When I was on 16x7 +42 205/60 I was getting 42mpg to gallon on my trip to school. Now on 17x8 +35 235/45 I get 33 mpg.... Anyone have any solution for me? Is the computer on the car off? I bought the car so I could get ridiculous gas mileage, I love the new rims more than I love my GF, and would hate to have to take them off.

 

first make sure your GF doesn't see this post...

then id start with tire pressure!!

 

Lol that's a harsh post
as for your mpg this is the effect of various problems.
Those 17in rims r bigger and wider unless u shelled out serious cash they r probly heavy. The tire size also effects this u should of gone with 235/40-17

added weight to the wheels sucks.

 

Wheel weight is probably a large part of it... tire pressure might also be an issue. Not to mention the speedo is going to be a tiny bit off - so that could affect mpg readings - not sure how much though.

 

He actually went down in overall diameter, but only by approx 0.3 inches. Would cause the speedo to read just a tad faster than you are actually going.. which in reality would make it seem like mileage is better, not worse. However, the speedo is only off by less than 1 mph at 60mph (aka 2% at 60mph) so not much of an effect there.

As far as the rest, wheel weight affects rotational inertia. This has zero net effect at cruising speed, but affects how hard it is to change wheel speed (accelerate or decelerate). So if they are a lot heavier, then that can be part of it.

You also are running a lot wider tire, and possibly tire pressures that are off, which can both lead to more friction, meaning lower efficiency.

But 9mpg I would not see being 100% due to the changes you mentioned.

 

I have about 35psi in the tires, and to be honest, the new wheels are lighter lol

 

tire compound can play a big difference in mpg. That along with new tires that are not yet broken in. I think the stock tires were made for mpg (hard compound) and not performance.

 

That should not account for 9mpg, so something else is up.

How are you measuring fuel economy? It should be like this:

1) Fill up the car and record mileage
2) When you fill up again, record mileage
3) Divide miles traveled by the amount it took to fill up in step 2

Average that over multiple tanks and you have an idea of your fuel efficiency. Even doing it over 1 tank tells you very little, since it can fluctuate a lot from tank to tank.

 

It could just be that it's colder than it usually is Flo-Rida. Last winter it was 20 degrees colder than a usual Chicago winter. My Summer to Winter Ratio was like 37 MPG - 27 MPG.

**Edit - I want to add that I have remote start and burn a lot of gas in the winter **

 

You sure the wheels are lighter?

 

Even if the wheels are lighter, you have more rotational mass (extra width) at a diameter that is farther out. It requires extra rotational force. Imagine spinning a stone on a foot long string, and on a 10 foot long string. Which takes more effort?

You may also have stickier tires. Hard all season grand tourings, for example, will give you better mileage than some stickier summer rubber because there is less friction because they don't grip as well.

I think it's bigger tires+ wider wheels, with the forces at work there that are doing it.

Maybe you have changed your driving style too? I upgraded wheels and tires and took corners faster and hooked up from lights better and I would find it more exciting, which obviously affected my mpg.

 

But if the rim is a larger diameter and he is running a lower profile tire, there may not be any more rotational mass concentrated further from center than before. Tires tend to weigh as much or more than the rim, so he would have effectively removed some mass in the tire while moving the outer rim portion further out, hence having some sort of net cancelling effect.

But, the wider tires definitely play into it. So that also ADDS more weight to some degree.

The overall net effect of less sidewall + more tread width + larger rim diameter, will tell you what the overall inertial effect is.

Keep in mind though that the inertial effect ONLY hits efficiency when you are speeding up or slowing down. At cruising speed, the weight of the wheel and tire have no effect on how much fuel you burn or how much power is needed to keep the car moving.

 

I agree, chances are you've significantly increased the weight of your wheels (rim+tire) and so it takes more energy both to accelerate and to brake. That translates into more out-of-pocket cost.

 

From what I've read, aftermarket rims typically weigh more than OE, not less. So although the looks may be improved, typically, fuel economy goes south. As far as inertia is concerned, again from what I've read, taller rims tend to have higher inertia, again cutting into fuel economy. The point of people tending to be heavier on the throttle after making mods is also 100% valid. And there's nothing like being heavier on the throttle to eat into gas mileage.

 

Not sure if you are agreeing with me or not really

My point is that inertial changes have zero effect at cruising speed. You are correct though that they do have an effect, but like everything else, that effect will be worsened in stop and go traffic and will almost go away on long, highway cruises. At a constant speed, it takes no more energy to sustain that speed with heavier wheels than it does lighter ones.

Most aftermarket rims I have seen are lighter than stock or at least the same weight, given the same size rim. But, if someone buys cheaper rims or larger ones, then of course weight will suffer. I take a different perspective since I would never buy rims for my car that increased unsprung weight, but that is just me.

And like I said above, you have to take into account the weight of the tire and rim. An RE-11 in a 225/45/17 for example, weighs in at about 25 lbs. If you were to run a smaller, lower profile on a new rim, you would potentially lower that. Not by much (maybe a lb or two), but if the rims weighed only slightly more, then you would negate the inertial changes as long as overall total diameter is the same.