Tire size, Tach and speed
 

Ok after being gone for a while I ended up buying a an 88 740GLE wagon. Nice car but i noticed something. The pervious owner had replaced the 185/70/14 tires with 195/70/14. I know not a huge difference but when im doing 65 the tach is reading around 26-2700 rpm. Is thins inthe ballpark of being correct or is the learger tires throwing off the tach reading that much. My Minivan and truck even though they have a v6 runs around 2-2100 rpm. even the 4cyl Kia optima doesnt run that high at that speed. Any thoughts?

That's normal for your car. A v6 or a v8 will generally turn slower as will newer cars geared for economy.

I don't see how the width would affect the tach. Diameter, yes, but contact patch, no. You're only talking about 1cm of difference in width from the original tires.

Ok...that little change in tire size, that insignificant change...well let's just see...

185/70/14
sidewall 5.1"
diameter 24.2"
circumference 76"
revolutions per mile 834

195/70/14
sidewall 5.4"
diameter 24.7"
circumference 77.7"
revolutions per mile 815

Assuming you're speedometer was accurate originally, now when you're traveling 60 mph according to the speedo, you're actually traveling 61.4 mph, meaning the speedo is 2.3% off.
A much better size would be a 195/65/14 where the speedo is barely affected.

And no, I'm no tire guru...I have a tire calculator web site in my favorites...
http://www.miata.net/garage/tirecalc.html