You stop between St. Louis and Blackhawk? There’s your problem, get a biigger vehicle with dual tanks!
Fewer stops is good but I’d guess we could rarely if ever save a stop by slowing down a few MPH, a lot of people need the occasional stop to stretch and such anyway. Worth giving some thought while planning a given trip though. On the other hand, sometimes it seems that between regular traffic, tolls and endless construction on the highways in this part of the country there isn’t much full-speed cruising to begin with.
It is worse than that. The reduced speed changes the stop from 2 to 1. A couple of years ago I switched from diesel to gas for reasons not worth discussing. I bring a 30 toyhauler because it works better for family with limited mobility. I bring a golf cart for same reason. So 11,000 lb trailer, 1000 lb golf cart, GMC 3500 with flat bed(heavy), 8.1 liter gas, and 4wd results in about 6.75 mpg when behaving and 6 when not. At 30 gallon tank I do not go far between stops.
On the other hand it sits in garage when not pulling the trailer. Even at $120 in extra gas per race for about 4 times a year the like new truck with 83k miles for about $8k still worked out as the better deal than similar diesel trucks. And it had higher tow rating to boot.