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Old 10-27-2012, 12:08 AM   #21 (permalink)
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Quote:
Originally Posted by YeahPete View Post
I have to add this. When I switched to smaller tires on my cavalier the mpg and the speedometer were effected. It showed me as getting better mpg and my speedometer shower me as going 2-4 mph faster.

So if a larger diameter tire actually shows mpg gains, the gains are also multiplicative because it should decrease your mileage because it shows as your speedometer registering a slightly lower speed.

Anyone understand what I am getting at?
Larger tire shows a decreased speed and decreased miles on odometer.
Smaller tire shows increased speed and increased miles on odometer.

Was this factored into the mileage gains?
Hmm. I was under the impression that increasing the size increase the speed, and usually increase the mileage.

I mean, the cars calculate by axle turns. If one had 1 axle turn with 15s, then they would go so far on one axle turn. If one increase size to 20s, the cae moves way further for one axle turn.

I think the quoted may not be accurate. :/

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Old 10-27-2012, 09:47 AM   #22 (permalink)
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As I am the OP on this thread, I'll say that the ScanGauge was re-calibrated between testing the OEM tires and the "tall tires". As I recall, the OEM tires used -3% setting, and the "talls" +7%. In both cases, the ScanGauge error was less than 0.1 mile in 10 miles. AFAIK, an improperly adjusted ScanGauge will affect both MPH and MPG readings.
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Old 10-27-2012, 10:01 PM   #23 (permalink)
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Did you use a tape measure and measure to compare the actual circumference of each tire?

The new tires are taller, giving greater circumference and therefore greater distance traveled per revolution.

Also, a new tire has thick tread depth, but an old tire has thin tread depth, further increasing the difference in diameters/circumferences/distances traveled per revolution.

OTOH, an old tire with thin tread may have less squirm and rolling resistance than a new one, or being smoother, less aero drag.

Could be, the difference in mileage is attributable to more variables than just differences in nominal height, or that once the new tires wear down, they won't be as fuel efficient.
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Old 10-28-2012, 10:51 AM   #24 (permalink)
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Per the original post, the tires were measured with a tape measure at 40 PSI cold, just prior to mounting the wheels on the car for testing.
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Old 10-28-2012, 03:12 PM   #25 (permalink)
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Quote:
Originally Posted by UltArc View Post
cars calculate by axle turns. If one had 1 axle turn with 15s, then they would go so far on one axle turn. If one increase size to 20s, the cae moves way further for one axle turn.
Yes, which means the stock gauges would be under-counting mileage and speed when a larger diameter tire/wheel is used. The dash shows one mile traveled, when perhaps 1.25 or 1.33 was actually traveled.
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Old 10-28-2012, 03:34 PM   #26 (permalink)
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Quote:
Originally Posted by California98Civic View Post
Yes, which means the stock gauges would be under-counting mileage and speed when a larger diameter tire/wheel is used. The dash shows one mile traveled, when perhaps 1.25 or 1.33 was actually traveled.
So for one to go from their current gauge to accurate, they would need to increase, right? It is greater, not lesser.
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Old 10-28-2012, 04:09 PM   #27 (permalink)
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Quote:
Originally Posted by SecondWind View Post
Some work I just completed...

Edit by admin: see attached spreadsheet for raw data and full text; some of the spreadsheet text is quoted below...
I got 7% better mileage after fitting Energy Savers, so the increase in mpg is likely due to the tyres' profile, rather than their size.
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Old 10-30-2012, 09:13 AM   #28 (permalink)
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Larger tire showes your going slower. Less RPM per original tire size.
Smaller tire shows faster. More rpm compared to the original tire size.

Euromodder you are correct. Smaller profile less air drag, less surface area touching road. It could just be a better tire as well.
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Old 10-30-2012, 10:29 AM   #29 (permalink)
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Quote:
Originally Posted by chrs View Post
This is interesting--the taller profile of the vehicle should make aero drag worse, but apparently that's minor compared to the gearing effect. Too bad it's not as easy to swap gears as it is on a bicycle.
Well. since the newer tires are 2" larger in diameter, and a 1" difference in radius (obviously) Calculate in the flex of the sidewall with 40 psi.. The car maybe only about 3/4 inch taller. If there is any extra drag from the larger frontal area, I think the total circumference benefits will outweigh them.

But , good thinking about this. Its one of the overlooked variables.
Specially in the offroading scene. LOL
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Old 10-30-2012, 11:44 AM   #30 (permalink)
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It is my belief that long term proper testing is the only way to really know how a tire will affect FE on any given car.

I think the tires load rating (and loading), PSI and aero drag likely play more of a role than their size in terms of FE. (although they are also related)

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