You are calculating the distance in miles traveled in 1 hour. To do this, you need the circumfrence of the tire (Diameter * PI), the gear ratio, and the rpm of the crank.

For a 26 in wheel, assume a 26 in diameter (approximation) and you get a circumfrence of 26*3.1415 = 81.6814 in. The gear ratio above is 38/10 = 3.8, or the tire rotates 3.8 times per 1 turn of the crank. So for each turn of the crank, the distance travelled is 3.8 * 81.6814 = 310.3894 in. If the crank turns 75 times per minute, you travel 23,279.2 inches per minute. Over on hour, you travel 23,279.2*60 = 1,396,752 inches. There are 63,360 inches per mile, so your speed is 1,396,752/63,360 = 22.045 mph.

Hope that helps.

Edit: DOH! Must read post date....grrr.