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Old 06-08-2008, 11:47 AM   #1 (permalink)
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Larger diameter tires = better FE?

I drive a N/A 4EAT Subaru Forester and love it. Now I am about to move to a different city and need to commute about 30Km to work and back every day. My car feels like a little powerhouse and on the highway the only thing that annoys me is that it downshifts too readily when passing, when it would probably be quicker and more efficient to just stay in 4th.

Since I've owned this car it has been making steady improvements in FE, some of it due to my changing my driving habits based on knowledge I gathered from this website and others.

And now let's get to the point. Since this engine feels pretty strong and effortless I have a feeling that if I go with slightly bigger tires so it can rev lower I'll be able to squeeze a few extra MPG out of it, especially on longer trips. What do you guys think?

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Old 06-08-2008, 12:18 PM   #2 (permalink)
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Don't go too big now. Remember that wheel weight (this includes tire weight) count two times as normal weight. Also bigger diameter will mean that your car will be sitting higher up in the air and you don't want to destroy your aerodynamics.
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Old 06-08-2008, 12:29 PM   #3 (permalink)
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Quote:
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Don't go too big now. Remember that wheel weight (this includes tire weight) count two times as normal weight. Also bigger diameter will mean that your car will be sitting higher up in the air and you don't want to destroy your aerodynamics.
What aerodynamics!?

This is a bit of a brick. I'm not even worried about that at this stage. I just feels that the car needs longer gears because the engine is powerful enough that it will rev lower, especially in cruise. Someone recommended that I just get the larger diameter tires instead of doing expensive transmission work (IF at all possible). I figure that a larger diameter in a low rolling resistance tire aired up to the max recommended on the sidewall would probably work out pretty well, however I don't feel that I know enough to be able to make that decision without some advice. Also, the tires are a pretty expensive proposition as well so I thought I better check with you guys before committing.
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Old 06-08-2008, 01:01 PM   #4 (permalink)
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I think the mod will help very much. Reducing revs will help you more than most of us because you're driving 4 wheels through 3 diffs!
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Old 06-08-2008, 02:02 PM   #5 (permalink)
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Literally hundreds of pickup drivers report that going to bigger tires makes their MPG go down.

The rotational moment of inertia goes up with the square if outside diameter increase. that makes you accelerate four bigger "flywheels" as you leave every stop light.

If you want to slow the engine down - a valid fuel economy practice - slow down or regear.
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Old 06-08-2008, 03:12 PM   #6 (permalink)
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Quote:
Originally Posted by Big Dave View Post
Literally hundreds of pickup drivers report that going to bigger tires makes their MPG go down.

The rotational moment of inertia goes up with the square if outside diameter increase. that makes you accelerate four bigger "flywheels" as you leave every stop light.

If you want to slow the engine down - a valid fuel economy practice - slow down or regear.
Have those hundreds of pickup drivers adjusted for the offset odometer reading of the distance travelled?

Also, outside of stop and go traffic, wouldn't those larger flywheels be beneficial, like on the highway?
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Old 06-08-2008, 04:38 PM   #7 (permalink)
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This topic is currently being discussed here:

http://ecomodder.com/forum/showthrea...ires-2827.html
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Old 06-08-2008, 05:49 PM   #8 (permalink)
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bigger tires make for lower revs at actual MPH, make sure to adjust 1 you speed acordingly and 2 when you calculate tyou MPG adjust for the added tire size...

keep them narrow as stock or narrower, wider makes more rolling resistance.. this is where trucks get MPG drops they usually go from a 8" wide tire up to a 10" or 12" wide tire.. thats where they get nailed.. and many don't think about the added diameter and just go by there odometer..

some of there trucks tires are 20% bigger than stock se for ever mile your odometer says you went you actually went 1.2 miles.
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Old 06-08-2008, 06:51 PM   #9 (permalink)
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my dad switched to larger tires on his isuzu trooper, drove back and forth to work (the same as he had always done) and had an increase in mpg. same mileage and less fuel used. scientific enough to know it was better, now dont ask me how much of an increase exactly because that was as scientific as it was.
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Old 06-08-2008, 08:56 PM   #10 (permalink)
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Quote:
Literally hundreds of pickup drivers report that going to bigger tires makes their MPG go down.
That's because truck guys go bigger and fatter, not just bigger. We're not talking about switching to swampers here. His city mpg might suffer but his hwy mpg will improve.

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