Tire Pressure and Speedometer Calibration

[RidingOnRailz]

Tire Pressure and Speedometer Calibration


Curious question:

In a new build vehicle, what pressure are the tires maintained at for calibration, or at least verification of correct operation of, the vehicle speedometer -

the maximum cold pressure according to tire sidewall, or, vehicle manufacturer OEM specified cold tire pressure?

The reason I ask is because if I drive, IE, 56mph in the right lane of a 55mph zoned highway, other drivers are either zooming up to my trunk, or, warping by me in the lanes to my left. My tires are kept at, or at 1psi above, cold pressure specified on the vehicle OEM door pillar decal.

[CapriRacer]

Originally Posted by RidingOnRailz

Tire Pressure and Speedometer Calibration


Curious question:

In a new build vehicle, what pressure are the tires maintained at for calibration, or at least verification of correct operation of, the vehicle speedometer -

the maximum cold pressure according to tire sidewall, or, vehicle manufacturer OEM specified cold tire pressure?

The reason I ask is because if I drive, IE, 56mph in the right lane of a 55mph zoned highway, other drivers are either zooming up to my trunk, or, warping by me in the lanes to my left. My tires are kept at, or at 1psi above, cold pressure specified on the vehicle OEM door pillar decal.

Speedometer calibration is done at the placard pressure. But the RPM (revs per mile) value is given to the OEM by the tire manufacturer - AND THEN the vehicle manufacturer makes adjustments - and they tend to have the speedometer read a bit low, because if the speedo reads high, the vehicle manufacturer could be legally liable for someone getting a speeding ticket!

But on a more practical note: People tend to speed, so judging your speedometer accuracy based on the traffic around you isn't a good idea.

A better idea is to use the Interstate mile markers. They are at least reasonably accurate. But use several over a long distance. I like to use 10 miles, although it is sometimes difficult to maintain a steady speed for 10 minutes - even with cruise control!

[RidingOnRailz]

Originally Posted by CapriRacer

Speedometer calibration is done at the placard pressure. But the RPM (revs per mile) value is given to the OEM
by the tire manufacturer - AND THEN the vehicle manufacturer makes adjustments - and they tend to
have the speedometer read a bit low, because if the speedo reads high, the vehicle manufacturer could
be legally liable for someone getting a speeding ticket!

But on a more practical note: People tend to speed, so judging your speedometer accuracy based on the
traffic around you isn't a good idea.

A better idea is to use the Interstate mile markers. They are at least reasonably accurate. But use several
over a long distance. I like to use 10 miles, although it is sometimes difficult to maintain a steady speed for
10 minutes - even with cruise control!

Thanks for that explanation.

Although I think the part about the speedo reading high/low is reversed.

You'd want the speedo to read(indicate) high, maybe 1mph/2kmh. That way, if a driver is driving at an indicated 60mph, they are actually doing 59.

In any case, I'm doing things right, for the most part.

[shorod]

Originally Posted by CapriRacer

A better idea is to use the Interstate mile markers. They are at least reasonably accurate. But use several over a long distance. I like to use 10 miles, although it is sometimes difficult to maintain a steady speed for 10 minutes - even with cruise control!

I'll offer that most people these days have smart phones, and if not, they probably have a standalone GPS navigation unit. There are multiple applications available for smart phones that will display speed based off the rather accurate GPS receiver in a modern smart phone, and many stand alone GPS navigation units also have an option to display vehicle speed. I suspect that even a dated GPS receiver will provide a more accurate speed reading than counting mile markers over time. But even if someone opts to use a GPS receiver to compare against the displayed speed on a speedometer, a relatively constant speed should be maintained for several seconds since it can take a GPS unit several samples to update the calculated speed.

-Rod