitchelin Commander II tires on your bike. They wear like iron over 10,000 miles with ease, cost less to buy and handle and ride smooth. I love those tires.

 

The short answer is an MU, or 140/85, alternatively 140/90. You may find it rubs your belt guard, in which case you can trim it, to provide clearance. It's unlikely you will get a 150/80 in due to clearances.