Speedo Error, Adjustments other than tire size? Options

[Jamie]

"A 195/65/R15 has a rolling diameter of 25.0 inches and turns 809 revolutions per mile. It has the lesser circumference for 31 mm (1.22 in), which is 1.5% lesser than the 165/80R15. The actual speed will be 1.5% lesser than the speedometer reading.In the relation to the 165/80R15., the 195/65R16 has the lesser overall tire diameter for 10 mm (0.39 in)."

...based upon the above #'s and my fuzzy math, the 65 series tires should only cause the speedo to be off by less than 1 mph at 55 and only about 1.2 mph off at 75. I only bring this up because I would say, only anecdotally, that many folks here prefer a tire between a 55 and 65 series, many times based upon performance and availability of 15 inch tires, and with 65 series only being off by that much, it seems almost inconsequential. Granted the discrepancy gets worse as you head down to 60, 55, and 50 series

...perhaps my calculations are off, ...sometimes I do have to take my shoes off to count to #'s greater than ten.

I've been running Michelin 205x65x15 on original 5.5" steel wheels for several years, and at 75MPH my speedo is only about 2MPH different than my GPS readings. Of course I'm not sure how accurate the GPS readings actually are? (IMG:style_emoticons/default/confused24.gif)