Let's consider this whole situation.

First of all, how do you know you're speedo is reading 3-4 mph too high? Are you comparing your speed to another car? Did you use a stopwatch on either a measured quartermile set of lines, or mile-markers, on the highway? Or did you use a GPS?

Also, at what speed did you observe this error. I ask this not only because a 6-8% error (assuming you're only going ~50mph -- the error would be less if you're going 80mph) is pretty high, and unusual if you're using the stock tire size of 185/70-14. But also, if instead you're claimed error is observed at 75 mph (rather than 50 mph), your error is quite less, only about 4-5%. We have to know at what speed you're seeing a 3-4 mph error, in order to calculate the speed correction needed.

Second, assuming you have the aforementioned error, the only way to fix this is with a change in tire size; the speedo cannot be adjusted, nor can you swap a gear in the tranny because your speedo is controlled by a toothed wheel in the differential that sends electronic pulses (the days of driven cables are gone).

If you want to use the Virgo rims (i.e., use 15 inch tires), we'll calculate the size you'll need to cancel your speedo error. Yes, the 195/60-15 tires that are OEM with the old turbo'd cars' Virgo rims are virtually the same diameter/circumference of the standard 185/70-14 tires. So here goes:

Both the original 185/70-14 tires and 195/60-15 tires calculate to the same diameter -- either
(185 * .70 * 2) + (25.4 * 14) = 614.6 mm
or
(195 * .60 * 2) + (25.4 * 15) = 615.0 mm

Because your car, for some reason, is showing a 7% higher than actual speed, you need to increase your diameter (or circumference) by that amount:
614.6 * 1.07 = 657.6 mm
This will make your tires turn 7% more slowly for any given actual car speed, and you'll see a concomittant reduction in the speedo's reading.

205/65-15 tire would be [(205*.65*2)+(25.4*15)], or 647.5 mm, a 5% increase. This might be quite sufficient if your error was at closer to 75 mph than at 50 mph -- again, we need more info to advise.
On the other hand, 205/70-15 would be [(205*.7*2)+(25.4*15)], or 668 mm, an 8.6% increase, which would be what you'd need if your 3-4 mph error was seen at 50 mph or below.

Otherwise, you can do what I do. I have 205/55-16 tires on 16x6.5 Hydra wheels, a 632 mm tire with a 2.8% error, and I have a GPS more or less permanently sitting on my dash to tell me exactly how fast I'm going.