Tires and Speedometer accuracy...
 

Speedometer has been off in my truck ever since it was new. Had the dealer check on it and was told it was within the spec range so ford wouldn't do anything about it. According to my GPS, the speedo is off by a little over 2 mph at an indicated 70, about 3.5%.

Today I went and bought some new tires, went up to a 285/70R17 from the oem 245/75R17 size.

My speedo is now almost perfectly on the money, maybe a half needle width off at 70 now.

This of course has also lowered the rpm's a little for a given speed.

Hope this all translates to a little better mpg's... :)

Check the speed in engineer mode. Before, it was probably close to exact.
The speedometer is programmed to be fast. That was my experience with OEM tires.
Now your truck is probably reporting slower than it is even though your speedometer now appears more accurate.

I check it by the GPS that I know to be very accurate.

By truck I meant truck computer, not the speedometer.
Your GPS matches your speedometer now but you truck is now wrong.
That's my experience.

With OEM tires, my speedometer was off like yours but the truck speed matched two GPS units.

As compared to two GPS's that nearly match, my speedometer has always indicated about 2.5 mph high at 65 mph with the stock Michelins. :-huh

I don't understand, the truck is "now wrong" ???

It was "wrong" before with the stock size tires, and now has been corrected with the taller tires, now the speedometer matches the accurate GPS readout.

Yes, exactly what my situation was before adding the taller tires.

I believe ford purposely sets the speedometer up to read fast, causing miles to accrue quicker on the odometer, making the warranty miles run out sooner.

Actually most people find that the odometer reads several percent low on the 2011+ trucks.

Every person I've talked to or read forum comments from has claimed just the opposite, like me their truck's speedometer reads faster than actual mph and of course the odometer is fast too.

My dad's new F350 also reads fast.

The speedometer on my all stock-OEM Michelins 2012 F-350 has always been right on the money as indicated by two handheld GPS units and roadside radar speed signs. I like the idea of going into engineering mode to see what the computer actually "thinks" the speed is compared to the speedometer. I'll have to try to remember that.
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2012 F-350 Lariat 6.7 CC SRW LB

The point I'm attempting to make is the truck speed and speedometer are two separate readings.
By truck, I mean the truck computer (that you cannot see in normal mode).
My understanding, mileage, MTE, etc. read off the truck computer speed not the gauge.

https://www.ford-trucks.com/forums/1...ml#post9695824

You think that the speedometer and the odometer have to be calibrated the same.....how cute.

I drove by a roadside speed radar yesterday. The speedometer was right at 45MPH and the radar read 43MPH. Thought it was kind've weird but, it makes sense they design the speedometer to read faster.

My speedo is off, my odometer is on the money.

Mine reads 2 mph fast too.