I havent actually worked it out but i have a leyland daf and it does about 22-25 mpg i reckon. Its not too much worse than when i was towing with my discovery
I have a 12T and get around 17mpg! I think thats quite good!
Probably cos unladend it weighs around 7.4T
A 7.5T will have to work a bit harder if it has 2 horses in it.
With one medium sized horse I reckon I get around 18-20, with two horses 15 and with three around 10-12!
It's a Leyland Roadrunner with a cummins engine.