akirby Posted July 2, 2008 Report Share Posted July 2, 2008 Tire size has an effect on the finally gear ratio to the road. Smaller tires give more mechanical advantage vs. larger tires. Larger diameter tires (has nothing to do with the wheel size) cause the engine to run at a lower RPM than smaller tires at a given speed. Therefore, everything else being equal (rolling resistance, weight, etc.) a larger tire should yield better FE than a smaller tire. However, I don't think the difference would be noticeable under most conditions. The point was that smaller tires won't improve fuel economy and could actually decrease it. Quote Link to comment Share on other sites More sharing options...
cbEdge Posted July 2, 2008 Report Share Posted July 2, 2008 I thought that for all things being equal .... engine, transmission, etc... that 3 things effect MPG the most: 1) Driving Style 2) Weight 3) Friction With #3 being effected by the tire size, there would seem to be no physical way a wider tire could get better MPG then one that was narrower. Quote Link to comment Share on other sites More sharing options...
akirby Posted July 2, 2008 Report Share Posted July 2, 2008 I thought that for all things being equal .... engine, transmission, etc... that 3 things effect MPG the most: 1) Driving Style 2) Weight 3) Friction With #3 being effected by the tire size, there would seem to be no physical way a wider tire could get better MPG then one that was narrower. Wider? If you notice the tire sizes above, they go from 235 to 245 so you're only gaining 10 mm of width (4.2%) going from the 17 to the 18 or 20 inch wheels. That is not enough to make a noticeable difference in fuel economy. In fact the tire compound and tire pressure have WAY more impact than 10 mm would. Most of the friction comes from the air and trying to push a brick through it rather than from the tires. That's why the mileage drops so much when you go from 65 to 75 or 85 - takes a lot of power to push that air out of the way. Quote Link to comment Share on other sites More sharing options...
Fifth_Ford Posted July 8, 2008 Report Share Posted July 8, 2008 I have been reading a lot of topics concerning MPG, but I don't see any mention of the average speed of the vehicle (nifty little feature on your computer). The average speed is what really sets the MPG (ie. city stop and go vs Highway). THere is obviously going to be differences between people depending on how fast they are really going and how long they remain idle for (ie lights and traffic). The computer keeps track of this. Here are my latest stats: City @ an average speed of 18-21 miles/hr I am getting 12-14 MPG Highway @ and average speed of 55-62 miles/hr I am getting 21-23 MPG. Quote Link to comment Share on other sites More sharing options...
McEdge Posted July 9, 2008 Report Share Posted July 9, 2008 I have been reading a lot of topics concerning MPG, but I don't see any mention of the average speed of the vehicle (nifty little feature on your computer). The average speed is what really sets the MPG (ie. city stop and go vs Highway). THere is obviously going to be differences between people depending on how fast they are really going and how long they remain idle for (ie lights and traffic). The computer keeps track of this. Here are my latest stats: City @ an average speed of 18-21 miles/hr I am getting 12-14 MPG Highway @ and average speed of 55-62 miles/hr I am getting 21-23 MPG. Matches my stats. I have a AWD SEL+ with 10,000 Miles. Somewhat annoyed my highway is 21-22, but I have AWD and when measured, also had A/C on Auto. So, it makes sense. Quote Link to comment Share on other sites More sharing options...
thehume Posted July 10, 2008 Report Share Posted July 10, 2008 (edited) Wider? If you notice the tire sizes above, they go from 235 to 245 so you're only gaining 10 mm of width (4.2%) going from the 17 to the 18 or 20 inch wheels. That is not enough to make a noticeable difference in fuel economy. In fact the tire compound and tire pressure have WAY more impact than 10 mm would. Most of the friction comes from the air and trying to push a brick through it rather than from the tires. That's why the mileage drops so much when you go from 65 to 75 or 85 - takes a lot of power to push that air out of the way. The extra weight with more rubber and more metal on the rims X 4 has got to factor into fuel economy with larger tires. Edited July 10, 2008 by thehume Quote Link to comment Share on other sites More sharing options...
akirby Posted July 10, 2008 Report Share Posted July 10, 2008 The extra weight with more rubber and more metal on the rims X 4 has got to factor into fuel economy with larger tires. It can but I thought the other poster was specifically referring to the extra width only. Quote Link to comment Share on other sites More sharing options...
Fifth_Ford Posted July 10, 2008 Report Share Posted July 10, 2008 Matches my stats. I have a AWD SEL+ with 10,000 Miles. Somewhat annoyed my highway is 21-22, but I have AWD and when measured, also had A/C on Auto. So, it makes sense. I have the same vehicle (SEL AWD). Good to hear you have the same stats. Couldn't believe the difference between city and highway. Thought it was just my car Quote Link to comment Share on other sites More sharing options...
polak_slawcio Posted January 8, 2010 Report Share Posted January 8, 2010 I go to zero miles to empty almost every tank, just cuz I hate going to the gas station and standing in the cold........Counter-intuitive I know It does not go negative. I drove about 5 miles once when it said 0 miles to empty and even then the most I have filled up with was 19.7 gallons. Start saving some $$ for a new fuel fump if you keep this up I would hate to the next owner of your car.... Quote Link to comment Share on other sites More sharing options...
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