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2011 Kia Sorento, 08 Toyota Tundra, 1956 VW Manx buggy, 1958 VW rail
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I posted that I had put on a size bigger tire. my son says the extra weight of the tires will hurt gas mileage. i thought that it would slow the engine down a little and help gas mileage. we just drove from anaheim to pheniox driving 78 and 80mph and got 22mpg. we have gotten 24 and 25 at 70 to 72 so my v6 does not like the higher speeds. on the way back at 75 (TO GET BETTER MILEAGE) on flying J gas, my mileage dropped to 19 at Indio and finally to 20 at anaheim (DOWN HILL TO A LOWER ALTITUDE) Why did i lose mpg on the way home driving slower. (poor gas?) by the way-- why wasn't the 2011 V6 fwd included in the poor mpg kick back from KIA? thanks, sam.
 

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Cerato S Hatch Auto
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There's 1.5% difference in overall diameter, or maybe 3.5% if you're comparing worn 235s with new 245s.

Looking at 3.5%, that would compound to a 11% difference in observed fuel economy. That's 1.035 odometer error multplied by 1.035 squared (due to aerodynamic effect of a faster than indicated speed). It's 4.5% even for new tyres.

Wind conditions might explain your "downhill" observations.
 

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2011 Sorento AWD V6 SX Cherry Black
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The 2011/2013 Sorento V6 FWD & AWDs meet the tested National government standards for fuel economy, city/highway and combined, so no problems. Infact if you can't beat the EPA highway mileage you are hammering it too hard, just enjoy the speed.:D
 

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Cerato S Hatch Auto
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You added weight, and a larger tire to push. Why would you gain milage from this?
These are minor considerations, dimensional effect on final drive ratio is more important.

One car that I inherited had entirely inappropriate low-profile tyres (100 km/h was nearly 120 km/h indicated, with 8% odometer error). Larger wheels and higher-profile tyres transformed the car's cruising ability and brought the instruments more into line with reality.
 

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2015 Sorento
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Your using an example of a car with the incorrect tire size on it going to a more correct size. We are talking about a car with stock tires and then putting on larger ones. I understand your point, to an extent, as weight and overall dimensions play a large part in fuel economy. But, my statement stands, it make no logical sense, and as he posted, no factual sense either as he did loose MPG.

You also did not slow anything about the engine down, just created more load for the engine to push. Actualy, since it will need more power now, you may even see higher RPM, or engine speed for the same vehicle speed.
 

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2011 Sorento V6 AWD
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HA, if I thought I could save gas and get better traction with barely wider tires I would have done that right out the door. More weight, more ground contact, and throwing off your gauges just to name a few. Just for a tiny bit more tread/traction that you wont even notice. Don't see the point myself.
 

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Cerato S Hatch Auto
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You're using an example of a car with the incorrect tire size on it going to a more correct size. We are talking about a car with stock tires and then putting on larger ones. I understand your point, to an extent, as weight and overall dimensions play a large part in fuel economy. But, my statement stands, it make no logical sense, and as he posted, no factual sense either as he did lose MPG.
My example was simply to point out that tyre size can have a big influence on performance. The OP with somewhat larger tyres will get worst consumption figures, partly because he's asking more of the engine with the higher gearing together with an incorrect odometer. Engine RPM won't have a great effect on fuel consumption at high speeds, since air resistance dominates.

I did suggest that the anomalous fuel consumption might be due to the prevailing winds.
 

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2015 Sorento EX V6 AWD Ebony Black
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IMO, The bigger tire has a bigger circumfrence, (total distance traveled) is more with each revolution you're going, so your speedometer, & odometer read less then your actually going in speed, & distance, hence the false lower mpg reading.

75mph may equal= actually 77mph

You, & your, speedometer/odometer may think you've traveled 750 miles in 10hrs, but in actuality it's 770miles. Figure out what your actual speed is at 75mph with a GPS, or measured mile, then add the extra miles to your calculations.

Plus, "bum" gas (it's out there) doesn't help much, been there, done that.
 

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Cerato S Hatch Auto
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IMO, The bigger tire has a bigger circumference, (total distance traveled) is more with each revolution you're going, so your speedometer, & odometer read less then your actually going in speed, & distance, hence the false lower mpg reading.

Plus, "bum" gas (it's out there) doesn't help much, been there, done that.
You're getting the idea. IMO the main thing would be the aerodynamic effect at the increased road speed. It goes up as the square of the speed.

As for effect of wind: We had a car once with a top speed of 140 km/h and we encountered head-winds of at least 50 km/h (trees bending, dust etc. etc.); the car would only maintain 100 km/h with the accelerator flat to the floor and the fuel gauge started to plummet. Only way we could make headway was at 75 km/h in 3rd gear, so we decided to take a break from driving.
 
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