Speedometer is way off

[REMC]

I felt like my speed was slower than what the car displayed, so have been comparing to Waze. At low speeds there is around 3 mph (5 Km/h) diference, at 70 mph is around 6 mph (10 Km/h), what seems like a lot to me.

Has anybody checked in a more "accurate " way?

Mine reads Km/h, no mods so far.

 

[misterquad]

You are going slower and the mpg is reporting higher than actual too. The 4Runner do the same thing. When I swapped my 265s for 285s on my 4Runner, the speedo was corrected. I tell myself that they designed it this way on purpose because they know many of us will be up-sizing to 285s.

 

[TakaraLC]

Mine is correct based on those signs that display your speed.

GPS based speed is also not accurate if the road is completely flat.

 

[spdrcr05]

I replaced the stock tires with 275/65/20 Pirellie Scorpion's which are theoretically 1" larger in diameter. My speedo and my Waze/iphone GPS were spot on before, but now my truck reads 1-2 mph slower than indicated on GPS.

Regarding Takara's comment on GPS accuracy .. the wisdom of the crowds (i.e. the Gemini AI search tool) says this -
"

• Good for most situations:
  Under normal conditions, the GPS speed reading on an iPhone will be very close to your actual speed.
  
  
• Factors affecting accuracy:
  • Signal strength: Poor GPS signal can lead to less accurate speed readings.
    
    
  • Weather: Heavy rain or dense clouds can interfere with GPS signals.
    
    
  • Environment: Buildings, tunnels, and dense foliage can block GPS signals.
    
    
  • Road curves: GPS might not perfectly capture sharp turns, leading to slight speed discrepancies on winding roads.

 

[NoMe cruiser]

. I tell myself that they designed it this way on purpose because they know many of us will be up-sizing to 285s.

285 is the width of the tire which should have zero impact on Speedo readings.

 

[NoMe cruiser]

Mine is correct based on those signs that display your speed.

X2

 

[misterquad]

285 is the width of the tire which should have zero impact on Speedo readings.

I hope you are joking. Or do you really not understand how to calculate a tire height?

 

[NoMe cruiser]

I hope you are joking. Or do you really not understand how to calculate a tire height?

Sorry but 285 is not the height of the tire. It is the width in mm’s. The second number, commonly 65 or 70 is the aspect ratio which is actually a percentage of the tire width that designates the height of the sidewall from outer rim to outer tire. The third number is the rim diameter in inches. 285/70/18. That is how I always looked at tire sizes. How about you.