Adjusting 4.0 tach for use with 2.5

[benjy_26]

Hello all.

 

I recently installed a full dash cluster in my '87 Sportruck. The cluster I obtained came out of a '88 XJ with a 4.0, so the tach is off (by 33%, I think). Now, I know that the proper way to adjust a tach is to install a reference tach, start the engine, and adjust  the on board tach to match the reference. What I was wondering is this: since Jeep only used a 4 and a 6 cyl engine in these vehicles, would turning the adjustment knob all the way over to the opposite end of the adjustment range get me the correct readings, or should I just do it the proper way?

 

Thanks in advance! 

[eaglescout526]

I did this with my blue cluster. I turned the pot dial all the way to the left for four cylinder after installing it and fiddling with it to see where the right RPM was.  I thought I would’ve needed a reference tach at first but after I got my REM the tach was correct to what the REM was saying. + or - 100 RPM. All the way to the right is for 8 cylinders.

[benjy_26]

Cool! Thanks! 

[JYau]

I saw this thread a few months ago and thought wow perfect, I can adjust my tach to accommodate my 2.5. So I finally got around to it this week. However, I turned the pot dial all the way to the left and at idle the needle wouldn't even go into range, it still sat below scale. So then I turned the pot dial all the way to the right and now it is better. It sounds and looks like it might be riding a little high like +500 rpm.