I'm trying to hook-up a Defi BF Tachometer and as expected, part of the installation requires a connection to the engine speed. The thing is, the harness comes with two possible tacho signal wires: 1) "Engine speed input" and 2) "Built in rev adapter input". According to the instructions, only one of these is supposed to be connected to engine speed signal but I'm not sure which. The only other info I have are two graphs as described below...

Graph 1 = <V1> 4.0V and above; <V2> 0-1.5V = blue wire
Graph 2 = <V1> 1.4-4.0V; <V2> 0-1.0V = light-blue wire

Anyone know what this stuff means, and/or which wire I should use?
(I thought I'd ask here before calling up NS (makers of Defi products).