i just threw on a set of 18's on my xa and noticed the gas mileage has gone down dramatically. i used to get about 330 miles to the tank. now i'm getting about 280. i knew i would lose some mileage, but that seems really excessive to me. anyone else experience this or could there be a problem i'm not aware of?

 

You're all lucky. I live in Florida, city driving, AC and pedal to the metal. Around 25 MPG.

 

I don't know what your area has been like but when mine goes down even though I don't think I use it all that much, my boyfriend reminds me of when I have been using my A/C.

Maybe you were using it more on this last tank than you normally were...

 

Here's a little tidbit for you...

I'm running 17's, and with 215/45/17 tires I calculated a 3.76% increase, so the miles traveled are approx. 3.76% more than what your odometer reads. Your 18's would generate higher numbers, but you'll have to do the math yourself w/your tire size.

Sure, it's a small number, but at least it's in your favor.

 

You need to see how much more each tire weighs. Factor that in to the increased contact patch, assuming there is one...hmm...I forget how to calculate energy needed to overcome the frition Well, it's high school physics so it shouldn't be too hard to find the formulas on the Net somewhere. That should give you a a start. The frictional coeffcient will vary depending on what surface you're driving on, but assuming either concrete or asphalt should give you a pretty reasonable theorectical figure of how much more energy it requires to move the 18's. I'd suggest measuring the contact patch of the tires to give you a more accurate figure.

 

well i aint complaining about my mpg my last tank was around 30 mpg and it still costs me under 20 dollars every fort night

 

i drive about 300 miles per week so i want to make sure my mileage is on par. the 18's are adr's and weigh a ton. i think they're 7.5 across. again, i just hate to see it go down 50 miles a tank. a definite ouch. i wonder if there are any mods to make the car more fuel efficient?

 

Going from a 15" to an 18" tire will change everything. When your spedo says you're doing 50mph, you could really be doing 60mph

 

Here's a tire calculator I found for you.

It, along with some basic math, should allow you to figure out exactly how many miles you REALLY went the last time you figured out your MPG.

http://www.miata.net/garage/tirecalc.html

 

You have no idea what your talking about......

With 215/35/R18 tires and TRD wheels, Im still averaging 33 MPG....

Also, stock xA/xB tires are 24.1 inches tall, and guess what- with 18" wheels, you still get 24.1 inches tall tires with 215/35/R18 tires.......

This is a common misconception.......

Do you think Toyota would offer 18" TRD wheels if it would throw off the speedo? - I dont think so......

 

I have some idea

And that's why I said "could" in my first post and included the specific calculator that I did in my 2nd post.

I didn't link any of the many calculators that only ask for # of inches, I linked the one that takes into consideration exactly what tire you're using, including the width, aspect ratio and the diameter because it can accurately determine what difference the new tires will make.

Perhaps I shouldn't have said it "will change everything" and instead should have said "could change everything", but I hardly think that I "have no idea" what I'm talking about.

 

spoken like a true gentleman.

i tanked up yesterday so i'm going to see what i average this week. maybe i just got a bad batch of fuel last time. the gas station station i used last time is sketchy at best.

 

I know I know, "Mileage may vary." But if it's a difference of 32 mpg in city (which the manual is rated at) and a real-world 25 mpg, isn't that a bit fraudulent? I mean, that's off by 22%!!

 

Even though I normally get around 25, I can get 30 if I drive it like an old lady. (No offense, old ladies)

Johncal

 

How's that going to happen the circumference of the tires will be the same, If you are changing the circumfrence of the tire then you would be changing the speedo

 

Yes yes yes. We've been through all this. As I said previously, I did say "could" in the following sentence and the post I followed it up with included a link to a calculator that takes into account all aspects of the new tires to determine what difference they might make.

I'm sorry that my first sentence says "will change everything" and not "could change everything" which was my intended meaning.

Why isn't anybody picking on sakwt01 who wrote about his 17s and this 3.76% increase and then said "Your 18's would generate higher numbers"? Don't you wanto to give him a hard time for not saying "could generate higher numbers"?

I hope you can forgive my typing transgressions but, if nothing else, I'm glad my post could bring a smile to your face

No everyone move along... nothing to see here

 

True, the size and weight may be the same (you'll have to check the details of each wheel/tire combo for that info) -- something that is largely not taken into consideration, and rightfully so due to the math involved in the finding the answer: The weight distribution on a TRD wheel will be different than the steel stock wheels that you've replaced.

The weight on a steel tire is mostly at the center (closer to the axle) and gets lighter as you move outward (toward the tire), thus requiring less force (and less fuel) to get the tire to make one full rotation. Conversely, if you move the weight to the outer edge (as most upgraded wheels have) it requires significantly more force to turn the wheel the same rotation.

Think of an ice skater doing one of those insane spins around and around -- as their weight is centered they spin more quickly, and then as they move their arms (weight) outward the spin slows down. It is the same thing here -- more force is required to move the same weight when the distribution shifts. Maybe it isn't the best analogy, but the best i could think of right now.

Another factor that may have some impact is the increased friction of a wider tire (and different tred) -- increased road friction could compound the impact of shifting wheel weight distrobution.

Bottom line -- you get fewer mpg w/ your new tire/wheel setup... which is often the price we pay for a finer looking car.

 

I agree

 

Well easy mods to increase mileage would be lightweight wheels, but lightweight wheels cost serious money. Not sure who make wheels again, SSR (They were bought by Tanabe after bankruptcy earlier this year.). For 18s I think you're looking at $2k and up. But then again if you sell the ADRs that will offset the cost somewhat.

I just got 17s and I want to see how I do on this tank. If I lose a lot of mileage I may opt to put the 15s back on and sell the 17s and save up and order some 16" SSR Competitions from tirerack.com. Those wheels weigh about 9 lbs each. Anyone know how much the 15s weigh? I'd like to know what the baseline wheel weight is.

Also a less sticky tire compound would increase your fuel economy, although I tend to think I don't think this will be significant, but I could be wrong.

 

my mileage sucks soooooo hard. i'm getting just over 250 miles a tank. what the hell? my next mod will be anything to give me better mileage. carbon fiber hood and some drilled rotors to reduce weight should help up the mileage. no alterior motive whatsoever there.