I recently upgraded my 2003 G35 wheels and tires. It had original OEM setup and I liked the look of the 18" 350Z wheels. I bought them along with some low profile runflats. I may be crazy but since then I have noticed a decrease in fuel economy. Being my daily driver and driving the same route daily I have a feel for the cars rate of gas consumption.

I went from the stock 03' 6-spoke 17" wheels with 225/50-17's all round to 225/40-18 on front and 255/35-18 in the rear. I believe the rears are economy impacting and according to the tire calculator are .83" smaller than the OEM size.

http://www.discounttire.com/dtcs/infoTireMath.dos

Am considering replacing the rears with a taller profile tire that would be closer to OEM diamteter and hopefully restore my original MPG. With only ~2MPH difference@65 MPH seems hard to believe it could bring on the decrease I am seeing. Any thoughts on this?

What about you guys upgrading from 17" to the larger 19"? Do you notice in increase in fuel economy going to a larger diameter wheel/tire assembly than you had previous?

 

taller lowers the rpm at any given speed, but the differences of the tires your mentioning aren't too crazy... however, the 18" wheel is probably heavier, and the run flats are generally heavier tires as well, so the weight difference is probably what your noticing the most.

 

Thts possible - what would you consider excessive? A 5 lb difference or more?

 

hell if I know. wheels vary so much. on my truck when i went from alum. 16's to chromed alum 18's, I lost 2.5 mpg. Some of this was also tire weight and the tire was a smidge taller, but from one combo to the next, that was the loss. i feel most of it was wheel weight. if the extra weight and mpg loss is too much for you than it's excessive, if it's not, too much for you, then it's not excessive.... thats an 'eye of the beholder' deal....

 

Understood - what I meant to say was how much heavier can a rim/tire assembly be before it becomes noticeable on the gas gauge.

It does make sense though, the tire calc computes only a ~2.1 mph difference @ 65 mph. That means when the speedo is showing 65, the engines turning over like it was doing 67mph. This seemed negligible to me and I really hadn't considered the affect of weight on the cars fuel economy.

thanks for that

 

i don't know the math but have read elsewhere that weight of a tire and rim, as well as the wider profile, can have 10% or greater impact in mpg's. try a search.

 

how are you calculating your mileage? Due to your tire sizing, your odometer is now off by ~3% too high, so if you use that to calculate your mileage, it will be automatically be lower without any other changes. Once your factor in the increased weight and rolling resistance due to the wider tires, then its easily plausible that you lost some gas mileage

 

regardless of what came on your car, the correct diameter for factory tires is 26.31". What you have currently is 25.08 front and 25.02 rear. The fronts are off 4.68% and are turning 33.8 more revolutions per mile than stock, and the rears are off 4.91% and turning 40.8 more revolutions per mile than stock.

The circumference of your current rear tire is 78.6 inches x the 40.8 revolutions per mile to make up the difference to cover the same mile = 3206.88 inches / 12 = 267.24 feet.

With those rear tires your car has to turn the rear wheel 40.8 more times to cover 267.24 feet PER MILE to be equivilent to a stock size tire.

How many more revolutions of your engine at highway speed does it take to turn your rear wheel an extra 40.8 times? You can fugure that one out, but .......

YES you are losing economy, YES you are losing performance, YES your spedometer is telling you that you are going between 4.91% and 5.161% faster than you really are depending on your speed

And not just moving forward.........Your braking performance also just got worse. Your rotor speed Front and Rear is also the same 33 - 40 revolutions per mile faster than it was stock, so you need more braking force to stop it, which means that your 60 - 0 times / footage are worse than they were. You'll eat up more pads faster because every time you need brakes it will take more to do what less used to do.

People who spend thousands on go fast parts to make up tenths of a second in the quarter mile, and then run a tire this much smaller just threw the cost of their mods out the window.

 

Wow - thanks for that. I was thinking of replacing the tires for a taller profile to restore the original wheel geometry but clearly there are other factors like wheel/tire assy weight, increased rolling friction from the wider tires, etc.

I do like the wheels though. Will run these tires for awhile and when it comes time to replace them will focus on tires that restore the cars original wheel assembly diameter.

One last question does come to mind relative to diameter only.

Obviously smaller diameters are counter-efficient to the cars original mileage as the engine is working harder now to maintain the speeds you drove previous. If I were to select a tire with a larger sidewall height than original, could it be substantial enough to overcome the negative affects of the heavier weight and wider tire than original?

 

Taller is a tradeoff in the other direction of course. Meaning that you usually gain fuel economy and mileage at highway speeds but lose torque and low speed performance as you have changed the gearing. VDC will also engage a little more as the geometry is changed in that direction because it reads traction based on the size of a smaller tire. I personally run slightly taller tires in the rear when I can't get the exact diameter that I want(255/40 19 instead of 255/35 19) because I like to know that on the interstate when the tach is running low RPM's in overdrive that I'm actually getting the best fuel economy that I can, and the tires is turning as many RPM's per mile as stock or slightly less(790 or lower RPM's per mile). With a plenum spacer, intake, exhaust etc. you'll never miss any low end performance and you'll still get as good or slightly better fuel economy and performance than stock. So if you are fudging 1/2" fudge it in the taller direction. Of course dropping the car brings other problems because that 1/2" might be the difference between rubbing over bumps or not. Thats what started the itty bitty tire trend, again what looks cool overrides sanity.

When people here talk about wheel weight and performance I have to laugh. Yes it does matter but you have to think about how much tire you need for what you are trying to do. Most here simply shove as much tire front and rear as they can fit under the car because they like the look. Thats usually much more tire than they will ever need. Most will talk about weight savings while packing a 245 or 255 on a 8.5 - 9.5under the FRONT! It's not a formula one car people.....but again the looks cool factor overrides everything. Given the weight, performance trade off a 225 or 235 front is more than enough tire even for a track car if it's a good soft compound summer tire. More than that on the street with the added size comes added wheel weight, added tire weight, added wind resistance with wider footprint, additional rolling friction, etc. and you are losing performance unless you live to take +1G turns all day long which we rarely do.

The Forced induction guys or anyone making 350 - 400 HP need to get it to the ground so 275 / 285 is not a bad choice, plus it looks really good. But again what is practical for a normally aspirated car making less than 300HP when weight is a factor or fuel economy? Personally I like 255/265's on light 9"s or 9.5"s for looks, but a 245 is really all the tire you need there.

Speaking of light wheels, stocks are heavy as heck and most anything is an improvement. Try to stay under 25lbs for the wheel & 25 lbs for the tire or you'll be back to heavy as stock. It is possible to shave nearly 100 lbs off the car by chosing the right wheels & tires.