"At the M.I.R.A test rack, situated in the heart of the U.K., when Rover put their T.3 through its paces, it achieved a 0-60 mph time of 10.5 seconds, with 18 seconds recorded to reach 80 mph. At 60 mph fuel consumption reached 14.3 mpg, dropping down to a mere 12.8 mpg at 80 mph. These fuel mileage figures are much poorer than they actually seem simply because Britain’s imperial gallon equates to 4.5 liters of liquid vs the smaller U.S. gallon size of 3.7 liters."

https://www.hemmings.com/blog/2019/08/12/turbine-powered-rover-the-experimental-jet-t-3-gas-turbine-car/

It seems you should get more performance with that kind of fuel consumption. I don't know how much improvement could be had with more development (and a lighter body), but the gas mileage is far below modern standards. Some folks might like one for the novelty value, though.