Shake down cruise - 5 mph over ?

[mfm22]

Sunny and 65* couldn't resist - played hooky for 2 hours from work

Oh what a glorious day totally enhanced by 986 fun . I put the top down to see how wind felt without the screens - wasn't bad but screens do help

Noticed that I'm moving along at a good clip but cars are passing me . Well I'm not racing just cruising but I felt my speedo was not accurate - I'm doing what I read as 75-80mph and being passed ? this is a 55mph zone and plenty of police around .

So I paid particular attention going by a street with radar read out on the street
30 mph it will say slow down and give speed - my speedo was at 40 the machine was at 35 .
Not sure whats throwing the calibration off - 17" wheels came from factory . tires seem standard height
I had a similar thing happen when I put taller tires on my 4 runner but it was the reverse - I was going faster than speedo showed

any ideas ? Cheap thrills if nothing else