Originally Posted by Jimbo17
One reason for using the larger size tire is fuel mileage as it changes your gear ratio.
Sorry, you are incorrect. The diameter of the WHEEL has NOTHING to do with the outside diameter of the tire. Most 16"-18" tires today are low profile and have outside diameters in the 25"-26" range, which is SMALLER than most of the 75-series 15" tires you'll find. The real reason for larger wheels today is because cars have larger disk brakes. There's only so large a rotor and caliper you can fit in a 15" wheel. This is compounded by the fact that all FWD cars (and now most RWD cars) have a large amount of positive wheel offset to clear the wheel bearings. This effectively pushes the rotor and caliper further into the wheel, lowering clearance and requiring an even larger wheel.
Of course, this is significantly off the O.P.'s topic, so to get us back on track...
I can't say that I've ever seen a 185/80-15 tire. This tire would be about 26.6 inches in diameter and has a section width of about 7.25". Alternatives that are more commonly available are 195/75-15 (26.5" diameter and 7.7" section width) and 205/70-15 (26.3" diameter and 8" section width).