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.