tach adapter

[cwagner at info2000.net]

I have another project that I am working on for a friend.  He has a 
Toyoyta vehicle that had a 4 cyl engine that he dropped a v6 into.  
He wants to use the factory tach in the dash instead in getting a 
new one.  Does anybody have any ideas for a tach adapter to 
convert a 6 or 8 cyl signal to a 4 or 6 cyl tach?  I am hopping that I 
could make one thing that can do all this, like convert a v8 or 6 cyl  
to a 4 or 6 cyl signal in one package by changing some dip 
switches.  Would a 555 or a op-amp work on something like this.  
If so, does anyone have any diagrams or schematics?