Anyone else notice that their speedometer is off with factory tires?

[Scott R Nelson]

Slow by 1 mph...

Same here, at all speeds. I find it kind of weird that it isn't off by a percentage.

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[Fly2Win]

Mine is pretty much dead on with Waze, but not sure they are the authority on speed accuracy.

 

[donkeygoatrunner]

I drive by two or three radar speed limit signs every day. Speedometer is consistently higher by 2mph, regardless of what speed I’m driving.

 

[Stampede.Offroad]

...
Based on what I've seen, and from discussions in the Forscan threads (for adjusting tire sizes), the computer is set for a tire circumference smaller than what actually exists when new.
...

That sounds backwards. If the circumference is larger than what the computer thinks, you're traveling farther per unit time than it would predict, thus having a higher velocity in reality.

If however they used the theoretical 35" size rather than the real smaller value, it would over-predict as is observed, and this phenomenon will only get worse as the tire wears down.

 

[Jhuff]

That sounds backwards. If the circumference is larger than what the computer thinks, you're traveling farther per unit time than it would predict, thus having a higher velocity in reality.

If however they used the theoretical 35" size rather than the real smaller value, it would over-predict as is observed, and this phenomenon will only get worse as the tire wears down.

Ugh, this stuff is so easy to get flipped around in the head. I need to draw it out and do math to confirm .

What you say sounds right, and might explain why the Sasquatch Forscan size was so small. Maybe it's designed to be slow/conservative at first, and finally be more accurate (or at least not inadvertently speeding) when the tire is bald?

My Sasquatch circumference in Forscan equaled 33.25“, and the tire was advertised around 34.4“ diameter; my actual speed was 3mph slower than the dashboard.

Now I have a headache.

 

[foreWard]

Mine is off by 2-3mph as well. Annoys me, but I compensate by speeding everywhere. LOL

 

[Skott]

I've noticed when driving I will have cruise control set, and Waze on my screen will usually say my speedometer is 1MPH slower. So there's a bit of a difference in my WildTrak.

Likely due to 35 inch tires?

 

[puckett_jw]

I drive by two or three radar speed limit signs every day. Speedometer is consistently higher by 2mph, regardless of what speed I’m driving.

So I initially tested at 65 & it was off by 2 (sometimes 3). I assumed that this was a percent off, so I assumed that it was off different amounts at different speeds. I did more testing today & it is ALWAYS 2 off. It does not feel like a tire ratio issue.

 

[Bronco Bri]

So I initially tested at 65 & it was off by 2 (sometimes 3). I assumed that this was a percent off, so I assumed that it was off different amounts at different speeds. I did more testing today & it is ALWAYS 2 off. It does not feel like a tire ratio issue.

I noticed mine is about 1-2 mphs off as well. I have Sasquatch package.

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