Like I said, it definitely is not the tire diameter. You can compare your size to stock size (245/45-17 or 225/55-16) here: Tire Calculator
With either size compared to your size, the error (revs per mile) is less than 1%.
With your tire size and gear ratio the 7/21 tooth speedometer gear combo should get you within 3% of dead accurate (and actually, it any error should be reading slow, not fast). The chart you saw is correct for what you needed, but still you might find this useful at some point: Speedometer Gears
Okay, so consider this: Say you are actually going 40 MPH when the speedometer says 45 MPH, and I have no reason to doubt this is accurate because your method is a solid one, imho. That error equates a 12.5% error
(simple calculation, but you can see it here
). Now, the error produced if the drive gear was an 8-tooth and not a 7-tooth would be exactly 12.5%...
All 94-95 T5s came with an 8-tooth drive gear, no exceptions. I really think you do in fact have an 8-tooth drive gear. The stock 8-tooth drive gear is a pale yellow too (from the T5s I've had apart). Why would this have been ever changed in your trans
P.S., I actually have the exact same setup you have, 275/40-17 tires, 3.73:1 rear gears, and a T5. I am using the 7/21 gear combo and it's almost 100% accurate.