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Old 04-09-2007, 08:26 AM   #1
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Inflating tyres - effects on MPG

I'm sure others have come across this, but still worth considering.

Overinflating tyres (within safety limits... usually!) is known to cause an increase in MPG. However, it also has an effect on the recorded mileage that will actually 'hide' some of these gains. This is simply because the effective diameter of the tyre increases, if it inflated at a higher pressure (since there is less deformation at the contact patch).

I worked out a few weeks ago that my replacement speedometer was 4% optimistic in the distance travelled, but now I have my tyres at a higher (but within the safety limits) pressure, it actually underestimates the mileage by about 1% (worked out by digital map data + an actual journey I did yesterday). I won't be correcting my MPGs at the moment as that would make
things more complex, but for those who have changed their tyre pressures, it might be worth rechecking the accuracy of their mileage meter. This is because I have noticed a nice increase in MPG, even when using the speedometer mileage readings that appeared to 'lower' my MPG by 5%.
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Old 04-09-2007, 08:46 AM   #2
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Rrreeeeaaaallllyy? Wow... Anyone with a GPS see the same thing?
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Old 04-09-2007, 10:05 AM   #3
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You may want to try marking your tires with something similar to the sidewalk chalk kids play with and try measuring how far the car rolls per revolution. With steel belted radials, the change in diameter likely would be negligable. The contact patch with the road would be smaller with higher pressure, but the diameter would not get bigger.
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Old 04-09-2007, 10:41 AM   #4
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That's what I mean - the 'effective' diameter is that of the bottom of the contact patch to the centre of the axle, and this gets less with overinflation. If the tyre was at 0.5psi pressure, then the effective diameter would only bit slightly more than the wheel rim itself.

When I changed the speedo a few hundred miles ago, it was clearly optimistic, but now that I pumped my tyres up, it is almost perfectly accurate.
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Old 04-09-2007, 10:48 AM   #5
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That's interesting I've done this on 3 cars and have not noticed any change with different pressures. What size tires are on Bluey?
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Old 04-09-2007, 11:49 AM   #6
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At present, with a nominal 40psi front / 38psi rear (vs the 'standard' 32psi and sidewall max 44psi), my Scangauge-reported distance is within a fraction of a percent of GPS-reported distance. The odometer report, however, is around 1.5% fast. (*sigh* I should adjust my gas log down some day.)
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Old 04-09-2007, 11:53 AM   #7
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Its semantics. Doesn't amount to a bean, much less a hill of beans in the real world when over all mpg, miles and mph are looked at.

My OD,,, trip A/B are all on spec with the Navi in the Civic. The SGI doesn't agree. Unless I adjust it it wants to read long by 15 miles or so on a 600 mile tank. Do the basic math and its not worth the time or energy. Since the ECU replacement in my car. Ive been within 1 or 2 tenths in my fuel useage from pump to SGI. Before ECU replacement... it could be way long or short. I wanted to run over the SGI before I got to the meat of the issue.

If your car has a good reverse p-trap for the evap system. Do full top offs and work from the OD if its close to right and call it good. Anything else is mind games with no clear cut result...

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Old 04-09-2007, 12:59 PM   #8
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Ill check with my gps
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Old 04-09-2007, 01:54 PM   #9
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Yet another thing to test with a SG: Someone do some runs @ 45psi compared to the same route @ 32psi. Check out the differences in odometer readings with the trips.
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Old 04-09-2007, 02:50 PM   #10
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"Sure the diameter doesn't grow appreciably BUT the distance between axle and road rises with increased pressure, effectively increasing the "diameter"."

I guess I should have been clearer, the effective diameter (distance between axle and road) may change, but the radial belts keep the circumference from changing, effectively meaning there should be little change in the accuracy of the odometer/speedometer by changing tire pressure.
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