Higher compression = better efficiency, but as I understand it has presented challenges meeting pollution control standards here in the US. That's why cars in many other countries are getting significantly better MPG than similar models here. And cars like the new Fiats have to be de-tuned for import so they meet US emissions, but it leaves them less than impressive on MPG numbers. Being an autocycle the Elio has a different standard to meet, so that may be where some of it's improvement comes from. Also, it's not an 'engine' technology, but removing 1 wheel reduces rolling resistance, the reduced cross-sectional area REALLY impacts aerodynamic drag, and weight is always a HUGE factor. So maybe restricting the question to 'engine only' misses many other pieces involved.