Hi there,
I had searched tis site for info on tire sizes to fit with 20" rims and had no luck so felt I would share my research and conclusion.
I have an 04 Rodeo, as far as I know this info should work for 98 up 2wd
As I have managed to gather, just about every tire outfit recomends a
275 -45-20 for this application which I ordered from my local dealer, taking their word for it. I then began to research this size of tire through many different brands and found out that these are generally 1.25" to 2" larger in diameter than the stock tire.
My rims are 8.5's and as most 20's are and also taking into consideration that most offsets stick out further than stock it was apparent measuring physically and visually that with this dia. of tire I would rub the back of the wheel well at a 1/4 turn of the steering wheel and hard.
I made the decision then to go with a 275-40-20 which is slightly smaller in dia. than stock and although at the right position they rub ever so slightly on the back it is still functional with minimal care.
If I had gotten the 45's I know now it would have been a disaster.
Hope this helps anybody in the market for plus sizing.
Have a good 1!
JC

Where did you find your info? If your Rodeo came stock with 245/70-16's (~29.5") a 275/45-20 (~29.7") is barely a quarter inch or so larger in diameter.
Tire Size Calculator (http://www.miata.net/garage/tirecalc.html)

My size for the stock 275-70-16 is based on the physical measurement of the tire diameter at the edge of the tread. Sorry for not being more clear on that. This is a more standard profile tire so the overall diameter which is typically spec. is at the center of the tread pattern. (my tires: Bridgestone Dueler with about 70% tread left with even wear measure 28 5/8")
The problem area for rubbing the fender well is the outside edges. On a low profile tire the tread surface is wider and flat, the spec. diameter is consistent across the
entire width of the tread, Physically measuring the tire, the 275-45-20 is wider tread width and larger diameter at the outside edge which is enhanced by a wider rim which sticks out further.
The 40 series tire being a smaller diameter compensates for this enough to be able to drive more normally with very slight rubbing.
Of course all tires vary by brand and type and the offset of the rims will make a difference too! I will try to shoot a couple pix showing the clearance (barely) soon.
I am not claiming to be a tire expert. Just sharing what I've learned from my experience.
My 40's work and as I said any bigger would rub bad.
Also I'm looking into the removal of the inner fenderwell and possibility for clean modification to reduce the major contact area.