Originally Posted by moparado

Thats hard to believe, a +/-7 mph speedometer tolerance?
Is there an official Ford document showing this?
Could come in handy in court for a Barney Fife type speeding ticket.

Originally Posted by Green_Bean

My '13 F150 had 275/65R18 tires from the factory. I just replaced them with new Goodyear Armortrak tires of the same size. I've noticed that since new, my trucks speedo is just a little off from my Garmin GPS. For instance, when my Garmin shows that I'm doing 60 mph...my speedo shows around 62-63.

Is the speedo and odometer both ran off of the same source? I'm just wondering if the odometer is putting more miles on the truck than I'm actually going. I know that 2-3 mph is not much...but over thousands of miles it would add up.

I was also curious if ForScan has the capability to adjust the speedo setting (tire size). I have ForScan installed on my laptop and have the USB ELM327. Although I have not even hooked it up to my truck yet.

Originally Posted by marshallr

The odometer/speedmeter isn't 100% accurate, they never have been. You're off by 3% at most and that is probably as close as it gets. There are just too many factors

The diameter of the tires are the biggest difference. Even 2 tires in the same size from different manufacturers will have slightly different circumferences which will throw the odo/speedo off.

And tires change with wear. A set of All Terrain tires with 40,000 miles on them will be about 1" shorter than when new. My Tacoma was right on the money with my GPS when the tires were new but read 2-3 mph faster than the GPS right before I replaced them.

My son is an electrician and travels to the same construction site daily for months until that project is finished then changes. A couple of years ago he told me that for the last 4 months it had been exactly a 100 mile round trip to work and back. But the last few weeks his car was showing 95 miles round trip. I pointed out that he had recently bought new tires for his car. Same size,but the diameter of the new tires with full tread was enough bigger to account for 5% fewer miles.

Originally Posted by Eduskator

As I said, best thing to do is to compare your actual vehicle with a GPS. Depending on your findings, you'll be able to adjust your speed to stay within the limits.


https://www.theglobeandmail.com/glob...rticle4348596/

Originally Posted by Rnlcomp

Here's an exerpt from the 2013 shop manual.

Pinpoint Test G: Incorrect Speedometer Indication

Normal Operation
The Instrument Panel Cluster (IPC) receives the vehicle speed signal from the PCM over the High Speed Controller Area Network (HS-CAN). The PCM receives the vehicle speed signal input from the transmission Output Shaft Speed (OSS) sensor. The IPC monitors the vehicle speed signal input from the PCM and commands the speedometer with a corresponding movement of the pointer.

The IPC provides a tolerance which allows the gauge to display between 3% lower and 7% higher than the actual vehicle speed. This means that with an actual vehicle speed of 96.6 km/h (60 mph), the speedometer may indicate between 93.7-103.3 km/h (58.2-64.1 mph). Factors that could potentially affect the speedometer accuracy are incorrect tire size configuration and axle ratio configuration.




.

Originally Posted by bassJAM

Maybe I should have taken my truck to the dealership when I got it, as my speedometer read almost 6% lower than my actual speed, and the tolerance is only 3% (at 70 indicated, GPS read 74). I ended up fixing it with an SCT X4 tuner, but it ticked me off that I had to because that was a recipe for a speeding ticket.

To answer the OP, I did check my odometer against a GPS app, and changing my speedometer made no changes to my odometer, so they must be controlled separately.