Speed Limit Setting Mystery?

[Echo]

Hello all.

First time I've played with the Speed limit thingy in the MMI and have an odd thing going on.

I set it at 60mph and tried it out. I didn't get the ding or warning at 60mph or just over.

The speedo kept going to approx 67 - 68mph, and then I got the ding and red warning symbol at the top of the DIS.

So I set it to 70mph and had another run. The ding and warning went off at approx 77 - 78mph.

The 8 came with 20 inch alloys and 265x40x20 tyres. Because of this, I think the speedo was out slightly, eg: Speedo reading 70 mph, when the car was only doing around 65 mph, I think.

It's now on the original spec 17 inch alloys and 235x55x17 tyres.

Do you think all this, original spec 17's to 20 inch wheels and back to original spec 17's has messed the speedo up and giving false readings when the speed limit is set?

Speedo or MMI speed limit setting, which is the true reading I wonder.

Hope all that makes sense, ta.