There wouldn't be a speedo error if you properly profile your tires such that the diameter of the tire remains the same.

here ya go

The way a tire's size specifications works is as follows:
My 1990 740's factory tires and wheels are 185/65R15.

This means that the width (if you're looking at your car straight on, it would be left and right) length is 185 millimeters.

The 65 is 65% and indicates your sidewall thickness measured from the rim. 185 * 65% = 120.25 millimeters. The sidewall goes around the tire, so to find the total thickness of the sidewall you multiply by 2. 120.25 * 2 = 240.5

The R indicates that the tire is radial.

The 15 indicates the rim size. To convert 15 inches to millimeters, multiply by 25.4. This tells me my rim diameter is 381 millimeters.

Add the rim diameter and the sidewall thickness together to get your total wheel + tire size. This is 381 + 240.5 = 621.5 millimeters.

Your new rim + tires should have this as your total diameter.

So for example, my tires are wider now and 195. Because of that, my new tires (using the same rims) are 195/60 R15 which gives me a total tire diameter of 615 millimeters, which is closer than 195/65 R15.

I'm not sure what the profile is on your tires, but I would guess that your overall diameter is the same as mine, in which case your options for 15" rims and tires are:

195/60 R15
205/60 R15

Punching in those numbers into the tire calculator link above, you will see that the speedo difference is very, very small. :-)

Now, as to whether or not the turbo rims themselves will properly fit on your car or not (lugnut pattern and brake caliper clearance), I don't know.
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'