Speedometer Off?

[mwalsh9152]

Has anyone else noticed that their speedo is off? On my trip home from picking the truck up, my speedo was off 2MPH. Truck showed 80, but GPS showed 78MPH. Went past a roadside indicator today which is spot on with all my other vehicles, and it showed 39MPH with the truck showing 40.

 

[CJM]

I install Distance Measure Computers in vehicles for road survey work. When I get a good calibration on the
unit I am usually within +/= 3" on a 1 mile course. I have found most every vehicle speedometer reads over by
1-2 mph. Even had one that was reading 5 mph over. I have never had one that reads under.

Also, check your tire inflation. It can actually make a bit of difference in you speedometer/odometer readings.

 

[jadmt]

You can adjust with a procal snap or similar device.

 

[Pronto]

When I was adjusting my speedo with AlfaOBD, I used two different GPS apps on my phone to check my speed. Strangely, they did not concur and were off from each other by 1.5 to 2 mph at highway speed. I ended up splitting the difference and it seems to be accurate. I can't discern any errors to make me want to make further adjustments.

 

[jadmt]

When I was adjusting my speedo with AlfaOBD, I used two different GPS apps on my phone to check my speed. Strangely, they did not concur and were off from each other by 1.5 to 2 mph at highway speed. I ended up splitting the difference and it seems to be accurate. I can't discern any errors to make me want to make further adjustments.

that is odd. I use Ulysse speed app and two Garmin stand alone gps and all three are spot on.

 

[jadmt]

with the pro cal snap (alpha obd may too i dont know) it shows you what the tire size the factory has entered so you just adjust from there.

 

[mwalsh9152]

thanks, I wasnt sure if anyone else had noticed anything. In all the other vehicles I drive regularly, the GPS on my phone (either my work iPhone, or my personal Android) and the speedo always are the same. The roadside sign down the road from my house always tells me the same MPH as the phone and my cars as well. Both my phones would show 78MPH while I have the cruise set to 80 on my truck. Guess I will have to pick up an AlphaOBD like I planned anyway.