F100 tyre size

[Rambo_the_Ambo]

I live in Australia and have a 1978 F100 ex-ambulance. Basically it is a F100 cab with a custom fiberglass shell tacked on the back. It is a 302 with a C4 and 15" rims with a standard (I guess) height tyre.

My problem is that, acording to my speedo, it is showing about 10% faster than it is really travelling. i.e. If I am going at 110kph (around 50-55 mph) by the speedo I am only going 100kph.

Also, when I travel at around 100-110kph the 302 is fairly high on the revs (no tacho, but it just sounds high).

So my question is "Which standard rim/tyre configuration for these beasts are normal?" and "If I plan to take it on a 3 hour trip at open road limits (around 60-65 mph) would the revs that the engine is working at make me insane with worry?"