This is a handy tire size calculator for comparing different sizes:

http://www.miata.net/garage/tirecalc.html

According to the manual for my 240D, the original tire size was 175 SR-14 (similar to 175/80-14). Your new tires are 185/70-14, which are 3.3% smaller diameter than stock. This corresponds to the speedometer error that you measured 2.5/2.42=1.033. I use 185/75-14 on my 240D, which are a little taller and almost identical to the stock diameter. Personally, I don't think 3% is a big deal. What you have done is reduced you final gear ratio a little, which will give up a little better low end performance and limit your top speed a little. I would just use the tires you bought and decide if you want to do something different next time. Also, I would keep an eye on the front tire wear, some chain stores don't really know how to do a proper alignment on a benz.