Was your speedometer reading correctly before you toke it to the dealer? If so, it may have been "tuned" by the previous owner. Dealers typically install any updates when you take them in for service (At least my dealer does). If that is the case, it overwrites the previous "tune" back to a stock tune. Is your speedometer off now?

Just a thought.

 

I installed 5 Star's 93 performance/tow tune after installing 33s. My speedo was off by 4 mph. Also, I have 3.15 rear gears (luckily I have the 10 speed so it's like a 3.55 rear with the old 6 speed). The tune made my truck feel better than when it was bone stock. I chose the tune over re-gearing because of price.

 

I'm not sure if the speedometer is off, haven't tried to check it. Haven't been pulled over though and I usually drive 10 over so it may be close. I plan on purchasing a tuner in February so we'll see what happens. Thanks for all the input everyone.

 

I downloaded an app called "speedometer" and got on the highway and checked my GPS speed versus what my truck was reading out. Mine was 4 mph off. Once I tuned the truck and adjusted the speedometer my truck drives amazing now.

 

A GPS even one on a smartphone app will give correct speed, on average, to compare with your gauge. The most accurate adjustment I have done was via FORScan. I had posted this into an earlier thread.
The mention of a dealer doing a tune while in for service and resulting loss of MPG could be due to a computer reset, now requiring the vehicle to relearn your driving style. But as several have commented the wheel tire combo, exterior add on's, and lift, are the largest contributors to MPG loss. KM

"Here's a few links if interested in FORScan correction for the speedometer. Very helpful, and got me to a 1/2 MPH error at 60MPH, checked against 2 separate GPS readouts. Thanks to the members who figure out and post such helpful information. KM
Ford built as Tire Calculator
Correct your speedometer error"