Hi all. I need to get new tyres, and I am just scoping opinion on the correct sizes for my K75.
Its a 1988 K75S with rear and front discs. Haynes says the rear should be 130/90/17, and the front at 100/90/18.
At present, I have fitted a 130/80/17 rear, and a 100/90/18 front; both are Bridgestone BT45.
The speedo reads 5mph slow at present. Has this got anything to do with the small discrepancy in tyre spec? Can I go with a 130/90/17 at the rear? Does it make any difference?
Thanks.

If you're up for it download Karamba from the downloads section to a laptop, and using the detailed instructions there you can correct the speedometre's error. Have a look around 'cause there's a whole heap of info and suggestion recently on tyre sizes and other people's experiences within this forum.

On a K bike with those wheels I'd use 110/80-18 front and 140/80-17 rear, and I'd use Pirelli Sport Demons. But youse guys likely have a strict MOT to adhere to so using the 'plus one' tyre sizes I've suggested (and used) might not be on, even though they work a charm, and unless you slip the inspector a few quid to look the other way? Lower profile equals the same rolling circumference but slightly wider contact patches and a bit more rubber on the greasy roundabouts and alleys of London Town can't be bad for a rider and his machine.

But to answer the question more specifically the aspect ratio (the '80' bit in the middle) of the current back tyre is having a negligible effect on your speedo reading.

And do the oversizes mean having to take off brake calipers to remove wheels?

You will always have to remove both callipers to remove any width front wheel, but as regards the rear, no, the approximately 10mm wider tyre will slide in, but unbolting the calliper and sliding it up and away makes ingress and egress of any wheel/tyre combo easier. Also, with ABS models the sensor ring is in the way so the calliper is best removed anyway. But it has to be mentioned that, tool-wise, what's easy for some is a dread for others.