OK, I have 15 tires on my 68 with powerglide. Assuming the same gearing (in other words, the only variable is tire size), according to the math, a 14 to 15 switch should only result in an approximate .07 mph increase in the speedometer per mile per hour. In other words at 10 mph, I'm actually going 10.7, at 20mph/21.4, etc. If you are experiencing a consistent 10 mph difference as you increase speed, then there has to be another variable. The tire size alone does not seem to add up to the total problem.