I have recently bought one of these myself but don't have much experience with it yet. I was told the speedo on mine did this - but it hasn't yet. The speedo sensor is mounted on the diff housing just above where the o/s drive shaft enters it, but I think on the Hdi model this also supplys info to the engine management system so if the fuel consumption computer is working, its not likely to be the problem, most likely its a fault in the instrument panel itself, these are not serviciable I am told, so a replacement one from a breaker may be required.

This also maybe the reason for your low temperature reading but I wouldn't worry about that as long as the heater is working OK.