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Delta says some 550s may be different, but I have no knowledge.
The reason I ask is because I bought an after market matching speedo and tacho. The speedo looks like it's reading pretty close as they usually work off front wheel diameter anyway but I'm not so sure about the rev counter, but since I'm not going be red lining the bike every where it's not so important. There is a marking on the rev counter dial reading 1:4 so maybe it was intended to fit a twin and actually showing half the true rpm?