Can anyone else running 205/60/16 tires, can let me know what your speedo error is? Mine looks like about 3% which seems to be high.

 

Try using this tire size calculator to compare the effects of your change...it should answer all of your questions:

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

Recognize that with the stock wheel/tire set-up your installed speedo was most likely already reading about 3% too fast at highway speed.

 

If you really want to know, check against a GPS, otherwise just assume 2 or 3 mph. I just GPS whenever I make a tire change. The actual spedo change will depend on the specific brand and model tire coming off AND on the specific brand and model tire going on, AND the pressures you run in them. On Saturday I garaged a new (571 mile) set of OEM Bridgestone summer run flat 205/45 17's at 38 psi and had 215/45 17 Michelin AS's at 39 psi mounted and my speedo went from 3 over to 2 over.
I have been GPSing all my vehicles and to my surprise have never seen a % error - they're all absolute. Even my cable driven '84 Westphalia is 4 over at 15 mph, 4 over at 45 mph, and 4 over at 85 mph. Honda Accord, Civic, Kawasaki, Suzuki motorcycles all absolute.

 

85 mph in a Westphalia? I had an older model that was so underpowered I don't think I could ever achieve that speed. In fact, when driving into a headwind at highway speed (i.e., 60 mph) a strong gust could knock it down 10 mph, even with the accelerator pedal mashed to the firewall...

But I digress OT. Good advice on calibrating with a GPS.

 

Sharp eye, wondered if anyone would catch it!!!

The miracle of Subaru engine conversions...depending on the one you pick you can go about a hundred million mph (or, seriously, 75 over Rabbit Ears Pass in fourth gear). Aero limits ME, though, to about 80 or 85 minus the sidewind speed...others mat be braver, but I'm often scared to go 65 for fear of being blown off of the road.

 

Using stock OE sized tires will give you 5km/hr too high. Changing to a different tire size might actually correct it.