goldme

I'm receiving my new W213 in the coming months. I just wanted to ask people who have changed their tyre size or rim size what the difference has been.

I have some experience with my current Audi A6. I have gone from 245/45/R18 to 255/40/R19. My fuel consumption went from 56 MPG (5 liter/100km) to 50 MPG (5.7 liter/100km).

Do you have any experience with changing tyre/rim sizes? And what were the sizes before, what were they after and what is your MPG difference?

ua549

First, how are you measuring distance? The vehicle odometer is not correct unless you had it re-calibrated to accommodate the tire change. The new tires rotate ~767 times per mile whereas the old tires rotated ~778 times per mile. Rotations per mile can vary depending on the specific tire manufacturer and model. Other tire characteristics such as rolling resistance also affect mileage. A good tire reference site is tiresize.com.

goldme

I keep a mileage log with an app. Every time I fill up I calculate the mileage. I don't use the car metrics. So it is actual mileage calculated by the difference in odometer and the number of liters that I refuel. The tyres should have the same diameter.

ua549

The tires do not have the same diameter. That's the point.

goldme

I understand that and the difference is just 1.1%. The difference in mileage is much more than that.

ua549

You are comparing miles from the smaller tires and different miles from the larger tires.
Rolling radius is also different and is influenced by sidewall stiffness. In addition 7/10 liter is a very small amount compared to tank size. A full tank can easily vary in quantity by several liters.

To be accurate fuel should be measured in units of mass, not volume.