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Discussion Starter #1
I just did a 225 mile trip, followed by a return. When I got there I finally (yes, I know I should have done this when I first got the car...) checked the tire pressure on my month old 2001 CVT Insight. Eyup, twas low (don't ask...). I filled them up properly and headed home.

Mileage dropped.

I got to thinking... This might actually be just a reporting glitch and could be confuing things. Let me explain:

When the tires are underinflated they're (loosely speaking) smaller than they should be, turning more often than spec'ed, and the car is misreading the rolling mileage. For example, when you travel fifty real miles it may think you've gone 55. So... when it looks at the gallon you used it'll report back 55 mpg.

Inflate them properly, and that same road trip "only" takes you, now fifty miles. Ok, you get better "real" mileage so you'll actually ue a tad less than the gallon you did before, but....

Net result: the _reported_ number with low tires had been 55 mpg (even the the "real" number was 50), and the new reported one is 52.

Am I making any sense? Thanks
 

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The problem with your theory is that radial tires have a very strong non-stretching belt under the tread. The effective diameter is very constant regardless of pressure over a pretty wide range.

I suspect that there was a prevailing wind, or the temperature was lower on the way back (or hotter and you changed the ventilation settings), or the traffic was different, or there's a significant elevation change. All these can have a very significant effect.
 

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When I finally inflated mine from 30 to 45, I had an instantanous result and noticable difference....about 7 mpg on the trip A mpgs....all on the same drive to & from work.
 

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Discussion Starter #4
Dougie said:
The problem with your theory is that radial tires have a very strong non-stretching belt under the tread. The effective diameter is very constant regardless of pressure over a pretty wide range.
- My head is starting to hurt when I visualize this. On the one hand, you're of course correct that the total length of the circumferential tire belt is pretty much the same regardless of inflation pressure.

- on the other hand, if the rim-plus-tire radius has dropped from say, 20 inches in a fully inflated unit to 19 inches, the " two-pi-r " circumference has dropped commensurately.

- Similarly, when one tire is underinflated, the change in rotational velocities is one of the tricks used to (on high end cars) flash a warning display at the driver.

- anyone have access to a set of calibrated motorized rollers? 'twould be interesting to compare car<->"real" mile readings with fully inlfated versus partial tires...

Dougie said:
I suspect that there was a prevailing wind, or the temperature was lower on the way back (or hotter and you changed the ventilation settings), or the traffic was different, or there's a significant elevation change. All these can have a very significant effect.
- actually, I had driven about 75 of the return miles, getting pretty much the same mpg readings as when I headed out. It was after that point that the displayed mileage dropped..
 

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Those high tech cars that flash low tire pressure warnings have little sensor/transmitter thingies on the rims of the wheels, inside the tire, that talk via radio to a receiver on the car body.

The radius doesn't have anything to do with it. The sidewall squishes around as the tire rolls, and the result is that the wheel, the sidewall, and the tread all go around at the same speed. Probably if the tire is at like 5 PSI you'll see a difference, but going from 35 to 45 is nothing.

Getting better mileage from higher pressures doesn't have anything to do with changing the tire circumference, it's because there's less drag because the tire flexes less.
 
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