In the ID3 155/70R19 were about 4% better in efficiency than stock 215/55r18 tires. Both were with fully blocked air cooling holes. 215 were A on rolling resistance, but the narrow ones ony C.
Actually difference was higher as the tire deflectors were 60mm too wide for these tires and still the first test showed 4% improvement. After I tuned the deflector size to match the narrow wheels it ofcourse got even better results.
In this case I would say it comes from aerodynamic drag. As the RR of that tire is said to be higher than the oem tire size.
On the tests 215 were with 3,5 bar and 155 with 4,0bar tire pressures. On tire pressure tests done to the 215 size it did not help much with those tires at least to boost the pressure, but I wanted to compensate the worse RR figure with little bit higher pressure.
The 155 have 0,84% bigger diameter, that was not taken into account in the tests. Meaning savings are that much more better.
https://www.willtheyfit.com/index.ph...h=5&offset2=40
I had also spacers in the car to bring the more out where oem tire is.