Re: XE 250 to 351 conversion - Tacho Mods
On the tacho in my XD dash there are 2 screws adjusting screws on the back and a trim pot on the side.
It aslo has a "VDO 8" sticker on the back. Are you sure you can use it on a 6 and vice versa? Definitely no 6-8 switch like most tachos. It obviously measures pulses per second and converts this to an RPM so the calibration screws may work for you. You'll just need a hand-held version connected under the bonnet to calibrate it - just guessing and worth a try.
6 cyl @ 1000rpm = 6 x 1000 pulses per min = 6000/min = 100/sec.
8 cyl @ 1000rpm = 8 x 1000 pulses per min = 8000/min = 133/sec
So your 6 cyl tach will over-read by 33% on a V8. ie: 1200 = 800rpm. 6000 = 4020rpm.
The calibration screws may wind it down for you. While you're there, there's also a calibration screw hole on top of the speedo that most people don't know about (covered by a sticker) - good for tyre/diff ratio changes. Gives you ~+/- 40km/h.
My old work ethic or push, pull, prod, turn ,twist until it does what you want.
393 Tremec TKO 9"