2004 2500 hd. Changed from 245/75/16 to 265/70/17. Per Miata.net tire size calculator and gps was 2mph off at 60 on speedo actual 62 mph. Speedo was correct before rim/tire change. Took to local dealer and thay said they made changes to correct rim/tire size but speedo is still the same. Dealer stated cannot do anything about it because it is a diesel. Anyone heard of this ? I thought it could be corrected. I know 2mph is not that far off but would like it to read properly.