OHTA.ca

Changed tire size on your vehicle? Wonder how much it affects your speedometer reading?

Get your stock tire size for your vehicle - driver's door of your vehicle, should be a white manufactur plate there, this includes weights, VIN and the proper tire size for the vehicle

go to this website www.1010tires.com go to "tire and wheel tech", then "tire size calculator"

http://www.1010tires.com/TireSizeCalculator.asp enter in your vehicle info, then select the tire size is now on your vehicle.

The calculator will compare the tires and let you know the difference, if any in your speedometer reading.

Of course all depends if your speedo is accurate to start with

cheers

HB

hwybear wrote:Of course all depends if your speedo is accurate to start with

What's the best way to do this? I've always thought asking an officer to tag your speed while coming along right at the speed limit would be fine, but I'm not sure they'd appreciate that while they're doing speed enforcement.

While not 100% accurate, most GPS units give real time speed information. Many highways also have mile (or kilometer) markers, so you could time yourself between markers and see if it matches your indicated speed.

Stanton wrote:While not 100% accurate, most GPS units give real time speed information. Many highways also have mile (or kilometer) markers, so you could time yourself between markers and see if it matches your indicated speed.

Would GPS units be a better measurement of your speed, than your cars speedometer?

admin wrote:

Stanton wrote:While not 100% accurate, most GPS units give real time speed information. Many highways also have mile (or kilometer) markers, so you could time yourself between markers and see if it matches your indicated speed.

Would GPS units be a better measurement of your speed, than your cars speedometer?

The instant answer is no, due to data transmission delays and interference.

Is it accurate?? most of the time, i would say yes.... cloudy days, bridges, tall building will affect the update rate and accuracy thou.... Just my 2 cents.

admin wrote:Would GPS units be a better measurement of your speed, than your cars speedometer?

I dont know if it would necessarily be better, but it might help clarify how accurate your speedometer is. As Reflections said, GPS signals can be problematic. Id suggest finding a nice flat stretch of highway on a clear day. Lock in your vehicles speed with the cruise control for a few minutes and compare your speedometer to the GPS reading. It should give you a good idea if theres any significant discrepancy.

At highway speeds Ive found both of my personal vehicles, with factory rims and tires, show my speed to be 5 km/hr faster than it actually is (better to overestimate I guess). At work, the cruisers Ive tested with calibrated speedometers have been dead on.

To the best of my knowledge, Canada has no regulations governing speedometer accuracy, however the US and European Union do. I doubt they build different speedometer parts for Canadian cars, so quite likely those standards get applied to our vehicles as well. Id also hazard a guess that most manufacturers would err on the side of caution and have the speedometer overestimate your speed to avoid possible litigation.

Edit: For added accuracy, conduct tests in an area which HwyBear patrols. When he pulls you over and advises you of your speed, thank him for contributing to the accuracy of your tests.

Another good one is "http://www.rimsntires.com/specspro.jsp"

good advise,

question: in the UK, while technally +1mph over the limit is illegal, "usually" its regarded as 10% +3 MPH to be considered speeding.

10% because a speedo can be that much out (the older ones where) and 3% is the width of the needle.

SO in a 70, you are given a "grace" of 70 +7mph +3mh = 80mph...

with radar they can still zap you at anything, but if your doing at least 80 in a 70, thats regarding as speeding in that zone..

Soooooooooo, is that the same here? (I doubt it eh!)

cheers

hwybear wrote:Changed tire size on your vehicle? Wonder how much it affects your speedometer reading?

Get your stock tire size for your vehicle - driver's door of your vehicle, should be a white manufactur plate there, this includes weights, VIN and the proper tire size for the vehicle

go to this website http://www.1010tires.com go to "tire and wheel tech", then "tire size calculator"

http://www.1010tires.com/TireSizeCalculator.asp enter in your vehicle info, then select the tire size is now on your vehicle.

The calculator will compare the tires and let you know the difference, if any in your speedometer reading.

Of course all depends if your speedo is accurate to start with

cheers

HB

I checked my GPS yesterday with my speedo; (standard OEM's)

and it was bang on,

I read somewhere that the older cars pseedos where not great anything y2k + are balls on accurate

Aren't smaller tires better for the winter?

george__ wrote:Aren't smaller tires better for the winter?

Narrower tires cut through snow better, but it's the overall diameter that you don't want to change to avoid affecting the speedometer.