In days of petrol rationing, there was considerable interest in the Index of Performance - the measure of cars making an improvement on its nominal assigned distance, based on engine size. Entrants had to choose to run on either gasoline (68-Octane), diesel or ‘’ternary’’ fuel (a blend of 60% gasoline, 25% ethanol, 15% benzole). All fuel was supplied by the ACO. Fuel, oil and water could only be topped up after 25 laps had been run, and ACO inspectors sealed the radiator and oil-caps after each refill. A spare wheel, fire extinguisher and toolkit had to be carried in the car and on-circuit repairs could only be done by the driver, with the onboard tools. Night-time (when it was compulsory for lights to be on) was defined as being between 9.30pm and 4.30am.