Is it a digital or analog meter? In any case you have to know what your meter is telling you when making a measurement. For most stuff in vehicle wiring you are just looking for either an open or closed state and the actual measurement is not important. I'm betting that if you're using a typical 3-1/2 digit DMM which means you're probably looking at a minimum resolution of probably 100mΩ on the 400 Ω scale and an accuracy of around +/- 250mΩ. Some can be more accurate (I have a Fluke 87 that has a special 4-1/2 digit mode that allows 10mΩ resolution and is good to +/- 0.2% + 2 counts on the minimum range), but it's still best if you calculate needed wire size and validate it using voltage instead of resistance. Most DMM have 10mV resolution on a 40V scale and their accuracy will be decent enough for practical use, say within about 50 mV at ~12V.
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