Speedo reading 2 mph higher than GPS

[playinboats]

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

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[bluelobo26]

Every car ever made has an inaccurate speedometer; anywhere from 1 - 5mph high or low, but usually high. Unless you have a police package car with a "Certified Calibration" speedometer.

 

[d7602002]

I like when my speedometer is off by a couple. Then I set it at 60 and I can say I'm doing 60 miles an hour but I'm only doing 58

 

[Glen Baker LLC]

hmmm

 

[DS_Ohio]

The Japanese motorcycles I've owned are closer to 10% optimistic. The Maverick is not nearly as bad, based upon my cellphone's nav readout of current speed.

 

[MaverickDragon]

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

As noted, few (if any) cars have a completely accurate speed readout unless they are calibrated.
Just a minor change in tire size, even due to wear, can alter the accuracy of the displayed speed.
IIRC, Forscan can be used to change the speedometer reading so it's spot on, otherwise an exactly accurate speedometer is unlikely.

 

[Glen Baker LLC]

As noted, few (if any) cars have a completely accurate speed readout unless they are calibrated.
Just a minor change in tire size, even due to wear, can alter the accuracy of the displayed speed.
IIRC, Forscan can be used to change the speedometer reading so it's spot on, otherwise an exactly accurate speedometer is unlikely.

Wouldn't tire wear affect a calibrated speedometer?
Is it recalibrated it every couple thousand miles?

 

[MaverickDragon]

The variation due to tire wear is part of the reason why police departments have a regular schedule for street patrol vehicles that use pacing for issuing speed tickets.
Anything that changes the circumference of the wheel/tire will affect it, including tire inflation pressure.

 

[tiktokbrainrot]

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.

Wouldn't tire wear affect a calibrated speedometer?
Is it recalibrated it every couple thousand miles?

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.

The Japanese motorcycles I've owned are closer to 10% optimistic. The Maverick is not nearly as bad, based upon my cellphone's nav readout of current speed.

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.

 

[Wahjahbvious]

I usually see ~1MPH difference in unmodified vehicles. More, if wheels or tire sizes have been changed without recalibrating.

 

[MaverickDragon]

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.

Can be held liable? Highly unlikely, as many factors outside of their control impacts that.

Federal law allows factory speedometers to read up to 5% higher or lower than actual speed at 50mph. If outside that range, a judge isn't going to issue a ticket to Ford, but an attorney might seek damages, typically in a class action, such as one that Honda settled.

Since most speedometers err on reporting a lower speed, it's a lot more likely that the error would be on the driver, typically for adding larger tires or wheels.

 

[Raymundo76]

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

Has always been +- about 1 mph for me

 

[HeyBales]

Back when phones were getting GPS receivers, and prior with actual GPS device - all 6 vehicles since then have shown under actual speed by that amount.

Even the Maverick has like 4 PIDs a scanner can get to regarding speed - and the one for use by the IPC is lower than the others.

It actually appears the system gets metric measurements and convert to miles - because my 65 mph reading on cruise control easily is 64 on the gauge speedo as it is clearly not halfway, as is the PID for speed, 64.3 or such.
I'm sure it's a rounding error from metric.

 

[lavaspill]

Buy some larger tires, that will straighten that problem right up.

 

[not null]

Yeah.
My snow tires are the same measurements as my all seasons.
With the snows, the speedo reads 2mph faster than gps.
With the all seasons, the speedo and gps basically match.

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