Living in the southwest desert heat myself, I can tell you I’ve never worried about the speed rating on my tires. I typically go w/ whatever size is recommended, and whatever happens to be on sale at the tire store. I drive A LOT. approx. 200 miles per day and have never had a blow out. I’d say the most important thing is, as others have mentioned, to keep the tires inflated properly; and replace them when they’re worn.