So after going from stock 255/65R17 to 275/55R20 it looks like my speedo is 6.2% slower.

When i go 60 I am actually traveling 63.7 - does that sound about right?

 

You are going bigger on tire and rim will need to get your speedometer calibrated. I will be going to the dealer tomorrow to get this taking care of. Supposedly it will correct the shifting points too.

 

less miles on your truck for the lifetime you have those tires on over 5 years think of the miles saved better re sale

 

is that hand calculated from tire specs or GPS. most speedometers read a little faster than actually traveling from the factory. i have had a GM truck that was 10% off according to my Garmin gps, even my Toyotas have read faster.