This may help some of us gain confidence in which info is correct:
FOR STOCK FACTORY GAUGES: I guess we all are familiar with the name of Eckler's for parts--hence we maybe trust them. Look at this link & see for sure that our trucks from 60-66 all used 0-30 ohm sending units. Did not go to 0-90 ohms 'til 1967 on the trucks, even tho' GM cars changed earlier.
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http://www.ecklerstrucks.com/chevy-t...1960-1966.html << Look up Part #: 129006 on Eckler's site.
Here's another supplier, Car Parts Discount, who shows specifically a 1965 GMC as having 0-27 ohm senders--I guess GMC was a tad more accurate & got it down to the "fine print", being more exact: their gauges prolly did not read beyond "F" when filled like our cheaper Chevies. Naturally, in the grand scheme, 0-27 is practically the same to such a circuit as 0-30 would be.
.....for GMC 1000-series (read their 0-27 ohm spec in description)
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http://www.carpartsdiscount.com/auto...html?3593=2627 <<
...for GMC 1500-series: (read their 0-27 ohm spec in description)
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http://www.carpartsdiscount.com/auto...html?3593=2627 <<
Hope some of this helps,
sam