250 cid rpm

[lgehring]

Good evening all
I bought a dollar box at an auction, in it was a napa balkamp elec tach. I installed it in my 1966 c20 with a 71 250,granny low 4spd, 4.57 rear end. The idle is about 700rpm which seems about right. At 65mph it reads 2100rpm which doesn't seem right. The tach had been installed on something previously. Printed instructions inside said for 6cyl remove lower red resistor. There were 2 resistors, both brown. I'm thinking 65mph should be more like 4000rpm, it sure sounds more than 2100. Any thoughts? Thanks Lowell