I have 195/65 R15’s and I am always impressed how accurate my speedo is when I drive by one of those roadside speed detectors with the big digital displays. The slightly larger rolling diameter appears to have made the speedo more accurate, more so than my regular car. I've never changed the speedo settings.
The 60/65 change in ratio equates to about 3.2%, so I agree, it probably makes your speedo even more accurate. I would carry on and fit your tyres of choice and then check speed against a gps or sat-nav. IF perchance the inaccuracy is too great, it is an easy job to recalibrate an electronic speedo. A simple search on this site will give guidance as it has been discussed in the past.
