I found a citation for something opposite.

http://en.wikipedia.org/wiki/Pie_chart#Use.2C_effectiveness_...

> Further, in research performed at AT&T Bell Laboratories, it was shown that comparison by angle was less accurate than comparison by length.

Also, my guess is that speedometers are circular because of technology available at the time first cars were built. I think I'd much rather see a bar as a speed indicator. I guess it could be tested; anyone knows any racing game which allows to switch between standard speedometer and some kind of speed bar?