It's definitely not the tires and Ford said they cannot get the speedometer any closer. What if anything can I do to get the speedometer closer.

 

most tuners can change a speedo but i have to wonder what's wrong and if whatever is wrong, will it effect the tuner's ability to change it? i'm guessing since you said it's not the tires, you have stock tires?

 

My tires are 285/45r22 and stock are 275/45r22. Definitely not enough of a change to be 15% off

 

yeah definitely not... even if you add the 2mph cushion most manufacturer's add (not sure 100% on ford specifically) 15% is insane plus i imagine that's adding additional mileage to your odometer. I have and edge and i can put almost any number (it goes off circumstance of the tire in mm's) so you have a ton of adjustability

 

What year vehicle? That's usually a pretty easy adjustment with a tuner but 15% off is quite a lot. How are you checking it? GPS? I would say use another GPS or vehicle pacing you to make sure the first GPS isn't off as well.

 

It's a 2010 f150 harley. I am using a gps that I know is accurate

 

Original owner? My first guess is gears.

 

I am not the original owner. If it's gears, is that a pain in the *** to get fixed?

 

if it's gears... get a tuner, you can adjust for that as well. on a side note, isn't the speed ring in the hub? meaning gears shouldn't effect it?

 

You can check gear ratio (roughly) a few different ways you can find on the net.

I'm solo most of the time (no helper), so I would tie a piece of twine to the drive shaft, and put an interval Mark on the twine for each rotation of the shaft. Make it at least six rotations long and tie a washer to the end to help weight it down. Leave it dangling. Mark a rear tire at the ground. Pull forward slowly until the tire Mark is against the ground again (drive straight on level surface). Pull the twine taught and count how many wraps and add the percentage between intervals.

E.g. my last used truck I bought had 4.88 gears in it. I counted 4 marks on the twine and 8/10 of the way to the 5th. Factory build sheet said 3.73 gears were in it. That explained all the pissed off people passing me on the freeway when I drove it home.

IIRC speedometer is reading from tranny. If gears are changed, get a tuner and program for smaller tires as you go up in gears to correct Speedo. I'm not sure what the sensor ring is doing in the diff housing, I could be mistaken. If so, it makes sense the gear change would not impact Speedo.