Rally dash oil pressure gauge issue.

I did have the incorrect sender in. I found my old one in my 318,and put it in. same deal only now the sender dopesnt go to high with ley on. I did an ohms test with engine running and it is 70 ohms and gauge reads low. I hooked up a snap on oil pressure gauge and I have 45 psi at idle. i am going to try a sender before going after the gauge. I seem to think or read that 70 ohms will real as "low" so I am thinking the sender is not working correctly.