Let's see here, if my series GP passes 30 amps and drops 1.5 volts that makes it a 1.5/30 ohm resistor. Or, 0.05 ohms. If you put a .1 ohm resistor in series with this you'll have a total circuit resistence of 0.15 ohms, or triple the resistence of the GP. Remember that a GP's resistence will change as it heats up so you won't get the full heat reading. That's why you need an ultra-precision low resistence resistor. But, for automotive work it might not matter.

Instrumentation is a b*cth.

M.