WRX odometer readings switching between 205/55/16 -> 225/50/16? Need explaination.
03-04-2004, 09:53 PM
#16
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Sorry, I didn't finish reading monkey's post. He is right about the rotational enertia. It was something I hadn't taken into account. More mass on the outside of the rim means a higher rotational enertia making the engine use more gas to accelerate the car from a dead stop. But once the HEAVIER wheel gets moving, its own enertia will help the engine accelerate it easier and it will sustain speeds easier. Kind of like a heaveir flywheel will seem like it is letting your engine maitain a speed as you lift off the throttle.
When people put really light increased diameter wheels on their cars they are taking away from the car's rotational enertia in the wheels and making it easier for the engine to roll the wheels off of a stop but reducing their acceleration.
When people put really light increased diameter wheels on their cars they are taking away from the car's rotational enertia in the wheels and making it easier for the engine to roll the wheels off of a stop but reducing their acceleration.
03-04-2004, 10:05 PM
#17
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Originally posted by rustokman
Friction does not depend on surface area. The formula for friction is Force of friction(static or kinetic) = Normal force (the force that is pushing againts the wheels opposite the gravity force, newton's law) * Mu (frictoin coefficient).
F = (mu) *(Fn)
Wider tires provide more of a distribution of toque placed on the tire by the road surface. Surface area of contact patch has nothing to do with the friction. It is his odometer that is off and the new tires are bigger and so they weigh more and the car expends more fuel to spin them.
Don't mean to be snotty here, I am majoring in Mechanical engineering and minoring in physics and math.
Friction does not depend on surface area. The formula for friction is Force of friction(static or kinetic) = Normal force (the force that is pushing againts the wheels opposite the gravity force, newton's law) * Mu (frictoin coefficient).
F = (mu) *(Fn)
Wider tires provide more of a distribution of toque placed on the tire by the road surface. Surface area of contact patch has nothing to do with the friction. It is his odometer that is off and the new tires are bigger and so they weigh more and the car expends more fuel to spin them.
Don't mean to be snotty here, I am majoring in Mechanical engineering and minoring in physics and math.
P.S., I am also an engineering major.
03-05-2004, 03:55 AM
#18
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Drat, okay to make it a complicated story short.
From switching from 205/55/16 -> 225/50/16, I did the following:
1) Reduced my fuel efficiency
2) Messed up my odometer readings. My odometer is showing more miles travelled than what it should be normally
Correct Me here at anytime guys
The reason for me to change from 205/55/16 -> 225/50/16 is I would get better grip without sacrificing too much, at least that is the general overall feeling Im getting here on the i-club board.
Below are somee specs:
Diameter for a RE92 Bridgestone Potenza 205/55/16: is 24.88
Diameter for a Yokohama db2 225/50/16: is 24.86
So your saying that with a - 0.02 difference in diameter (In the yokohama tires Im using now) is making my car "drink" extra 50 - 80 miles traveled? (assuming I can trust my odometer)
Because here on the boards, i guess Im getting conflicting information then. I was led to believe that my odometer readings would be barely effected with the change because the overall diameter was roughly the same. Not a 50 - 80 mile distance travel drop in efficiency.
From switching from 205/55/16 -> 225/50/16, I did the following:
1) Reduced my fuel efficiency
2) Messed up my odometer readings. My odometer is showing more miles travelled than what it should be normally
Correct Me here at anytime guys
The reason for me to change from 205/55/16 -> 225/50/16 is I would get better grip without sacrificing too much, at least that is the general overall feeling Im getting here on the i-club board.
Below are somee specs:
Diameter for a RE92 Bridgestone Potenza 205/55/16: is 24.88
Diameter for a Yokohama db2 225/50/16: is 24.86
So your saying that with a - 0.02 difference in diameter (In the yokohama tires Im using now) is making my car "drink" extra 50 - 80 miles traveled? (assuming I can trust my odometer)
Because here on the boards, i guess Im getting conflicting information then. I was led to believe that my odometer readings would be barely effected with the change because the overall diameter was roughly the same. Not a 50 - 80 mile distance travel drop in efficiency.
Last edited by useful; 03-05-2004 at 03:59 AM.
03-05-2004, 04:18 AM
#19
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It doesn't sound like anything is wrong with your odometer, your fuel efficiency is slightly lower. If it says you went 250 miles on one tank you went 250 miles.
rustokman is not using the friction formula correctly, like ban suvs says. Another example: if your tires aren't fully inflated, you get lower gas mileage. Why? Because of increased contact patch and consequently increased friction.
And as long as we're listing qualifications, i'm an ME as well and I finished my degree.
rustokman is not using the friction formula correctly, like ban suvs says. Another example: if your tires aren't fully inflated, you get lower gas mileage. Why? Because of increased contact patch and consequently increased friction.
And as long as we're listing qualifications, i'm an ME as well and I finished my degree.
03-05-2004, 04:52 AM
#20
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With that small of a change in rolling diameter, I'm nearly positive there are other factors at work. How many tanks on your new setup? Exact same city/freeway mileage split? No side trips for a little excercise for those brakes? Also, this is the time of year some areas switch from "winter gas" to "summer gas" formulas and it takes some time for the ECU to adapt. the extra rolling resistance is certainly one factor, but I don't buy it causing a 5 mpg reduction either.
03-05-2004, 08:56 AM
#21
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Originally posted by rustokman
Don't mean to be snotty here, I am majoring in Mechanical engineering and minoring in physics and math.
Don't mean to be snotty here, I am majoring in Mechanical engineering and minoring in physics and math.
oh and i have a degree in mechanical engineering so i think i beat you if you want to look at it that way.
the size tire he has on there is the same overall diameter as before, just wider. his odometer is not off. only way the odometer is off is if there is a difference in diameters
03-05-2004, 11:31 AM
#23
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Okay! The general consenous is the diameter of the 225/50/16 is about the same as the 205/55/16. So my odometer isn't off. Good! Clarification for everyone who reads this thread. ^_^
Well, I dont have any hard data on tanks on the pseudo-stock setup right now. What I can tell you is this:
On 205/55/16 Potenza RE92's, I filled up the gas tank on Sunday then filled it again on Thursday. Usually get to 300 miles by then.
On these 225/50/16 Yokohama db2's, I filled up the gas tank on Sunday, then filled it up again on Wednesday. Usually get to 250 by then.
My braking style hasn't changed for a while. This brake setup i currently have is the same ones I've used on my RE92's. I gotten roughly around 300 - 330 miles using the RE92s and this brake setup. So, im positive it has nothing to do with my braking setup. (though who knows with yokohama db2 sticker tires)
It slightly urks me to know that by switching to a slightly wider tire on a stock setup (2002 WRX stock, no power or suspension mods), that I'm losing 50 miles of travel time.
With that small of a change in rolling diameter, I'm nearly positive there are other factors at work. How many tanks on your new setup? Exact same city/freeway mileage split? No side trips for a little excercise for those brakes? Also, this is the time of year some areas switch from "winter gas" to "summer gas" formulas and it takes some time for the ECU to adapt. the extra rolling resistance is certainly one factor, but I don't buy it causing a 5 mpg reduction either.
Well, I dont have any hard data on tanks on the pseudo-stock setup right now. What I can tell you is this:
On 205/55/16 Potenza RE92's, I filled up the gas tank on Sunday then filled it again on Thursday. Usually get to 300 miles by then.
On these 225/50/16 Yokohama db2's, I filled up the gas tank on Sunday, then filled it up again on Wednesday. Usually get to 250 by then.
My braking style hasn't changed for a while. This brake setup i currently have is the same ones I've used on my RE92's. I gotten roughly around 300 - 330 miles using the RE92s and this brake setup. So, im positive it has nothing to do with my braking setup. (though who knows with yokohama db2 sticker tires)
It slightly urks me to know that by switching to a slightly wider tire on a stock setup (2002 WRX stock, no power or suspension mods), that I'm losing 50 miles of travel time.
03-05-2004, 12:17 PM
#24
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I think you need more data. The Subaru gas gauge is not a precision mechanism -- my yellow light comes on at any time between 300 and 330 miles, and can come on as often as 270. You need to compare ten full tanks before and ten full tanks afterwards before you can even begin to correct for the margin of error.
03-05-2004, 12:38 PM
#25
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ME degrees aside, common "mechanically inclined" sense applies here. Here are some thoughts:
The width of the tire changes most the shape of the footprint, generally making it wider and shorter. The actual contact patch has about the same square footage (if you will) which has little to no inherent effect on rolling resistance - although it can (and likely will) be argued as to how much "little to no" refers to.
The resistance to rolling is influenced by the deflection of the tire, sidewall integrity, and rubber compound. You could also argue wind resistance, but we'll leave that off.
Deflection plays a roll because of a number of things; especially (but not limited to) the resultant outer diameter. The deflection influences the true outer diameter of the tire, which plays into the final gear ratio which can fall outside of a car's "optimal" ratio for best gas mileage.
Also is the fact that stickier tires actually adhere to the road better - like using double-stick tape on the tires (to exaggerate the point). This could account for a decrease in gas mileage.
As well, the deflection will affect it in terms of rolling resistance because of sidewall composition. Some tires get hot in the sidewalls because of internal friction from deflection. This heat generated has stolen energy from somewhere. Adding air pressure can help remedy this, but at the cost of smoothness.
My guess is that the 5mpg reduction is due to your change in driving/acceleration habits due to increased confidence in your tires.
The width of the tire changes most the shape of the footprint, generally making it wider and shorter. The actual contact patch has about the same square footage (if you will) which has little to no inherent effect on rolling resistance - although it can (and likely will) be argued as to how much "little to no" refers to.
The resistance to rolling is influenced by the deflection of the tire, sidewall integrity, and rubber compound. You could also argue wind resistance, but we'll leave that off.
Deflection plays a roll because of a number of things; especially (but not limited to) the resultant outer diameter. The deflection influences the true outer diameter of the tire, which plays into the final gear ratio which can fall outside of a car's "optimal" ratio for best gas mileage.
Also is the fact that stickier tires actually adhere to the road better - like using double-stick tape on the tires (to exaggerate the point). This could account for a decrease in gas mileage.
As well, the deflection will affect it in terms of rolling resistance because of sidewall composition. Some tires get hot in the sidewalls because of internal friction from deflection. This heat generated has stolen energy from somewhere. Adding air pressure can help remedy this, but at the cost of smoothness.
My guess is that the 5mpg reduction is due to your change in driving/acceleration habits due to increased confidence in your tires.
03-05-2004, 04:50 PM
#26
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Originally posted by wrx ish
listen to ban suvs, he is correct.
oh and i have a degree in mechanical engineering so i think i beat you if you want to look at it that way.
the size tire he has on there is the same overall diameter as before, just wider. his odometer is not off. only way the odometer is off is if there is a difference in diameters
listen to ban suvs, he is correct.
oh and i have a degree in mechanical engineering so i think i beat you if you want to look at it that way.
the size tire he has on there is the same overall diameter as before, just wider. his odometer is not off. only way the odometer is off is if there is a difference in diameters
I said that only to get some credibility to my name so that people wouldn't think i am coming out of nowhere with these statements. Like I said I do not intend to sound snotty and I am not questioning anyone elses credibility. Ban Suvs, I respect your input on everything and I don't mean to insult you if I did. Same goes to you WRX ish.
03-06-2004, 01:53 AM
#28
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You didn't come off snotty or superior at all rustokman. The purpose of this board is to share information, and if you think you know a better answer to a question you are welcomed to submit it. At worst, giving an incorrect answer to a question just spurs a more thorough discussion of what's 'correct.' No harm no foul. 
03-31-2005, 04:51 PM
#29
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Rehashing old stuff
Just wanted an opinion about this same argument. What about lighter weight larger rims, but still maintaining the stock outside diameter. For example: 18x8 in SSR Professors. With a tire that gets as close to stock outside diameter as possible. Will there be a reduction in fuel mileage or gain ? Or none of the above ? My thoughts were that due to lighter weight it takes less to get it all rolling. And with a reduction of unspung weight, that should allow the car to move more freely, still allowing the engine to work alil less. Am I correct in my assumptions ? I only ask because i commute about 70 miles a day, and I dont want to have to shell out more money for gas just because I want larger than stock wheels.
Thanks
Joseph
Thanks
Joseph
03-31-2005, 04:59 PM
#30
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You would see a slight reduction in ,ilage with the lighter wheels if you kept very close to the stock 24.9" OD. However, the fuel saved wouldn't come within 4 figures of saving more than you spent on the wheels. Also, the correct size tire is 225/40R18, which will have a bigger contact patch. Even if you don't use sticky tires, you're still going to have an increase in rolling resistance and a slight penalty in mileage.
Also, the correct size tire is 225/40R18, which will have a bigger contact patch. Even if you don't use sticky tires, you're still going to have an increase in rolling resistance and a slight penalty in mileage.