I've got a Scangauge II fitted to mine. I bit more than a reader like posted above, but it can give 4 engine read outs on the screen at a time (I have voltage, water temp, intake temp and RPM showing on mine) and stick it on the dash somewhere as it's quite small and slim
Quite a handy bit of kit. You can even set it up to show average and maximum speed, RPM, fuel consumption and a load of other neat tricks
They go for about £100 on ebay but for the money it's got alot of good uses - plus you can read and re-set fault codes on the move