That's odd - bigger tires corrected my speedometer

So about 2 months ago I replaced my stock 245/65R17 BFG Rugged Trail tires after I got stuck in the snow from the freak Halloween storm here in CT.
I replaced my tires with Nitto Terra Grappler AT 265/70R17. With the old tires on the car my speedometer was readding about 4-5MPH high on the highway according to my GPS and was never very accurate at any speed. After I repleaced the tires, according my to GPS, the speedometer is dead accurate.

I guess Ford wanted me to put bigger tires on this truck all along

 

[turboexplorer]

Same here. Mine is dead on at 63mph with 31'sthe and reads 71mph when gps shows 70. So mine is more or less perfect with my 31's.

 

[bbh4r4l]

Hopefully my speedo will be correct when I get my 31's in a week or two. When I turn on my gps the speedo is off by 4 mph.

 

[flutyr525]

I had a similar issue with my 02 ex but my speedo would read low according to my gps with the stock tires and now it reads high with the 265/75/16's that are on it now. So I guess my ex wants bigger than stock tires but not as big as my current rubber lol!

 

[College Student]

darn your lucky mine now shows a 10mph slow, so i show 60 when im going 70

 

[bigfoot8931]

UK owner here. I have 255/70/16s in my 1999 my wife's 2000 and father in laws 2000 Explorer The North Face. All 3 vehicles read 5 mph over. So when we are doing 70 mph the speedo reads 75. I have verified this with my TomTom GPS Satnav and my GPS in car CCTV (SIV Camera). Any idea on what size tyres would correct the speedo readings, bearing in mind we want to keep the North Face 16" alloys?