Matty,
Couple of things to consider. First, I assume that the 40 and 55 psi you mentioned were at idle? Depending on where you are getting your readings, the spread might tighten up at higher rpms. I have tested my pressure guages and found them to be within 5psi vs mechanical on the motor. Fuel pressure guages are notoriously fickel so I keep mechanical ones on the motors right on the fuel regulators. Were both your readings on the same side of your oil filter? If not, that could be a place to look. I had a brand new Fram filter collapse 2yrs ago which gave really funky pressure readings. If these things check out I would look again at the guage/sender setup.
- Marshall
1978 Magnum Starfire "MAYHEM"
1994 Donzi 22 Classic 540/Blackhawk - Sold
1982 Midnight Express 32 SS