Can I put different sized tyres on my rim?

Q. I have a bike with stupid tyre sizes – 100/90 x 19 front and 130/80 x 17 rear. I’m looking at the possibility of sticking different tyre sizes on the same rims.

I’ve done this before but what I need to know is how far can you go with each measurement (width and aspect ratio) when putting different sizes on.

I’ve been told that you can go up by 10 on the width, but you need to down by 10 on the profile, ie. 100/90×19 would be 110/80×19

It is only recommended that you go up/down by 10.
Philtheidiot, MCN forums

A. As one figure is a linear measurement and the other is a ratio they can’t be transposed. If you look at Bridgestone’s BT45s which are widely sold, the 130/80 x 17 is 638mm wide and fits rims between 2.5 inches to 3.5 inches with 3 inches the optimum.

A 140/70 x 17 is 627mm wide and fits rims between 3.5 to 4.5, with 4 inches the optimum. Stick to the manufacturer’s recommendations.