A couple of days ago I posted an issue I was having with my 56 truck. the temp gauge was pegged at hot but the engine is not running hot. I did quite a bit of research on this website (which is great and I am new to) and found that I am not the only person with issues when it comes to sending units. I have some good information that I wanted to share. The sending unit in my truck is new and has been replaced twice. It has been stated here that the 55 was a mechanical gauge which we know is different from 56 and on. If you walk into a parts store they will give you a sending unit that is compatible for 56 and newer, which lies the problem. Aftermarket senders will work on 57 and newer in most cases. However, 56 required a different and lower resistence in ohms. I talked to a guy at Airtex which is now Wells. He pulled a book from the late 60's and found that the earlier book stated these differences in the ohms but his computer does not. He says the common part is the Wells TU-5, which have compatible numbers with Airtex. The 56 has its own sender which is wells TU-4 commonly found at Autozone. TU4 has the correct resistence and should work properly with the original gauge and they are made in USA from brass not used refrigerator parts. I hope this info will help someone else with the same proble. Thanks.