I've seen speedos that show a speed faster than your actually traveling but I've noticed since I got the truck that the speedo is showing a speed SLOWER than I'm traveling. I verified with my iPhone, radar detector and speed sign today that its off. At 65 mph it says I'm doing 62-63. At 75 the gap opens a little wider at 4mph.

What could cause this? I'm running stock tire size and the tires only have about 5k on them and wear evenly.

 

Very common.

A lot of people who go with 285's instead of the stock 265's report a more accurate speedo.

Stewart

 

I had noticed the same thing with stock tires, it would always read about 2 mph slower on the speedo than my GPS. At the time I just thought the GPS was off a bit, it was a cheapo plug in model. Since then I have changed my wheels and tires going with 285's and installed an internal mirror GPS that is high quality. Now when my speedo says 65 my gps also says 65. My guess is the programming folks had a small calculation error somewhere that made it through the quality control folks.

 

Strange. Yours is backwards from the norm. With stock tires my speedo reads a hair faster than actual ground speed. Increasing tire size increases ground speed relative to what the speedo reads.

 

Mine matches my gps. I know my windstar was out about 1-2 mph my wife's fusion was out 1 mph but it seems good now. Too many variables to take into consideration to get it bang on, just think about new vs worn out tires it will change your speedometer reading by 3% then factor in differences between tire manufacturers. I would guess that they calibrated the speedometer using the tires it shipped with from the factory @50% tread. Making your speedometer read 1.5% low when new and 1.5% high when worn out.

 

thats exactly my issue i'm 2-3mph fast with stock sized tires.

 

I'm dead on with 285s, stock read slower by 2-4 depending on speed.

 

This comes up often. On my stock ex w/6L

1. The speedo and odometer are often 3 mph below actual at hiway speed.

2. The mph and trip distance read via the odb2 port and displayed on my sg2 reads the same as gps.


Folks who go with 285-75-16 tires report speedo becomes accurate, but fuel mileage goes down.... since the computer now under reports distance traveled thus lower mpg.

Vicious circle... sort of explains why ford "gauges" have no scale, to worry anybody.

 

Thanks for all the responses. I'm not that worried about it but my OCD kicks in and it drives me nuts that it's different.

Did the F-250's of the same years as the Excursion ship with 285s? Perhaps this is why the discrepancy?

I love how I have to un-Excursion my Excursion to make it what it was intended to be - a SuperDuty SUV. Springs, tires, etc

 

no 99-04 super duty ever came factory with 285/75 16's.

some did come with 285/35 16 but they are almost identical to 265/75 16 in height.


i watch my GPS more then i do my speedo, its easier to see since i dont have to look down.

 

With my stock size tires (265) my speed is 1.5% below gps speed. You would think all the speedometer's would be slightly below actual speed, to avoid speeding lawsuits.