[bmercer]

wyocobra

My math skills aren't too bad and my instincts are telling me there is something wrong here. Let's, for ease of calculation, say that your tires are 26 3/4" tall. This would make their circumference very nearly 7 feet. 75 mph works out to 6600 feet per minute, and if you divide that by your tire circumference, you will get about 943 rpm. This is the speed the wheels need to turn to go 75 mph. Take into account the original gear reduction of 3.73:1, and multiply this by the wheel speed and you will get the driveshaft speed, which, in this case, is about 3520 rpm. Assuming that your transmission is 1:1 in top gear, that should be your engine speed. Your 5100 rpm would suggest to me that you're not getting into top gear. Is this a possibility?
Also, with the change to the 3:00 gear, your engine speed can be calculated in the same manner. 943 rpm wheel speed multiplied by 3.00:1 will give you about 2800 rpm.
Tire size will obviously have some effect on this as will other variables such as transmission slip, tire slip, tire growth at speed etc.
3.00:1 should still be fine in a light car like a Cobra and your top speed should be frightening.