Remind me again what happens when you fit a slightly too large tire?

I know I’ve seen this topic covered before, but damned if I can find it. So apologies if this has been covered a bunch already. 

When I settled on the next set of tires I was gonna get I bought the rear with every intention of getting a front shortly, and then one thing after another and now I still have the rear, but they don’t seem to make the front in the size I need anymore. But I can get one that is slightly wider and so I’m wondering what the downsides are. My stock tire size is 110/80-17, but the one I’d be looking at is 120/70-17. 

I guess that logically speaking the beads would be pinched more than they should be, resulting in a more egg/oval shaped profile, rather than a true radial. But with the 80/70 ratio, would that really make a difference? And is 5mm on each side all that noticeable? What other issues are there to be aware of? 

I own a '91 Zephyr 750, which was the "KZ-of-the-'90s"....and it came with the front size 120/70-17 from factory,

As the height should be same, speedometer would not be affected............I would only be concerned with fender clearance on the sides........the bottom forks on the Zephyr are 153mm apart.  

If this helps.

How wide is your front rim?
The 110mm width or 120mm width is just an approximation meant to be exact. A 120 can vary in width between manufacturers.
5mm per side may or may not be enough to deform the tire enough to make it dangerous and that also depends on how you ride it.

You can check with each tire manufacturer, usually on their website, as to what tires are suitable for what width rims. They list that. You may find a manufacturer that will specify a 120/70 for the rim you have where another won’t. Happy hunting.

This calculator may come in handy.
www.tacomaworld.com/tirecalc?tires=110-80r17-120-70r17
 

blipco wrote:

How wide is your front rim?

A quick Google search says that my front rim is 2.5” and the tire I’m looking at has a 3.0-3.5” recommended.

Cue sad trumpet sound.

The rim size is marked on the rim.

Over doing the tyre size will cause weaving and instability, the below chart shows permissible and ideal tyre/rim combinations
 

On my stock rims I ran 4.10 front and 5.10 rear back in the day. Now I'm running Shinko 230 110/90-19 front, 130/90-16 rear with my new alloy wheels. I tried the 100/90-19 but it would wash out on dirt roads too easy and was unstable at high speed. I tried the 120/90-18 on my old wire wheels with the 100 front. It was better but it went away quicker. The 16" rear wheel helped the stability and the last tire lasted longer. 
Steve