Interesting Dave, since the 185/65 is almost an inch shorter(.9).
2.9" shorter per revolution. 96% of the taller tire.
To add to my previous post, I believe Carey also sells a VDO GPS speedo. The Speedhut speedo is close to the usual repro gauges, but you need the whole set for them to match perfectly.
If you have the typical VDO gauges, the speedo from Carey is your best bet.
The VDO and Speedhut both have an LCD odometer, so if that bothers you and you need the analog/mechanical version, you won't be opting for the GPS version.....