Better gas mileage with 15" or 16" wheels

[SUBARU3]

Your thoughts??

 

1.8 Impreza FWD 5 Speed. If the wheels go from 15" (195- 60/15) to 16" (205-55/16) do you think the gas mileage will improve or deteriorate? Both sizes are alloy wheels.

 

I'm getting mixed information.

 

Todd

[audio_file]

Your thoughts??

 

1.8 Impreza FWD 5 Speed. If the wheels go from 15" (195- 60/15) to 16" (205-55/16) do you think the gas mileage will improve or deteriorate?

 

I'm getting mixed information.

 

Todd

 

it really all depends on rotating mass (weight)

if the 15's are significantly lighter, then your actual mileage will decrease by going with a heavier 16. if i remember correctly, each lb of unsprung weight added (wheel, tire, brake rotor, etc.) is the equivalent of 8lbs of extra weight on the car . . .

 

i notice that you are likely going to have a slightly larger overall diameter on the 16's even though the "height" of the tire is a smaller percentage, the actual piece of rubber that is used will be wider, thus the smaller percentage (55 vs. 60) being relative, won't really make much difference.

 

hope this helps

chris

[SUBARU3]

Boy, you need a college course in "wheels 101" to figure this out! :-\

 

I guess it may be a wash since the 16" is heavier, but the height is larger.

 

Thanks, Todd

[Reveeen]

a college course in "wheels 101"

 

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

 

Once a mass is rotating, it takes very little to maintain it, a lighter mass in stop and go driving will improve mileage, but in highway driving the mass becomes less important. Either way, a difference of 2.7%, wouldn't merit spending $10 to achieve.

[EVOthis]

thats a pretty intresting little calculator there.....

[aircraft engineer]

any time you can increase your rolling radius, the number of turns of the driveshaft will be less to go the same distance.

 

Rotational mass is about as unimportant as it can be here - the relative weight differences are minimal unless one is made of cast iron and the other feathers.

 

I used to run 14 inch on a Pinto designed for 13 without any issue except for rear wheel well clearances in jounce ("jounce" is when the rear end goes "up" - the other way is "rebound").

 

So, now, there's the MAIN problem - it's "likely" that the tire to wheel well clearances are going to shrink to where you can't run chains if you need them.

 

I know that on the 97 Impreza, the standard size TIRE was 195-60/15 and going to just 205-60/15 reduces the wheel clearance enough to where the tires (chains, actually) won't clear the front wheel wells IN TURNING (it WILL run straight ahead, though without rubbing). With a wheel size increase, it makes it even more of an issue unless you can raise the suspension a "bit".

 

IF you can go without chains, try the wheel/tire combo on flat ground steering wheel stop to stop and see what rubs. No rubbing, good to go. You might need to go down a size on the tires

[Olnick]

the relative weight differences are minimal unless one is made of cast iron and the other feathers.

 

So which is heavier, a pound of cast iron or a pound of feathers?

 

(Sorry, I couldn't resist!)

[johnceggleston]

the 205/55/16 will fit and clear fine as long as it is a subaru wheel or has the same offset. this is the same size wheel and tire they run on the 97 GT which has the same wheel clearnce as the legacy L.

 

the circumfrence of the 16" tire is about 2" larger than the 15" tire. larger tires should give you better gas mileage but slightly less power. if you are now getting 25 mpg, in theroy you should improve to 25.68 mpg with a size change of 2.7%. this assumes the reduced power doesn't greatly affect your 'around town' mileage.

 

making the change for economy reason may be misplaced, which wheels look better on the car?

[SUBARU3]

Well, they are both genuine Subaru alloy wheels. The newer ones are the 02 WRX wheels and are brand new. I got them for $50 @ a yard sale! They came off the car and went to the garage with only 400 miles on them, when the owner bought custom wheels. The kid then totaled the car in 6 months, so there was no use for the wheels again. They just sat in the garage under a sheet. Yep, a good deal for me!

 

I have these WRX wheel and tires on another car, but never thought about the mileage issue till I thought about putting them on my FWD Impreza. I'm driving it the most now as it's getting 27 around town and I just got 39 on the highway last week. (I maintain my cars perfectly, run mobile 1, etc.)

 

It's amazing the difference in gas mileage between my (2) 95 Imprezas L's. One is 2.2/auto/AWD/sedan and the other is 1.8/5speed/FWD/coupe. I hope Subaru goes back to 2WD cars on some of it's line!

 

Todd

[3eyedwagon]

I think you have all overlooked THE ONLY logical choice here.

 

Sawzall out the wheel wells, and slap on some 22"s. WITH "spinners"!

 

The continual rotating mass of the "spinners" will improve both the gas mileage, and handling, with no ill effects!!!

 

Also, the massive spinners will also create a gyro effect, allowing you to set the cruise, and climb in the back for a nap on those boring, long, straight stretches...

 

I mean, my facts can't be disputed. Even the major auto manufacturers are offering these giant wheels as standard equipment in an effort to meet the stringent EPA fuel consumption guidelines.

 

PLUS, the kids will think you are sooo cool, and isn't that really why we are all here? :cool:

[Setright]

I'm running 205/55R16 on my MY2000 Impreza. No clearance problems.

 

 

I don't think you should bank on getting any serious mpg improvement. I certainly haven't....but of course I'm running either winter tyres or R050A's which get bad ratings in the tests regarding rolling resistance.

 

 

Bear in mind that your odometer will show ever so slighty fewer miles, and that will make your mileage look worse

[aircraft engineer]

So which is heavier, a pound of cast iron or a pound of feathers?

 

(Sorry, I couldn't resist!)

 

har har hardy har har. Yeah but try a pound of gold versus a pound of iron. (Troy versus Avoirdupois - and there IS a difference)

 

I'm actually a "metric measurement proponent" - a gram is a gram is a gram. Too bad the aircraft industry doesn't want to go that way - makes calculations a LOT easier. Of course we COULD just fly another Mars probe into the planet...

 

You KNOW I meant "made of feathers versus made of steel"

 

<grumble grumble...>

 

(by the way SUB3 - since the 1.8 has been out of production for a while, you have an older model. The older Impreza front wells aren't very "clearance friendly". I meant the part about about doing the equivalent of a torque bind test (figure 8 lock to lock) to verify clearance. Since it doesn't snow all that often in Ft Worth, you most likely don't have to contend with the chain issue. There isn't any appreciable difference in the wheels as far as "inertia" and likely a minimal increase in basic weight as well. That extra weight at an extra 1/2 inch of radius doesn't really matter since it's all in "rotation". When I was doing mileage calculations, I was using a hand held GPS for getting the distance and comparing it against the odometer to get the conversion factor)

[idosubaru]

15" and 16" wheels can have the same diameter so i'm going to assume the 16" wheels have a larger total diameter with the tires installed.

 

if your experience is anything like mine, you will get noticeably higher highway mileage assuming everything is equal (not winter or knobby off road tires). i've gone from 14" to 16" (with an appreciable increase in tire diameter) on my XT6 and noticed a significant highway mileage increase.

[OB99W]

The two main factors that wheels/tires influence relevant to fuel mileage are inertia and rolling resistance. All other things being equal (assumes outer circumference of 15" and 16" tires would be the same, the 15" and 16" wheels have the same mass, etc.), the 16" wheels/tires could provide slightly higher gas mileage. That would happen because the 16" combination would have shorter (and therefore stiffer) tire sidewalls, resulting in less flex and rolling resistance.

 

However, "all other things" often aren't equal. For one, the 16" ones may be more massive (have more inertia). While in theory the energy it takes to get wheels turning could be largely reclaimed by allowing the car to coast to a stop (rather than braking), that's not very practical in most situations. Therefore, to what degree the driver coasts when slowing, rather than having the throttle open and then braking at the last second, determines how much of that energy is recovered. Of course, that's true no matter what the wheel/tire mass, but the more massive, the more it matters. Stop-and-go driving tends to reduce or eliminate any advantage of the 16-inchers, while highway miles might show a couple percent improvement.

 

Another consideration is that some tires (especially certain "high performance" ones) have higher rolling resistance than others. That could negate any advantage of the switch, and result in an actual decrease in gas mileage.

 

[...]I'm getting mixed information.[...]

It's hard to predict exactly what would happen for the reasons I gave above, and probably some others I didn't mention.