Originally Posted by mach4

Fitting size will have absolutely no effect on pressure. Liquids are non-compressible so the pressure will be the same regardless the line (or fitting) size. If you check the orifice size on the gauge it is going to be a lot smaller than 1/4". If it were to have an effect, you would need to order a gauge with the inlet size the same as the line size (or tank size, if you were checking the pressure of a tank).

Line size does have an effect on flow rates, but I can assure you that the orifice on the nozzle is much, much smaller than the line size going to the injector.