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"The BIG question is when did that brake rotor change take place. I don't know that year...."
There isn't any specific year when they changed from one rotor size to another. It depends on the options package of the car. One of the only assurances that it will require 15" wheels is that if the car has ABS brakes, it will have the jumbo rotors. If look through the Volvo 700/900 brakes service manual, it will list the different brake packages that could have been equipped on the various cars in a year-by-year layout. Needless to say, there are a LOT of caliper and rotor combinations that could have been equipped on them. However, the large diameter rotors were usually not equipped to the earlier '84 through '88-ish vehicles unless they had ABS. I don't have the books in front of me, but I believe it may have been around 1989 or 1990 that Volvo started equipping the larger rotors on the cars from the factory (but it depends on what options the car was equipped with). Also, there's a chance that the earlier cars could have the later large brakes on them if they were changed under warranty or upgraded at a dealership. -there are several TSBs that cover this operation.
Also, if you have two wheel/tire sets off the car and want to make a rough comparison of sizes, stand one of each next to each other. If the total tire height is pretty close to each other, there will probably not be any significant difference in speedometer accuracy. The accuracy of the speedometer is not determined by if you have 14" or 15" wheels on the car, it's determined by the size of the tires mounted to the wheels. For example, I have 195/70R15 tires on 15" Omega wheels on my 240 that I use for summer driving. My winter tires are 195/80R14 snow tires on 14" Dersus wheels. Thus when I have my 15" wheels on, my speedometer reads a few mph fast, and when the 14" wheels are on, it reads about 2 mph slow. Take the factory tire size on the tire placard (on the inside of the driver's door or the underside of the lid in the center armrest) which is what your speedo is calibrated for, and plug it into the tire calculator with the new tire size you're planning to use and it will tell you how much (if any) your speedometer will be off.
God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 270k miles.
'88 Black 780, PRV-6, 149k miles.
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