To what extent does changing from 14 inch to 15 inch wheels effect the speedometer/od

Size of the rim has nothing to do with it. It's the height diameter of the tire. There are plenty of online gear/ tire/ mph/ rpm calculators.

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

http://www.americantorque.com/graph-rpm-vs-speed/

http://www.blocklayer.com/rpm-geareng.aspx

Rani (Rainydayauto) found a cool one once with a B body Rallye dash display

THIS MIGHT BE IT!!!

http://vexer.com/automotive-tools/speed-rpm-calculator