TobyV wrote:For example, I would bring in a rule whereby they could only have so much fuel to start with and they have to complete the race on only that much. Every year you could lower it

Just MHO!
The FIA did exactly that in the 1980s, with Formula 1 cars being rationed to 250 litres per race (approx 3.6 mpg) to 220 litres in 1984, and 195 litres in 1985 (4.6 mpg). At the same time, the FIA also introduced a fuel consumption formula to sports/endurance racing (Le Mans) under "Group C" rules, which allowed pretty much any engine but a ration of fuel. From memory, this started at about 5 mpg and ended at about 7 mpg.
Group C was a wonderful formula for car manufacturers, because it gave them the complete freedom to pit large capacity stock-blocks against pure-bred racing engines. Stock blocks could be directly related to actual production cars, with tremendous marketing benefits. Anything went, as long as it could handle the fuel consumption.
You might not be impressed with 7 mpg, but those cars were covering 3,300 miles in 24 hours — an average of 138 mph —and not just reaching but holding 240 mph for the best part of a minute, lap after lap. It lead to the development of computer controlled integrated fuel injection and ignition systems, pioneered by Bosch Motronic on the TAG Formula 1 engine and Porsche 956 Group C engine. It was immediately transferrable to road cars. Initially, only on the most expensive cars: BMW introduced it on the 732i in 1984, with a 20% improvement in fuel consumption on the previous year's model: same body, same mass, same engine, significantly better performance. By 1987, the BMW 525e, with a 2.7 litre 6-cylinder petrol engine, was averaging 40 mpg. With petrol! Now even a humble VW Lupo has Motronic.
For all its benefits to ordinary motorists, the fuel consumption era was loathed by racing drivers, who had to nurse their cars around the circuit, and detested by fans, because racing became less about all-out, gung-ho, pedal-to-the-metal action, and more about tactics. Worst was to see their favourite driver run out of fuel on the last lap.