Speedo reading 2 mph higher than GPS

[rlhdweman]

Wondering if others found this. Tires are Firestone all season 225/65R17

This can happen with different brands of tires or even the same brand but a different model of tire, I replaced my continentals with another line of continentals & had the same experience, both were the same 225/65r17 & the speedo read 2 mph fast even though the replacement tires had an extra 1/4 of tread when new.

Sponsored

 

[Connect]

my speedo is regularly 2mph more than gps, sometimes even 3.

 

[Chops]

Back in the mech days, an incorrect speedometer meant an incorrect odometer too?

Now that both are computed - that is no longer an issue.? (Speedometer can be overstated independent of the odometer).

“Turning back an odometer” will require different techniques

 

[TheSEARCH]

When new my 2023 Hybrid xlt speedo matched gps maybe .5 mph off. But super close. So have my last four cars. To say that is normal I would disagree. BUT my snow tires Blizzaks were off by 2 mph which I hated. Now I replaced my OEM tire with Tour 54 tire. Off the exact same amount my snow were. I got the usb to obd plug and forscan and reset it so it was perfect. I find it unacceptable to be off by 2 mph. Mine would say 60 but really 58 mph. I say it is NOT normal to be off by 2 or more Maybe one at MOST.

 

[kevink]

Wondering if others found this. Tires are Firestone all season 225/65R17

Mine is probably 1/2 mph off at 70+. My car is about 3 mph off.
The 2 Ford pickups were essentially right on. My 95 Ford Taurus was way off the other way, I never looked into why. My BMW was about the same 3mph off you see.

 

[playinboats]

The thing I noticed was that when I compiled the fuel consumption with real data, that was off about 2 gpm So the miles driven was higher and time frame off. (Note) just completed a 2600 mile round trip

 

[Fred Piering]

Wondering if others found this. Tires are Firestone all season 225/65R17

Mine is also off by 2 MPH High.
On the other hand, my Mustang was dead nuts on!

 

[HeyBales]

The thing I noticed was that when I compiled the fuel consumption with real data, that was off about 2 gpm So the miles driven was higher and time frame off. (Note) just completed a 2600 mile round trip

While wrong distance being used for computes could affect that - that is almost entirely a different thing - and purposely done.
And that MPG and DTE readings can be improved by manually correctly them.
Sadly it does nothing for the mph.

But I've found the distance between mileage markers to be right on, but the mph to still be off.
They are different things in the computer modules.

Look up engineering mode threads for that manual correction for mpg and dte.

 

[MavUnicorn]

Google " What would account for a 2 MPH difference between calculated MPH per GPS and a car's speedometer? " It gives you an interesting science lesson.

 

[Glen Baker LLC]

I guess I'm lucky. With almost 28,000 miles on my original stock tires my speedometer is exactly correct.
Edited
If I had thought about it ahead of time. I would have brought my other two handheld GPS units.

 

[KenE]

All cars will read slightly faster. The reason is that, if a car is found to be underreporting speed, the car manufacturer can be held liable for speeding tickets.

The speed calibration is set when tires are new, so any amount of wear will slowly throw the number off.

Yes, which is why calibrated certification is expensive, and on police cruisers, their speed radars are calibrated daily to ensure accuracy. They also have GPS data backups to ensure accuracy.

In my experience, my german cars have been right on the money when new, and 1 MPH optimistic with tire wear. My 91 ford is WAY optimistic, I think something is up there, its 5-6 mph optimistic, and my work Maverick is right alongside the germans at 1 on new tires.

GPS speed vs Speedo readout is the best way ordinary citizens have to check accuracy. Just remember GPS indicated speed is only accurate on flat ground. Going up or down a hill introduces error because you're not going in a straight line across the earth. FORScan correction is the easiest way to correct.

 

[Hootbro]

My speedo was off 1 MPH with the factor Continental Pro-Contact all season 225/65/17 tires. Install a set of new Continental Cross Contact ATR 225/65/17 tires and the speedo is now spot on.

Makes me think Ford just uses a baseline speedo calibration for most models and luck of the draw where ones build lands as to how accurate the speedo will be.

 

[stevj]

Factory speedometer offset is simply lawsuit avoidance.
If the speedometer reads higher than actual and you observe speed limits, you don't get speeding tickets.
If your speedometer reads lower that actual speed from the factory, that's a class action lawsuit waiting to happen.

Steve

 

[MiMav]

I used to design test systems that tested instrument clusters. It was common to have OEM specified display tolerances that purposely read high, even when receiving digital data. 60 MPH -0/+2 MPH is a typical example.

Sponsored