2000 F150 Speedo error

[Shomeyrwit]

On a trip to a friends ranch last weekend I discovered that my Odometer is off by .05% So for every 20 miles it shows I actually drive 21 miles, which means my MPG has actually been better than expected.
When I bought the taller tires I expected to lose MPG and I knew it would affect the speedo but the truck usually stays home for Road trips unless I'm towing a trailer when the MPG is hammered anyway so I looked at my commute mpg assumed the approx 1mpg penalty was the tire penalty and went on my merry way.
So my typical mpg on the road is over 19 with a 19.25 for the 536 miles driven this weekend including a whole day on the ranch haulin tools friends and beer around 400 acres building paths and roads