That's an interesting scenario because we normally recommend going with the radial-type tires or bias-type for highway usage. The radial (R13) have more flex in the sidewall which should give it better performance on the highway and help keep the heat down. If I were you I'd double check the psi rating on the sidewall. We have the Kenda Karrier ST185/80R13 Radial Trailer Tire # AM10208 which is rated for 65 psi so it could be that the air isn't inflated to what the sidewall calls for -... view full answer...