How does propane burn hotter with fewer BTU than gas?
 

Had this conversation with my brother today and everything I have found shows I was wrong and propane has a higher flame temperature than gasoline. Came up because he was talking about sweepers in the warehouse where he works ran much hotter than if they were gas powered. How is this possible? From what I have found 1 gallon of gasoline has 125000BTU and 1 gallon of propane has 91700BTU.

Maybe it's because fuel in an ICE is metered by weight, not volume. Propane has about 22,000 btu/lb, whereas gasoline has about 17,5000 btu/lb. Or maybe it's because gasoline engines chill the incoming air charge through evaporative cooling, while propane is already vaporized before being mixed with the air.

as stated above, use weight instead of volume. The most convenient method for me is Gallon Gas Equivalent (GGE)
Gasoline gallon equivalent - Wikipedia, the free encyclopedia

Look how diesel shows up there, far above any other common fuel. It does indeed contribute to approximately 10% of the economy difference between a petrol and a diesel engine. The rest is due to the better efficiency of the diesel cycle.

A bit technical, but the diesel cycle isn't more efficient than the otto cycle. Diesel engines are more efficient because of increased compression ratio's.

+ less pumping losses because of no throttle plate
+ more energy content in fuel

Compression ratio is the dominant factor for thermal efficiency, but those factors help.

There is a better explanation of diesel versus gasoline engines here.

I think most of the reason is the air fuel ratio gasoline is 14.7lbs of air to 1lb of gas and propane is 15.67 to 1 so propane is burning one more pound of air.