My previous X350 had 20" Sepangs on it when I bought it in 2010, and the ride was really awful, it shook your tooth fillings out. So I swapped them for 18" Tucana and it was a lot better but still a bit knobbly on our awful British roads, OK in Germany, though where most of them are like billiard tables.

My current X358 is on 19" and I really don't like the ride so have decided to put on a set of 17" 'Elegant' wheels. These wheels were only really offered in the early years of the X350 range. On the last swap-out I was told by the local agent that I needed to have the wheel size entered so the speedo would be accurate. Anybody know what is actually entered ? Is it the tyre diameter or the wheel size ?

As long as the aspect ratio of the overall diameter of the tire is the same, nothing should be off. Technically as any tire wears, the over all diameter is smaller than when new, but it is a small enough percentage that it does not need to be bothered with.

You can plus or minus size the wheels all you want and not change any vehicle settings, as long as the overall dimater of the tire remains the same.

Thanks

It seems you are correct as I found a comparison site, and the speedo error with the smaller wheels is very small.

Speed 20 30 40 50 60 70 80 90

Actual 20.1 30.2 40.3 50.4 60.4 70.5 80.6 90.7

At the moment, the speedo over-reads by about 2 mph, so with the new wheels it will be spot on, or so it seems.

https://tiresize.com/tyre-size-calculator/

And the best bit is the tyres for the smaller wheels cost less.

Thanks

It seems you are correct as I found a comparison site, and the speedo error with the smaller wheels is very small.

Speed 20 30 40 50 60 70 80 90

Actual 20.1 30.2 40.3 50.4 60.4 70.5 80.6 90.7

At the moment, the speedo over-reads by about 2 mph, so with the new wheels it will be spot on, or so it seems.

https://tiresize.com/tyre-size-calculator/

And the best bit is the tyres for the smaller wheels cost less.

I have seen it alleged that the wheel size can be set using the SDD, although why there should be a need to I can't see, for the reasons discussed above.

I haven't looked for it using my SDD so I can't say if it's true, or not.