Travellers who road race base a good horse on if it can do 30mph in trot. They sold me a trotter in terrible condition because they thought he wouldn't reach the 30mph speed. Now he has matured and is in good condition I can tell you he certainly could! (Not that I would ever road race or even race on grass!) Even my cob can trot as fast as another of mine in a reasonable mid speed canter which is over 20mph. He powers along and inserts an additional flick which is hard to describe. You can't rise to it though and are thrown around all over the place!!!
I'm not convinced by the accuracy of these apps but did try one at the gallops recently. We definitely cleared 30mph easily - I was grinning like a madwoman and may have turned the air a bit blue when we pulled up!
About 40mph maybe 38 on a race fit flat bred tb many moons ago tested by the vet with a landrover on the beach that made my eyes water reckon the hairy one could do about 35 as she beat the flat bred ones when she was younger