[oldtamiyaphile]

I average 16mph (a NYC Taxi averages 14), so I'd suggest that my figures represent close to the worst case scenario. That's what so surprising. I could probably count the number of times I hit 60mph over two and a bit years on one hand. I've got a couple of years of fuel logs (separated into trailer/ no trailer) to back up my claims.

When accelerating at 80% load, the engine uses the same amount of fuel, regardless of weight (or gradient for that matter), it just takes longer to get there.

With the trailer, EOC was rare as generally by the time I got up to speed, it was time to slow for the next light. It put the engine close to BSFC just cruising on the open road too (very occasionally).

Stored momentum that's 100% energy efficient leaves you a lot of room to play vs a Hybrid/ EV where regenerative braking is only around 40% efficient. You can waste quite a bit of that stored momentum via the brake pedal and still come out better than the hybrid (and I think we can all concur that a Hybrid helps in the city).

The light flywheel is a good point, but even so I don't think you'd ever measure the effect of that extra 1-2kg. It's also a fairly expensive and involved mod on a FWD car.

My Fiat is infamous for going into enrichment at light loads, so whatever you can do to reduce load will pay disproportionate dividends. No doubt there are other specific instances where my experience may not apply, but for the average car that holds closed loop as best as it can weight reduction beyond the easy won't be worth the effort.