Im on the market for tires for 20" rims. The specifics are:
size- 20-8.5
PCD- 5-115
offset- +20
center pore- 74.1mm

now, my questions are:
what is the recomended tire size? brand?
& what would be the tallest & widest I could fit?

i was told 255 was the widest u could go without rubbing, which is what i have. but i have a feeling u could push it to alot higher if u wanna do some minor modding

i have 245/40/20 if that helps...
In my opinion there is no way you can go taller. Its reeeeally close.
Wider...maybe with a 35 wall height

looong4, I love how your car looks.

I went to that link some one posted here, that shows you what fits.
255-40-20 comes out as a match.
& 265-35-20 sounds cool, but I dont know if it will rub. Does someone knows the places where it woul rub?

i picked the wheel I wanted and the told them to try the tire size I wanted. They told me it wouldnt fit but after I talked them into trying it did.
Make the place you get them try the 265 on if it fits it fits. If they want to sell you wheels im sure they wouldnt care.

I looked at mine agian today, I think you could definetly wider but not taller.

Damn Kevin, that'* nicest Bonneville I've ever seen.

The same OD and a 255 width tire is about the limit.
IMO the 20" wheel makes the rotors look laughable.
You will notice a loss in braking and acceleration performance.

I dont notice any loss at all really. I dont really drive hard anyways though.
I also think the rotors look fine.

The larger wheels look great IMO but you just have to realize that the greater inertial effect will decease accel and decel performance slightly. With some cars it'* more noticeable than with others.

Larger rotors always look better with larger wheels but they also contribute to greater inertial effects but the greater stopping power offsets the decel loss............and 500 hp offsets the accel loss.

I'm with Bullet on the physics, and I'll add that ride quality suffers, though the effects are totally subjective. You have a nice start on the looks with such a good set of appearance mods already.