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Discussion Starter #1
Some time ago I replaced the wheels on my 92 Roadmaster for Impala SS wheels.
Car has got the towing option.

I used to have basic M+S 225/75/15 tires with standard steel rims. Treadwear of tires is 440 (hard compound)

I bought 17"x8.5" Impala SS wheels and used 255/55/17 tires.
Tires are FEDERAL 595 SS with treadwear 260 (softer compound then ...)

Gas mileage increased as follows.
old tires/rims - average on 12 tanks - 18.37 MPG (12.84 L/100Km)

new tires/SS rims - average on 5 tanks - 16.62 MPG (14.36 L/100Km)

Long highway trips (family Holidays) are not affected though with average 20 MPG (11.5 L/100Km) at 84 MPH (135 Km/h) in cruise.

I did not do any other noticeable modification that could affect those figures and values are visibly constantly higher with new tires.

cwm3

I knew larger wheels resist more but here it is painful.
 

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As you said rolling resistance plays a huge part in gas mileage, between the wider contact patch and softer compound you see the effect. I run Kuhmo ASX-255/50R17 which have a treadwear rating of 420, run at 38 PSI, and average 24(If I stay under 70 MPH) on the highway. Better mileage and not having to replace them after 20-30K.
 

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Don't forget that a 255/50/17 is over 1" shorter than a 225/75/15. That makes for many more revolutions per mile.
 

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Gas mileage increased as follows.
old tires/rims - average on 12 tanks - 18.37 MPG (12.84 L/100Km)

new tires/SS rims - average on 5 tanks - 16.62 MPG (14.36 L/100Km)

Long highway trips (family Holidays) are not affected though with average 20 MPG (11.5 L/100Km) at 84 MPH (135 Km/h) in cruise.

I knew larger wheels resist more but here it is painful.
I think you mean to say that gas mileage DECREASED (ie. fuel consumption INCREASED) with the 17" lower-profile tires--yes, that confusing language (English) can get in the way at times!

The treadwear rating isn't a direct indicator of how fuel consumption might be affected, since there are so many other variables in calculating consumption--road surface conditions, weather (ie. dry or wet, headwind or tailwind), vehicle load, tire inflation, average speed, traffic (congestion means braking & accelerating more, which affects total amount of time in OD/TCC applied), terrain, etc.

The fact that you've changed tire & wheel size, and overall diameter, does mean something, of course--in this case, it's a very minor change, just doing the math--maybe 6mm less diameter overall for the larger 17" package.

Are you measuring distance traveled by road markers or odometer?

Have you found any need to compensate for the tire size change with a new calibration of your PCM--is the odometer still accurate after the tire change?

If you have a "test" or "control" section of road that you're able to do comparisons, that means something.

Yes, the wider tire will have greater rolling resistance, and some (very) minor amount of greater wind resistance--no idea of the real difference between a 75 series narrow or 55-series wide(r) tire. The total weight of the 4 original wheels/tires vs the current setup would be interesting to know, too. Inertial "load" is probably working against your combination as well since I suspect the 17's are a bit heavier--I don't recall whether your car is 2.93 or 3.23 ratio, but it would not surprise me to see some actual improvement if you went to a 3.42 gear--for cruise conditions, the lower throttle opening for a given road load & speed, and the ability to keep the car in OD/TCC lockup longer would probably help your overall economy on long trips.

I hate to say this, but considering your location, what MAY happen at some point, is that someone who hates big American cars and their relatively high fuel appetite and the resulting "big" carbon footprint will force the issue--though I sincerely hope that you're not being harassed for your choice of vehicle.

At the end of it all--would you give up this car and it's fuel appetite for something smaller and with (a little) better fuel economy, with less room & comfort, for a car payment you probably don't want???

My favorite saying in these discussions: "You can buy alot of gas for a car payment....."

There is a caveat to this....governments have a way of acting to force or "induce" certain actions or behaviors when it comes to things like getting "old/dirty/gas-guzzler" vehicles off of the road. Japan imposes more strigent (and more expensive) vehicle inspections, the older a vehicle gets. At some point, the owner throws in the towel and dumps the old car, since the amount being paid to keep it on the road is equal to or greater than would be the cost of replacing it with a new(er) vehicle. I'm sure Japan isn't the only place where this happens....coming soon to OUR neighborhoods....I know, don't give 'em any ideas!

Thankfully, there are options!
 

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Discussion Starter #5
Hello,

Well I have 55 sidewalls which are not exactly matching the Impala SS which is originally fitted with 50 sidewalls.
This is to match with the original ratio of my car.
In fact between 225/75/15 and 255/55/17 the difference is 0.9 % only whick means that when driving at 60 MPH my odometer would show 59.5 MPH. This is not an issue at all.

I found my average calculation quite accurate because it covers more than one year and it is done always with the same conditions i.e. mainly the drive to work and back home (40 Km)
It is not highway unless I am on vacations. It is mixed city drive with some traffic jam and country side roads at 60 MPH. However France developped a big taste for replacing traffic lights and turn them all into round abouts.
A LOT, EVERYWHERE even in teh middle of a straight road ! To force people to slow down ...
We also have many speed limiters (big bumps on some roads close to schools or pedestrian areas).
Of course when you drive a 4000 Lbs car this is not compatible with economy ...
braking, accelerating all the time.

So originally the MPG of my car was quite acceptable.
Now I open the eyes on my new consumption with larger tires it is disappointing but nevermind !!!

I have a V8 in my mind since I am a kid (Dad had a Jeep Willys from WWII and a Simca Chambord with a Ford V8 inside !!!)
Not to mention that I grew up watching Starsky&Hutch on TV as well as "Dukes of Hazard" :D (I am 35 years old)

I made the choice to sell my Nissan S13 Stage1 and my Mercedes 300 SE and buy this car and live with it until gas is far too expensive !!! I still have a couple of years in front of me then !!!


So I am not complaining, I am just giving some facts because I trided to get those figures from the forum around 8 month ago and I didn't. So now I am able to share them with other members.

Concerning the risk causes by people with too much of a "green spirit" I am not afraid.
People here do not know it is a V8 under the hood and even less that the displacement is 5700 cm3 !!! (VS. 1900 cm3 average here)
They are more focusing on AWD trucks in Paris ... that is all ..

And generally people really like the US cars because they are awesome and exotic. People keep on lokking at the car and talking to me of how nice t is (and it is only a RMW !!!) And remember we don't speak German because the US freed us 65 years ago. This is deeply anchored in our minds and this is why we always have a big sympathy for anything that is coming from the Uncle Sam.

And the Buick is just a family car with wood on the sides so what is the problem !!!

To come back to GAS, I just computed that I spend average 2,000 EUR per year in GAS which would be 3,000 USD. Of course If I could buy gas in the US it would turn into 1,500 USD maybe ... This is for 10,000 Miles per year.

p.s.: Sorry for decrease/increase confusion ... MPG and L/100KM are not behaving the same at all :p
 

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So I am not complaining, I am just giving some facts because I trided to get those figures from the forum around 8 month ago and I didn't. So now I am able to share them with other members.
I just bought some 18's for summer and they are a lot heavier than my 15" steel winter wheels - I expect to lose some mileage as well.

p.s.: Sorry for decrease/increase confusion ... MPG and L/100KM are not behaving the same at all :p
We use L/100km as well, I don't know why it could have been km/L so the numbers would cross reference a bit. :confused:
 

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Shorter tires will screw up your math. Numbers will be off give or take about three percent. Most stock Buicks run a 28" tire.
 

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In fact between 225/75/15 and 255/55/17 the difference is 0.9 % only whick means that when driving at 60 MPH my odometer would show 59.5 MPH. This is not an issue at all.
His 255/55 are 28" tall.
 

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Heck yes you loos some mileage. I slapped some 20's on my wagon as soon as I bought it, and later a 20/22 combo. Knocked my fuel economy WAY down. I traded my set of 20's for $$$$ and a set of sedan alloys and my economy went WAY up, even higher than with the steelies and wire caps. Looks better , too.

I will not likely run anything bigger than an 18" again.







Unless, I just do.:p


Question to the OP: How do you like those federals? They're dirt cheap.
 

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Discussion Starter #10 (Edited)
FEDERAL was not really a choice.

In 255/55/17 one will not find many tires in Europe.

I could find them 120 EUR each, delivered from Internet sites (Germany).

But finally bought them from the UK (http://www.camskill.co.uk/) at 80 EUR each delivered to my door :D

I love the Internet !!!!! (some Continental were 300 EUR each !!!)

From what I rode the FEDERAL are Korean brand (or so) specialized in sport tires. They are a very good deal for the money. Good on dry, good on wet. No particular complaign.

I would buy them again and even would put them on my wife's Alfa Romeo unless price is too high compared to other local brands.

And the pattern is also very nice, a bit like the one of the Eagle F1 tires.
 
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