I would like to get a tachometer for my Mercury 75HP. The marine shop sells their tachometers for 125$ while princess auto sells tachometers for 38$. Is there a difference between auto tachometers and boat tachometers or does it all come from the same place. I feel like boat shops jack the price of stuff cause it has the word marine in it. I should also add that I currently have a tachometer on there but I think just the guage is broken. So I thought I would go into a auto store and purchase a auto tachometer and just replace the one that is there already connected to the motor. Is this possible?