[sheepdog 44]

I think diesels are bit deceptive when you're comparing cc's. That 800cc at 3,000rpm max has the same power as a 250cc gas engine at 6,000rpm? Despite higher displacement, it's being loaded just as efficiently because it has such a low max power. And it's a diesel, so bsfc is probably the highest you'll ever see. A 250cc diesel eliminating 500cc's of pumping losses would have an unusably low max hp. Even diesel motorcycles are in the 800cc range. You're car as is, is as efficient as it gets.

I think how to make the centurion more useable in the real world is a more pressing topic. If you put in a Honda IMA system, you would add maybe 200lbs. But you would have a faster 0-60 with 30hp instead of 17hp for accelerating.

Or just off the shelf electric conversion parts for a motorcycle would give you a plugin hybrid like the XL1. Attach a motor by half shaft to the rear wheels. The XL1 has 5kwh for 31 miles. 5kwh is about $2,000 in batteries. You could do 3kwh for $1,200 and maybe 20 miles of electric range. If you installed it right, you might get the weight addition to 170lbs +-20lbs. That would make the car perform 200% better.

Any engine with decent hp would give you 100mpg and make it useable at freeway speed limits.

We've already discussed an electric conversion.