Cars are getting faster...

[Clivey]

But I'm glad you raised that subject, as it highlights the issue.

[P4g4nite]

I think that instead of going faster and faster at any cost, the focus should instead be on racing that evolves economy to the next level. Don't set any limitations on number of cylinders or capacity only that the race needs to be completed with a specific amount of fuel. For safety and some sort of competitive order the chassis should be similar; otherwise some might bring a fuel-cell powered unicycles!!

I like that idea a lot, it's the ultimate control, only dish out 10L of fuel and give them 30mins to make laps.

[Matra et Alpine]

The Classic Le Mans uses the old "Index of Thermal Efficiency" figures to group the racers.

This Index was a title awarded and fought over for years in teh full 24h Le Mans races.
Along with the "Index of Performance".

so any "eco" class will just be a return to original Le Mans classifications.