When I had the stock size tires on my truck, the speedometer read about 4-5 mph faster than I was actually going ie.-read 45mph when I was actually going 40. I put larger tires underneath the beast (295/70R18 Trail Grapplers) and the speedomter is reading correctly. Would you change the settings in the SCT to reflect the larger tires or just leave it alone?

 

The SCT will adjust the shift points and etc. Shouldn't affect the speedo reading at all. Change the ratio a double check the mph reading to see if its the same.

 

I would call Mike and ask...

 

Its not like you are going to hurt anything by changing the ratio and reflashing the truck. You can put it back the way it was with another reflash. SCT usually doesn't mess with the ABS system because of liability concerns thus why they usually cannot adjust the speedo.