TOPIC:

My A4 has the speedometer with the 80 mph dial. The bike is faster than that. What happens when you go above it? Does the red needle just stay stuck in the maxed out position?
It crossed my mind to purchase the 160 mph dial, but of course that would have to be calibrated. What does it take to make this switch? Do I need a new gear at the wheel or is it internal in the speedometer?

Cheers.

The part number appears to be the same, unless I'm mistaken, for the speedo front wheel drive gear, for models A1 and A4, but the speedo part number is different. So I would say it must be some of the speedo internals which differ on the ridiculous 85 MPH versions.

No better word than 'ridiculous' Jim! I truly see no point in this considering the bike can go to whatever speed you take it to above it.
I will be annoying to look at the speedo but it will be sufficient for the speed limits here. I wouldn't be getting a whole new set just to have proper speed display, but sometimes we go above that for sure. Would be even more annoying for guys in Germany I bet. It will be annoying me when I'll take it to a track one day.
I wonder if anyone ever dismantled these gadgets to compare. There must be a way around it, and as you pointed out, it points to the speedo itself not the wheel gear.

I put a 160mph speedo on my 1980 B2 which cam with the 80mph speedo. There was no need for extra calibration as it all appears to be done in the speedo itself.

I used the term 'calibration' loosely, meaning just slapping on the 160 mph dial (they sell them printed) over the current one wouldn't show the speeds correctly. I understand now that there have to be a different ratio gears inside both speedometers and it's not the gear on the wheel issue. Which is too bad - it would be cheaper than buying a good looking 160 mph whole set of controls from eBay. Not that desperate, haha I can live with the one I have. Maybe one day, or perhaps I find how to reverse engineer and hack that speedo to show different numbers.

the gearing driving the odometer and trip counter will be the same for both as they need to record the same number of revolutions. I thing the 160mph must just use a spring that applies twice the spring force to the magnetic cup turning the needle?