Speedo error worse than estimated?

[Driveitlikeuboughtit]

Is this just down to the brand of tire and slight dimensional differences? Estimated error is 1.92% for a 265/35 tire on 18x9.5" wheels, actual error is more like 5% per comparison of the speedo and GPS calculated speed.

Or was my speedo off by about 3% from the factory and the tire speedo error exacerbated that? Just wondering what other people are seeing.

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[jasonjm]

GPS can be off by 1-2% as well depending on speed. Tire inflation could further alter by a tiny bit too, all adding to an additional percent. Really need multiple measurements to get an accurate determination.

 

[Driveitlikeuboughtit]

Yes - I've been tracking it for a couple of weeks now. Additional proof is from radar detectors that display your speed while telling you to slow down - I'm consistently showing a 4-5% error.

I have also compared odometer to Google maps in terms of miles traveled. I am showing an error in the 4-5% range here.

2% I could shrug off, but 5% is getting significant. An extra 50 miles every 1k traveled. I might have to run a larger tire/wheel combo just to true this up. I just wanted to know if others are sharing my experience or if this is a specific issue with the manufacturers I've chosen or the size combo of 265/35R18. I mean, if the same calculator shows a 4% error with a tire/wheel size, will that be doubled to 8% in reality?

 

[RedGiant217]

I would think that for wear and tear and maintenance purposes, it doesn't matter. When the car shows 1050 miles, it doesn't really matter how far you've actually gone because from a drivetrain standpoint, it's more about number of revolutions. So, if it bothers you, then get tires that are closer to stock diameter. But either way, stick to the maintenance intervals based on the odometer and not on whatever actual mileage is driven.

 

[rich33]

Is this just down to the brand of tire and slight dimensional differences? Estimated error is 1.92% for a 265/35 tire on 18x9.5" wheels, actual error is more like 5% per comparison of the speedo and GPS calculated speed.

Or was my speedo off by about 3% from the factory and the tire speedo error exacerbated that? Just wondering what other people are seeing.

Revolutions of a mounted tire under load will generally be about 3% more than the calculated number. Specs are based on an inflated tire mounted on the "measured rim" size and not installed under the weight of a vehicle. Per Tire Size.com

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