One way to check and see if the sender is the right resistance might be to check the resistance when it was cold (room temp) and then put it in some boiling water for a little while to warm it up. That might not give you the extreme ends of the temperature range, but I would guess that it would give you enough to determine if you have the right sender for your gauge. I would also think that you could watch the resistance change as the sender cooled off after being removed from the boiling water. I've pulled a similar stunt when trying to figure out if a thermal switch was working. Just a thought...
