My new Ranger's speedometer shows about 2 mph faster at 70 mph than what my GPS shows. Assuming the GPS is accurate, I wonder if Ford does this intentionally to make the gas mileage seem better than it actually is. Also, if the speedometer is showing too fast, then the warranty would run out before it should.

 

I believe all carmakers will tell you that their speedometers are only accurate to within 2-3 miles per hour.

 

my fathers 05 f150 reads about 5 percent low so i would assume you can expect it to be a little off one way or another.

 

most regular speedo's are only accurate to plus or minus 5% and it is a sliding scale towards worse on the old mechanical driven ones. The new pulse counters can be within 3%, or close to 0 on traveled distance, depending on how fanatical you need to be. Since the meter movement has tolerances, that is as close as you can get.

OTOH, you can get it calibrated for a specific speed, or even get a chart with 5 mph increments and be really accurate, but not good enough to beat a speeding ticket.

If you need it really accurate, stay with the GPS, signal accuracy to 3 MM.

 

The speedometer does not directly drive the odometer. So just because your speedometer is not reading accurately, that doesn't mean the odometer/miles driven is inaccurate.

 

There's almost no way to get the speedo perfect and have it stay there. Due to tire wear/inflation pressures affecting the tire diameter, perfection is a moving target. I'd say that being accurate within 2 mph is pretty darn good...

If you really want to chase that target, then a Bama program will let you specify what your tire diameter is. With the big tires I have on my truck, I've chosen to make the odometer as accurate as possible, and the speedo relationship ends "floating". In this case, a gps says my speedo is about 3-4 mph fast when the odo is right on (per mileage signs on the highway). An accurate odo makes my gas mileage calculations more accurate.

 

Right, you would have to keep correcting the speedo as the tires wear.
My 99 is right on the money to my GPS with my tires about 3/4 wore out.

 

tire wear can through the speedo 5%

 

According to the Owasso police department speed trap trailer, my speedo reads 3mph slow at 35mph. According to my hand held gps, at 65mph it's right on the money. Haven't check the gps to speedo at the slower speed.

 

Yeah, whenever you have a speedometer that measures revolutions to calculate speed, you will have a margin of error. The tire sizes can vary depending on pressure, brand, and wear. Some new cars now use GPS based speedometers for this very reason.