Ron,
The important thing is the resistance per foot. Altho very thin copper wire may have a slightly higher resistance, it will still be in the range of <1 ohm per foot.

The reason it's so thin on the sender is to give it better resolution (finer steps). You'll need to find wire that has the same resistance per foot and wind the same length to get a gauge that reads correctly.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120