I found a pretty sweet looking Mercury Marine tach for a 6 cyl with a 5k rpm scale on eBay for less than $50 last year (mounting kit cost more than the actual tach). They're out there, just gotta wade through all the other stuff to find them. I ended up searching "6 cylinder tach" after looking at too many V8 tachs that did not have the 4/6/8 switch...
PIC 
