Weight reduction - not always good?

[landspeed]

I was just thinking about it...

Pulse and glide is the best way to increase engine efficiency (versus driving at a constant speed in gear).

If your car had a mass of 1 gram, but was the same size, and you also weighted 0 grams, then...

If you drove at 60mph in gear, you would put out 20% less energy as there would be no rolling resistance - however, the engine efficiency would decrease due to using it at less load.

If you came out of gear, the car would decelerate to 2 mph in about 2 seconds, as the aero drag would completely overwhelm the momentum of the car.

If your car weighed 100,000 tons, then your FE would be about the same at 1 or 1000 mph, but, would again be very low due to low RR.

So, in reality, weight reduction is not necessarily good - in reality, there will be a point of maximum efficiency, which will change based on average speed.

I have no idea where this point would be, but it would be interesting to find a way to work it out!. I guess, you would need the RR, CdA, engine efficiency at load, engine efficiency while cruising, engine efficiency while idling, mass of the car, and do some complex maths to work it all out!