Assuming you have 31.5 stock tires right now, the error is equal to
38/31.5 = 1.2
Or, 20%. So if your speedo shows 50, you'll really be doing 60. You have a few options:
1) Get one of the tuners that allows speedo correction
2) Get a cheaper speed corrector, or visit the Ford dealer, I heard they will do it also.
3) Do nothing and just use the rule "take mph on speed, divide by 10 and double it" then add that to what the speedo shows.
For example, 60 on speed: 60/10 = 6...double that is 12....add that to 60, you get 72. So your real speed is 72 mph
Why would you want option 3? Your odometer will show less miles. If you correct it, then the miles your tires see is what is registered on the odometer, while your engine actually only did 20% less rpms/etc. Helps resale value, etc. The rule is very easy, you'll get accostomed to it FAST.