[red-beard]

Efficiency on the engine is great. If the compression ratio is raised to 18:1, then there will be a 15-20% increase in thermal efficiency, but I'll need to review the actual fired temperature and the exhaust temperature.

That does not directly ratio to fuel efficiency, but it will decrease fuel consumption.

One way to reduce fuel consumption is make lighter cars with smaller engines and run them closer to their max efficiency point. This was the point of the hybrid cars. What most people don't understand about hybrid cars is they use different tires, are smaller, weigh less, along with running the tiny engine near its maximum efficiency point, along with regenerative breaking.

Put a 70 hp engine into a 1000 kg car with tires pressured to 4 bar (instead of 2), and you'll be close to the fuel efficiency of a Prius, with a lot lower cost system. I had one of these in college, it was called a Datsun B210, except for the tires.

My 1977 Datsun B210, unmodified, but with a college dorm worth of junk in it, would do 39 mpg (6.0L /100)@70mph (112.6 kph). Add high pressure tires (4 bar) I expect I could get at least 5 mpg more out of it. Switch the engine from a 1.7L 70 hp to a 1.0 with FI, I bet it could do 50 mpg (4.7L / 100).