Hmm...
there must be a market for an iphone/ipad mini appthat'll show rpm (would need some kind of adaptor to work out the rpm) and speed (via GPS)...
could knock one up in less than an hour but it would be a bit inaccurate due to changing gear.
can set speed and rpm with no issues but then you'd have to do speed @rpm in gear x, means you could be doing 50mph in 6th gear at 4000rpm or 50 in 5th at 6000rpm so you wouldnt be able to accurately do it unless you had a way to set the gear changes whilst doing them or you would be at 5000rpm in 6th doing 60 when your phone says youre doing 20,000rpm lol.
Rpm + speed above are examples and may differ.