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Crows Nest has never had a working fuel gauge. I checked using Bruce's method, and its the sending unit. I went to West Marine, and got their basic unit. Rotated all the way down, it reads 1/4 tank, and it stays about 1/4 tank off the whole way through the range. So, back it goes and I'm going to try Isspro (yeah, I know, I should've started there). The tank measures outside about 22". The boat is a 1977, and its got the bigger tank. Anybody know what Isspro sending unit I should get? The 20" one?

John, If you have access to the sender mounting location i would run a dowell down to the bottom of the tank and get an accurate measurement. The pickups should be a bit off the bottom, Id go with the sender that is a similar # off the bottom, easier to err on the side of caution..BH

John,
You could also measure the out ou.tside end. I think my sender is 22 or 23"