At 10mph you're getting about 2 revolutions per second. With a single spokepov, that's too flickery to be called a persistent image. You'll probably want three spokes, and that only gets you the equivalent of a 6Hz update. And then both sides...and then two wheels. That's a lot of TLC5940s. 72 of them, actually.
You might not be happy with the brightness control. It's PWM, which will be visible on a moving LED. If you can achieve a 5MHz grayscale clock, you get almost 10 full PWM cycles for each of the 256 radial slices. That might be enough to not confuse the image you're trying to generate.
I know a guy who makes POV displays that have variable brightness, and he's actually using a simple resistor DAC for 3 bit brightness control on each LED. This doesn't use pulses to vary the apparent brightness, so it doesn't disrupt the virtual pixels on the POV display.