Can anyone help w/ 4.30's and speedometer calibration

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Anything over 4.10 gears you need a reducer type gear for the speedo. I read normal gears for speedos only went to 21 but maybe it is 23....if that is what Steeda said you needed,maybe that is why it is 23(higher than 21). I would ask about the reducer and see what kind of looks you get.
According to Diablo Sports it will not work on a 97 Cobra. Same car, same engine, but different computer.
You'll just have to calculate which plastic gears to use based on your tire height and rear end gear.
Unfortunately I have the pink 19 tooth(stock) speedo gear 3:31 rearend gears (3:27 is stock), stock tires (245 45 17's) and no speedcal installed. Yet my speedometer/odometer is off by 25%. If I drive one mile the odometer goes .75 miles. 45 mph on the speedometer keeps up with 60 mph traffic. The car had 4:10's in it when I got it, I had the gears replaced and figured I'd need to replace the speedo gear, I went to change it but the one i the car was already a 19 tooth pink gear. It seems there must be another way that the previous owner re-calibrated the speedometer, unless the 4:10's were stock(I doubt that).
That's an interesting idea that I had considered. I believe the Cobra should have a 7 tooth drive gear, A GT trany has and 8 tooth, if I'm correct the speedometer would be too fast with this setup. A 6 tooth ( if there is such a thing) wouldn't make a big a difference (a little over 14%) as I am seeing (20%). Any chance this Cobra has parts designed for the 98. There is no ridgid cable coming out of the side of the tail shaft where the speedo gear is, just wires.