Speed is 5-10mph off Tire size or sensors?

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Noviastar

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Feb 2, 2019
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Location
Oregon
Apart from other problems. I noticed a odd thing that the Speed is reading about 5-10 off from what I'm actually going. Ive only had the truck a few days. I did test drive it but was just kinda cruising with the flow you know how it is. But driving it more yep its off.

I haven't measured to see if the tires are bigger than the factory ones. But the guy at the auto parts shop did a comp scan and it said right front speed sensor was bad. Would that do it on its own?

I kinda need to nip this soon as its throwing the millage off and confuses the shifting. Well on a positive i might have 5% less miles on the engine!
 
Stock tire size for 3:73 gearing is 235/70-16 (29")
Stock tire size for 4:10 gearing is 255/70-16 (30.1")

2001-05 have ABS sensors but no speed sensors in the hubs. Speed sensor (VSS) is located on top of the rear axle housing.
ODO, speedometer, and shifting is affected by tire size. Larger diameter will read lower speeds and make gearing higher resulting in less bottom end power. Stock tire size is also listed on the drivers door decal.

https://www.therangerstation.com/how-to/axles/ford-7-5-8-8-inch-axle-tag-door-codes/
 

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