Speed adjust for '95 t-5

This might be a stupid question but I have had 3.73 gears in my car for years. the speedo has always been of. From my judgment it seems 15MPH fast, which is alot. I finally took my car to a speedo shop to get it adjusted and the guy told me that in this year car there is not a gear that you switch out to adjust the speed. that there is a sensor that sends pulses to the computer telling it how fast. the only way to adjust it is buy installing a box that changes the rate at which the pulses are sent.

Now this did not seem right to me, but I did not argue with him, I just got an estemite, which was 200 dollars for the box and 100 to adjust the speed and installation. My last car was a 89 Fox and I just changed out the gear in the tranny. It was a t-5, i'm pretty sure it's the same tranny as whats in my 95 GTS. so is this guy a complete idiot or am I wrong? if any has a 95 and has tried this, let me know what your experience changing it out was?

thanks!

thanks for the help guys, I checked the gear and it still has the stock 20 tooth in it. I order the 23 tooth from ebay and i'm going to try it. hopefully that will get me close enough so that it's live-able.