Temp & fuel gauges - what am I overlooking?

I suppose you did verify your fuel gauge to be the 13 ohm version. I think I stated before that this gauge is not as accurate as it 20 ohm version. I don't have any mfgr detail for these. Having said all that, you numbers are close. The dealers tester,,, 73 , 23 , 10 . You half is off for a 20 amp gauge. I can't say for your 13 ohm gauge. I can tell you that 34 ohms on 20 ohm gauges is 1/4 tank and 180 degree water. 13 ohms is good for 3/4 tank on a 20 ohm gauge.You might verify this with your temp gauge if it's good. You have to be wondering why 73 to 34 moves the needle a quarter scale yet 23 to 11 moves the needle a quarter scale also. The winding wire inside the gauge has a per inch resistance at 68 degrees. As this wire heats up its per inch resistance changes. Now you know.