Yes. Speed is calibrated by revolutions per mile and set for your factory size tires. Larger tires will make the speedo run slower. Depending on the size difference it can be extreme or just a few mph.
Edit: Your mileage is computed by the same input as speed, so both will be incorrect.
Look around in the driveline section of this forum, and you will find lots of useful information. There is even a page that tells how to recalibrate your computer using a Ford diagnostic tool.
Welcome to the site.
We put larger tires on our 2000 Explorer Sport so I know it throws off our speedometer (how fast you go) but my b/f says it throws off our odometer and tripometer (how far you have gone)too since the meters run off the same sensor as the speedometer. Is this true? Its kinda confusing for me 
