Tire pressure versus speedometer accuracy

[Tom Brown]

Using a ScanGaugeII and a GPS, I made an interesting observation with regard to tire size versus tire air pressure.

Readings were done by driving at 165 kph for a couple of minutes to exaggerate any calibration errors that exist. That's as high as I care to take it. Worse, I had to hold it there for a couple of minutes to let the GPS stabalize.


Tires: Pirelli Carving 195/70 R15.

Test 1 - pressure: 34 psi
Test 1 - calibration: 5% on the SG II (actual mis-calibration is estimated to be 5.3)
Test 1 - As I would increase speed slowly, the GPS would increment to the next increment of 1 KPH higher a touch before the SG II.

Test 2 - pressure: 40 psi
Test 2 - calibration: 5% on the SG II (actual mis-calibration is estimated to be 4.8)
Test 2 - As I would increase speed slowly, the SGII would increment to the next increment of 1 KPH higher before the GPS.


Conclusion: tire pressure makes some, but extremely little, difference to tire circumference.

 

[Tom Brown]

Here's an observation from the day's driving.

I was going to try 40 psi and return to 34 psi after fuel economy tests. I assumed the higher pressure would impare the winter driving performance of the tires. Now, I'll probably just leave the extra pressure in the tires.

I live in a cold climate. We've had -20C in the last 24 hours and some fresh snow.

From what I can tell, there is very little difference in winter driving performance at the increased air pressure.

 

[Joe_Meehan]

Yea, very little difference. With steel belts, it's not going to stretch much. The difference really ended with radial tyres.

 

[RalphVa]

Higher pressure might help in the snow IF the middle of the tire is pushed out. Your tests show maybe this doesn't happen.

This is because skinny, higher pressure tires do better in the snow. Bigger, wider tires will act more like skiis unless they have a very aggressive tread.

 

[RiceEater]

Centrifugal force on the tire tread is like 50 gs for speeds around town; > 2x at ~45 mph; to ~200 - 300 gs at freeway speeds (notice how fast the g forces grow). Before steel belted tires people just didn't survive driving too fast but even with steel belted tires the size still increases but it does make driving stupid a little safer.