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 unless 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.

 

[NoMe cruiser]

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

tire size designation, like "P215/65R15", indicates the tire's width (215mm), aspect ratio (65%), construction type (R for radial), and the rim diameter (15 inches) that the tire fits on, where the first set of numbers represent the width, the second set the aspect ratio, the letter indicates construction, and the last number is the rim diameter; essentially, it tells you the key dimensions of the tire to ensure proper fit on your vehicle.

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[misterquad]

Is a 265/70R18 the same height as a 285/70R18? No. The width has a direct relationship to the height. Therefore, changing the width does change the speedo.

 

[NoMe cruiser]

Is a 265/70R18 the same height as a 285/70R18? No. The width has a direct relationship to the height. Therefore, changing the width does change the speedo.

The tire width has no direct impact on tire height or diameter. The aspect ratio will ultimately determine diameter. You’re digging a deeper hole there bub.
You have enlightened me on why people are putting wide tires on a 4wd, some of them think 285 is the tire height.

 

[misterquad]

The tire width has no direct impact on tire height or diameter. The aspect ratio will ultimately determine diameter. You’re digging a deeper hole there bub.
You have enlightened me on why people are putting wide tires on a 4wd, some of them think 285 is the tire height.

Okay, lets do the math. Lets assume a 70 aspect ratio is a constant.

A 265/70R18 is 32.6 inches tall.
A 285/70R18 is 33.7 inches tall.
They both have a 70% aspect ratio.
The only number to change is the width.
The 285 is 3.38% taller than a 265.

Now back to original purpose of this thread.

If the speedo reads 70mph on 265s and the GPS says your speed is 67.5mph, that is a 3.70% error. Now switch to 285s and the speedo reads 70 mph, the vehicle is 69.8 mph.

You can go to tiresize.com and check my math. It is pretty close I am sure.

 

[REMC]

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.

I did my checking on a straight line, the weather wasn't bad, 9% is a big error. I've checked other cars before, 2-3% diference is pretty common.

 

[Altitude]

Is a 265/70R18 the same height as a 285/70R18? No. The width has a direct relationship to the height. Therefore, changing the width does change the speedo.

You can’t know whether 265 to 285 changes the height unless you also know the sidewall proportion of both tires.

 

[misterquad]

You can’t know whether 265 to 285 changes the height unless you also know the sidewall proportion of both tires.

The is the aspect ratio of 70%. It is the same on both as noted above.

 

[Altitude]

The is the aspect ratio of 70%. It is the same on both as noted above.

Then they aren’t the same diameter.

Here is a website that does the calculation for you, including speedometer inaccuracy. Pick the “tire comparison” tab.

Tire Size Calculator

The best tire size calculator. Use our tire size calculator to find tire size specs and suggested tire sizes. Then check prices for all available tire sizes.

tiresize.com

 

[misterquad]

Then they aren’t the same diameter.

Here is a website that does the calculation for you, including speedometer inaccuracy. Pick the “tire comparison” tab.

Tire Size Calculator

The best tire size calculator. Use our tire size calculator to find tire size specs and suggested tire sizes. Then check prices for all available tire sizes.

tiresize.com

Exactly.