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Old 02-26-2006, 11:11 AM   #1
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Why does running one size bigger tires increase fuel economy?

I remember this from way back in the day, but I still dunno why this would help? Can anyone tell me? I know larry is running this so mehbe he'll chime in.
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Old 02-26-2006, 11:47 AM   #2
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i asked a similar question a

i asked a similar question a while back, relating to swapping to a "taller" transmission. the concept is the same.

http://www.gassavers.org/forum_topic/transmission_swapping_for_mpg.html

if you put a taller tire on the car, it has the same effect: lower engine RPM for a given road speed = reduced losses to internal friction (engine, transaxle).

you just have to make sure you have enough torque to compensate for the taller tranny/tire. (for most cars this won't be an issue; for my car it is an issue)

when changing tire dia., you would also have to correct your speedo/odometer, or compensate manually when doing fuel consumption calculations.

i tried to come up with a way of thinking about this that makes sense intuitively, and this is what i got: are there times you wish you had an additional (higher) gear (e.g. 6th)? if yes, then you could probably benefit from a taller tire/tranny.
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Old 02-26-2006, 11:50 AM   #3
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Hmm. Interesting, still a

Hmm. Interesting, still a little puzzling to me though, but I think I got it.
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Old 02-26-2006, 12:12 PM   #4
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you got me thinking about

you got me thinking about "losses due to internal friction", and wondering about just how large or small they are.

a quick, easy and illuminating data-gathering experiment for someone with an instant FE readout would be to record MPG at a constant speed (say 40 mph), on a consistent grade, in gears 2, 3, 4, & 5.

would this not reveal the magnitude of losses from internal friction at various rpm for that car's engine & transmission?

and then, wouldn't you be able to plot a curve and estimate changes in FE from further reducing rpm with taller tires/tranny?

any takers?

if not, i'll try to remember to do it next time i saddle up the blackfly (in a day or 3 probably).
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Old 02-26-2006, 12:18 PM   #5
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What'd I do to make you

What'd I do to make you think that?

Quote:
would this not reveal the magnitude of losses from internal friction for that car's engine & transmission? wouldn't you be able to plot a graph and estimate further changes in FE from reducing rpm further (with taller tires/tranny)?

any takers?
Dan did his first (with corrections), I think, commute on the superMID m-1 and told me via pm that he hit 56 mpg, which is nice. Also, he said that times when he wanted to be in forth it was so much better to be in 5th, and that despite a bit a lugging he was getting 70-80 mpg in fifth from 35-40 mph.
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Old 02-26-2006, 12:26 PM   #6
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Re: What'd I do to make you

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Originally Posted by SVOboy
times when he wanted to be in forth it was so much better to be in 5th, and that despite a bit a lugging he was getting 70-80 mpg in fifth from 35-40 mph.
i believe that. i've seen similar figures in my car in high gear/low speed situations. (though i haven't yet specifically looked for my "sweet spot" for the absolute best mpg/steady speed combination).

it would be cool to know the relative MPG for the other gears at that speed.
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Old 02-26-2006, 02:12 PM   #7
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So if my stock tire size is

So if my stock tire size is 195/65/15 and now I'm using 205/60/15, how big can I go to get a mpg boost??? My rim is 6 inches wide.
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Old 02-26-2006, 02:21 PM   #8
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http://www.miata.net/garage/tirecalc.html

I came back to this website and I figured the less revs per mile you have the less the tire spins. So my old 195/65/15 tire was better fuel economy if I were to pump it up.

So...
195/65/15 Revs/mi:807

205/60/15 Revs/mi:817 -Current tire

205/65/15 Revs/mi:791 -Clear winner

So would using 205/65/15 give me more of an advantage since it spins less per mile?

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Old 02-26-2006, 03:01 PM   #9
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Re:

Quote:
Originally Posted by Compaq888
http://www.miata.net/garage/tirecalc.html

I came back to this website and I figured the less revs per mile you have the less the tire spins. So my old 195/65/15 tire was better fuel economy if I were to pump it up.

So...
195/65/15 Revs/mi:807

205/60/15 Revs/mi:817 -Current tire

205/65/15 Revs/mi:791 -Clear winner

So would using 205/65/15 give me more of an advantage since it spins less per mile?
Yes, but your odometer wont show that you have travelled farther. You would have to add 2% to your odometer reading when you calculate MPG. That doesn't mean your MPG will increase by 2% however. I wouldnt go to the larger tire size unless you need new tires anyway. If that is the case, you would be better off with 195/70/15 or 195/75/15 if you can find them.
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Old 02-26-2006, 03:06 PM   #10
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Re: What'd I do to make you

Quote:
Originally Posted by SVOboy
Dan did his first (with corrections), I think, commute on the superMID m-1 and told me via pm that he hit 56 mpg, which is nice. Also, he said that times when he wanted to be in forth it was so much better to be in 5th, and that despite a bit a lugging he was getting 70-80 mpg in fifth from 35-40 mph.
I did show 56 MPG but it wasn't my commute. It was a trip to the in laws which is even more FE friendly than my commute. What was really sweet was my trip from the gas station to home yesterday. 67 MPG with the ICE on only 52% of the 3 mile distance. It was downhill so not really fair. Sweeeeet!
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