next tire change for sure.I trust my Garmin's mileage calculations 100%. With my last tire change the speedo below 50 mph is dead on with Garmin. It tends to stray slower above that but not by much. Odometer and ECU mph is off by over 5% slow. This is good for the odometer miles racked up but you need to keep it in mind when doing maintenance or fuel mileage calculations.
The onboard mpg estimation were always high by 2-4 mpg. Now it's down to 1-2 mpg high.