Tire pressure and diameter
 

So I was searching through some EM stuff and found something that said that increasing tire pressure would increase the radius of the tire, thus requiring speedometer recalibration. It made sense that - even to a micro degree - the tire would be slightly larger.

I'm considering upping my pressure a bit. It's currently at door-frame spec (33psi), but the tires are rated for 44. I plan on starting with a jump of 2psi and gauging how much harsher the ride feels.

Will a speedometer recalibration be necessary, and is it easy to do?

If my understanding is correct, the speedo itself will read slower than I am actually traveling - correct?


Here's what I was talking about:
http://ecomodder.com/wiki/index.php/...o_max_sidewall

[/URL]/Increase_tire_pressure_to_max_sidewall

The difference would be marginal, I really don't think yo'd need to re-calibrate. Just keep in mind many car models are available with a variety of tire sizes, each just a little different. Even tires which *should* be the same size according to the designation ont heir sidewall can vary from each manufacturer and even between different tire patterns from the same manufacturer. For offroad cars the difference in diameter can sometimes be up to an inch!

Just increasing the tire pressure won't do much (if anything) to increase its diameter. Modern tires are all steel belted anyway, they don't 'balloon'.

+1

...FYI, the actual "rolling diameter" of the tire is typically about 97% of its' unloaded, 100%-circular diameter, and it's due to the 'contact patch' "spread-area" flat-spot on the tire bottom.

Ah, okay. Thanks all.

One follow-up: Will the odometer be affected at all?

If I note a small mpg improvement with this mod, I want to know that it's related to decreased rolling resistance (and longer coasts), not calculation error.

You can check the difference on a stretch of highway with mileposts. Record your odometer / tripmeter to the tenth at the first milepost (123.4), then again 10 mileposts later (133.4). The difference in tenths (observed miles, 10.X, vs. actual miles, 10.0) is your odometer error in %.

a great lil online tool for comparing tires and diameters...

Tire size calculator

With my tires at 44 psi my gps read 2.65% higher on average than my odometer.
After bumping them to 50 psi the difference jumped to 2.83%.

The change is small, but there is one.

I suspect these small discrepancies would also exist in other situations, such as the difference between driving a 2 mile commute on cold tires, and a 50 mile commute on mostly warm tires.

I don't suppose there are any ways around this, are there? My odometer is digital and only records whole numbers.

Maybe I can do this: Set the trip odometer at the first mile marker. Then, have a passenger watch it until it changes. If it's right on the mark, we should know. But if it's off, I have another idea.

OR

If I maintain a constant 60mph (cruise)on flat ground, we can use a stopwatch to record the time it takes for the odometer to register one mile. Then, we can use the simple Rate x Time = Distance formula to hopefully uncover any discrepancies. I suppose the above can be done at any speed, but 60 would be easy.

The only variable is the amount of time it takes for the passenger to hit the stopwatch. There would be a minuscule delay, I suppose.