useful

I have a bone stock (well except for calipers, rotors, and pads)
2002 WRX.

Anyways, I've noticed that when driving on my WRX with my old RE92 Bridgestone Potenza 205/55/16 tires and stock wheels.
I was getting between 300 - 330 miles (according to my odometer) per full tank of gas. I usually get the empty light pop on at 300 miles.


2 weeks ago, I switched out to 225/50/16 tires with the same wrx wheels. The tires on these are the Yokohama db2's. What im noticing is that on a full tank of gas, I am getting 250 miles (according to the odometer), before my warning gas light appears.

So basically, I am seeing a roughly between a 50 and 80 mile difference on the odometer each time I gas up. Is this normal?

I mean, thats quite a difference from what I get on the stock tires.
Would sticker tires and a (slightly) wider tire base give me such a big difference in odometer readings?


Thanks

- Ulysses

ish

iTrader: (7)

its not the odometer that is off. a wider tire reduces your gas mileage. you still have the same outer diameter so speedometer and odometer are not affected. you are just getting less miles per tank

04blkWRX

Here's a cool comparison utility: http://www.discounttiredirect.com/di...foTireMath.jsp

The difference in your mileage is due to increased friction with the road due to larger contact surface, that's all.

useful

Thanks, I just wanted to be sure.

I would have never have though that my gas consumption would change so much from a simple tire change. 50 miles goes a long way now with gas for 91 roughly around 2.30 - 2.50 a gallon right now.

Much appreciated.

- Ulysses

Sea Dragon Rex

Anybody got 185/60/16's I can use to commute with! I figure that will get me about 400 miles per tank!

j/k

ish

iTrader: (7)

thats a good idea

theworx

2.30 - 2.5 / gallon?!?!?!? Holy **** I'm glad I don't live in California. I bought 94 over the weekend in Ohio for 1.70/G.

useful

Quote:

thats a good idea

heh... wider you go the more gas you go..... wonder if i should invest in ice rally tires for the street.

Kevin M

iTrader: (12)

Die.


Yes, I live in California.

rustokman

I too live in Cali where i paid 2.47 for 91 octane....it sucks.

Oh yeah...wouldn't you want 225/45/16 if you went with 225 instead of 225/50/16. Just a thought. That would reduce the diameter back to stock and it would be more or less even. I thikn he is riding a slightly larger diameter making his odometer think he is driving slower and going less distance.

meilers

It is most definitely his odometer that is off, not increased friction from wider tires -- that's just nutty. His power transfer to the ground is *more* efficient than before, not less. A larger diameter tire will take slightly more power to turn, which might account for 25 of those missing miles, but at least 25 of those miles is still just having a tire with a larger diameter than the odometer is calibrated for.

Kevin M

iTrader: (12)

Changing tires on any car has 2 effects.

Effect 1: Increased or decreased rolling friction. A bigger contact patch combined with stickier tires means a reduction in the *actual* fuel efficiency of any vehicle.

Effect 2: Increased or decreased rolling diameter. A larger diameter tire will cause the odometer to roll slower and the speedometer to read low. A smaller tire will do the opposite- the speedometer will read high and the odometer will roll faster, making it appear as if mileage has improved.

These effects can cancel each other out, or build upon one another. The only way to know for sure is to get your odometer corrected for your tire size.

meilers

I'm curious about this. Gas is combusted to create the energy which you then transfer to the ground through friction to gain forward momentum. If you increase the efficiency of this transfer (by increasing the amount of friction) wouldn't that reduce energy lost to heat and slippage? Obviously, if you DECREASED the friction of the system (for instance, put on tires with a very hard compound and little tread) the wheels would turn more often as you slid around and you would also be wasting energy and reducing efficiency of fuel combustion... Obviously there is a cycle of diminishing returns, or we would all be driving around on glass tires!

Hmm. Perhaps a stickier tire takes more energy to pull the trailing edge of the contact patch off the ground? I'd think you would recover that extra energy "expense" in taking off from a standstill more easily and having to brake less.

I don't doubt what you're saying, BAN, I'm just curious about the actual mechanism that makes it happen. I find it tough to belive that when I switch to 225/45 tires I'm suddengly going to be cutting my fuel efficency by almost 5 MPG, from 22 to 17mpg.

Kevin M

iTrader: (12)

When your tire is rolling, it is constantly stretching and deforming around the contact patch. the stiffer the sidewall, the larger the contact patch, and the stickier the tread compiound, the more energy it takes to do all that. That's energy not being used to move the car.

Also, since even the crappiest tires are not slipping at all during normal driving, the increased efficiency hypothesis doesn't stand up. If it takes less than 100% of a tire's friction coefficient to move the car, it doesn't matter if it's 10% or 99%.

However, these losses are minimal. At most, 1 mpg on average, and it's unlikely that you will drive *exactly* the way you did on hte old tires. Our brains are just too good at adjusting to small changes like that and adapting to them. But, the majority of mileage changes come from the tire diameter, and something that hasn't been discussed yet, which is rotational inertia. Bigger wheels and smaller profile tires weigh more, and the effect is not linear as far as weight gained.

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.