Every 10 feet of speed is approximately 1 mph
Thomas Markov's answer has done the exact math for you, but in general, there's a quick and easy formula for getting an approximate answer, based on the normal speed in the travel pace table. At a normal pace, you travel 300 feet in 1 minute, which is 10 rounds. This corresponds to 30 feet per round, the normal walking speed of most humanoids. The table tells us that this pace also corresponds to 3 mph. This gives us a simple ratio for converting feet per round (the standard 5e unit of speed) to miles per hour: 10 feet per round equals 1 mph.
Based on this formula, your character with a speed of 85 would have a travel pace of 8.5 miles per hour, which is not too far off from the exact figure of 9.7. Obviously, it's not a perfect approximation, but it's dead simple to compute in your head: just move the decimal point one place to the left. That makes this method great for using during a live session, when pausing to reach for a calculator might kill the pacing.
If you instead want the character's speed while dashing or using some other form of movement, then instead of looking at their speed, work out how many feet they can travel in one round using the relevant means of conveyance, and then divide that number by 10 to get the approximate speed in mph.