Let's talk rotational mass savings

[KPC1.8TQuat]

So the fuel economy on my stock 1.8 q-tip has drastically gone down over the past few months. Thinking maybe I just needed a tune up, I recently replaced all 4 plugs, coil packs, and a new K&N drop in filter.

(Interesting to note the coils weren't R revision, they were something way older, and upon closer inspection turned out to be the originals that came from the factory. That's 75,000 miles on the same coils)

Anyway, fuel economy hasn't improved in the slightest. As for my driving tendencies, I baby the car and drive on average 50/50 highway/city. At it's peak I was getting about 430+ miles per 15 gallon fill up. Now I'm lucky to get over 300.

The only thing I can think of that I changed was this winter going from 16" A4 rims to OEM 17" S4 rims. Also for this next summer I was planning on getting some 18" RS4 reps, but obviously these will be even heavier, so was instead thinking of going with some 18" OZ ultraleggeras which are about 18.5 pounds per wheel.

Questions:
1) What is the difference in weight from OEM 16" to 17" wheels
2) What other steps can I take to lower the rotational mass of the car?
3) Would bad rotors effect fuel efficiency? My front rotors are beyond gone and I am just waiting for my ECS/Porsche Boxster rotors and calipers to come in.

Thanks for all the help guys

[03 silverbullet]

i would like to get 430 a fill up my car just has a GIAC chip and i only get 315.

Have you changed out the fuel filter yet ?

I dont think bad rotors would harm your gas mileage, unless you have a caliper hung up causing the pad to contact the rotor creating drag. You would surely notice that though.

if u get after market rotors that are cross drilled or slotted they would b a few ounces lighter.

The rim choice sounds good try to get the lightest wheel out there.

Let me know if u do eventually find out why the mpg went down. I have replaced everything but the GIAC chip which will b getting sent off to them for a new rom chip and updated flash.

[downhillracer]

I have 19's.. not the lightest 19's either and im getting about 22mpg city and 33 highway.. so im not sure whats happened with you... check to see if you have a boost/vacuum leak.. also did you gap your plugs properly?

[EBG 18T]

KPC - i think it might just be our local gas. I am getting 23-25mpg right now. I have had the best luck locally with Shell and the worst with exxon/mobil. My winter wheel/tire combo is also heavy (~12-13lbs a corner heavier than my summer setup) and it decreased the mpg a little but the loss of acceleration vs my summer setup was the most noticible.

[KPC1.8TQuat]

I have 19's.. not the lightest 19's either and im getting about 22mpg city and 33 highway.. so im not sure whats happened with you... check to see if you have a boost/vacuum leak.. also did you gap your plugs properly?

Yeah, plugs are gaped correctly. And my boost gauge is showing no leaks, I'm at 20 mmHg at idle once the car is warmed up, which is spot on. Just had the timing belt and water pump service done about 5,000 miles ago as well.

[dirtybrd]

You need a primary o2 sensor, CTS, or MAF. I'd bet o2.

[KPC1.8TQuat]

So, does anyone know what the OEM 16" and 17" rims weigh?

[03 silverbullet]

I have 19's.. not the lightest 19's either and im getting about 22mpg city and 33 highway.. so im not sure whats happened with you... check to see if you have a boost/vacuum leak.. also did you gap your plugs properly?

I have always got bad gas mileage and u suggested boost/ vacuum.

How would you check for a vacuum leak ? I have already done a boost leak check and everything seemed fine ?

[EBG 18T]

So, does anyone know what the OEM 17" rims weigh?

My OEM 17" sport wheels were about 25lbs.

[b6onboost]

Lighter wheels and 2 peice light weight rotors will significantly reduce rotating mass allowing your car to accelerate, brake, and handle better. The suspension will actually have an easier time handling bumps as well. Mileage will improve.

Bad mileage: Winter fuel blends, heavier winter wheel/tire setup, and driving the car cold will all hurt your mileage, even if everything else is mechanically fine.

[Kerschera4]

Bad mileage: Winter fuel blends, driving the car cold will all hurt your mileage, even if everything else is mechanically fine.

x2.. and i would also check your fuel filter, tire pressures, and O2 sensor. There are a whole bunch of things that contribute to poor gas milage in the winter. You switching from a 16" wheel to a 17" that weigh nearly the same is probably contributing a very neglagable amount

[vdubstreets]

KPC - i think it might just be our local gas. I am getting 23-25mpg right now. I have had the best luck locally with Shell and the worst with exxon/mobil. My winter wheel/tire combo is also heavy (~12-13lbs a corner heavier than my summer setup) and it decreased the mpg a little but the loss of acceleration vs my summer setup was the most noticible.

they put additives in the gas during the winter, that could lead to a poor gas millage comparatively to the warmer months.

[EBG 18T]

they put additives in the gas during the winter, that could lead to a poor gas millage comparatively to the warmer months.

my understanding is that ~10% enthonal is added for winter months.

[KPC1.8TQuat]

Well regardless, I need to seriously consider going with some OZ ultraleggeras instead of some RS4 replicas for summer I guess. That's assuming the RS4 reps are even heavier than my current 17" S4's for winter.

[OneBadAudi]

damn on a full tank of gas, i read about 465 miles, and i always use shell 93 octane...

[AudiA4_20T]

damn on a full tank of gas, i read about 465 miles, and i always use shell 93 octane...

dont go by what the screen says, it means nothing...

[EBG 18T]

Well regardless, I need to seriously consider going with some OZ ultraleggeras instead of some RS4 replicas for summer I guess. That's assuming the RS4 reps are even heavier than my current 17" S4's for winter.

my vmr 18" rs4 reps are my winter wheels and weigh ~26lbs. Thus the reason i use lighter summer wheels and lighter track wheels. The lighter wheels make a noticible difference in acceleration.

[AudiA4Turbo22]

with tires and full air each wheel weighs about 42lbs

[EBG 18T]

with tires and full air each wheel weighs about 42lbs

i wish. with my snowtires and these rs4 wheels i am at almost 51 lbs a corner. heavy stuff.

[AudiA4Turbo22]

^^ i meant for the stock 17s

[03 silverbullet]

When was the last time you changed the fuel filter ? The i just changed mine and when i asked the dealer why the filter was so much they told me it was because there is an internal fuel pressure regulator. Possible that just went bad causing the bad MPG

[EBG 18T]

^^ i meant for the stock 17s

Originally Posted by AudiA4Turbo22
with tires and full air each wheel weighs about 42lbs

Oem 17" sport wheels weight 24-25lbs. the Bridgestone Lm 25 in a 235-45-17 weights 25lbs according to tirerack. so that still nets out close to 50lbs.

To get a 42lb combo we would be looking at a OEM 17" wheel weighing 17lbs. I wish the factory would supply us with 17lb wheels in stock form, but since Ronal makes many of the OEM wheels we should feel lucky they only weigh 24-25lbs each.

[AudiA4Turbo22]

^^maybe the air was down, because when i shipped them UPS said each weighed 42.3lbs or something.

[KPC1.8TQuat]

Hmm... decisions, decisions.

[Skyler@Achtuning]

The OEM S4 Avus wheels weigh 29.3 Lbs, not particularly light however rotational mass of the wheel should not really affect gas mileage that much. If it did you would see manufacturers putting as light of a wheel on their cars as possible in order to better their CAFE MPG rating.
Tire choice will affect your MPG though, the rolling resistance is the main factor which tires/wheels would affect your MPG. Rolling resistance depends on tire compound, tread pattern, inflation pressure as well as suspension alignment and road surface.

[dirtybrd]

You are avoiding the problem and trying to fix it with wheels. You said your mileage has been decreasing. Have you changed anything to make it go down or has it done it on it's own?

[wolfdog]

You have to compare apples with apples. Wait until the summer mix of gasoline comes back out to get a better comparison. ALL gas stations have to switch to a crappier mix in the winter time. That combined with different operating temps will definitely affect your mpg. Whiel it may not be huge it's enough to throw off a nice comparison if you're experimenting across seasons.

I also agree with the fuel filter. My car behaved much better once the fuel filter was replaced. And, of course, the psi on the tires.

Finally, I doubt the difference between 16" and 17" would affect you that much. It's likely a combination of several things a la what I wrote above.


I thoguht this was going to be a flywheel thread when I read the title ;-)

[KPC1.8TQuat]

You are avoiding the problem and trying to fix it with wheels. You said your mileage has been decreasing. Have you changed anything to make it go down or has it done it on it's own?

Did you not read my first post?