yep get yourself a tuner and it should fix it right up, or just gotta remember that your goin faster than the speedo says, btw I assume your a fan of Eric koston op?

 

i bet your 18" and 19" arent any different with teh taller tires, i went form 19 to 18 and the speedo was right on.

 

The original size on the v6 was 215-65R16.

If his speedo is set @ the 16 wheel rate, then he is going 64 when his speedo reads 60 with the 20 inch tires. With the 235-50R18's he had first, he was actually doing 59 with it read 60. Go figure. Bigger, wider tire, more clearance at the axle, yet less diameter.

So anyways, when the speedometer reads 65, you are actually doing 69.8 mph. Hope that puts it in perspective.

 

Just checked with my gps earlier, and the speedo is spot on... OP I would try checking your speed with a gps before you do anything.

 

Snazzy move.