Steelies vs. Alloys

[GasSavers_Scott]

As Im battling finding the right tire size for maximum economy, my 02 Ford Focus auto came with giant factory 16X8 rally rims and they recommend a 205/50 R16. So rather than mounting these huge meats on my car I was planning for a set of 195/60 R15's. Now the question here is I can pick up a set of cheap steel rims at the junkyard, how much would I loose by going with steel over alloy? Some say you have a fly wheel effect with a heavier rim, others say rotational mass eats more fuel by having more unsprung weight. Right now the 16's have a hard ride and they're hard to turn in parking structures due to they're width. Whaduya think?

[theholycow]

Unsprung weight affects handling, but I don't think it directly affects fuel economy enough to be measurable, for mixed or highway driving. If you're doing mostly city driving, heavier wheels/tires may affect your FE. Are you sure the total weight of your proposed tire/wheel combo will be more?

The new tire size will be slightly larger (resulting in .6% additional speed). You mentioned in your other tire post that you needed shorter gearing, the new tire size won't help with that.

It may be possible to adjust your power steering to provide more assist so it's not so much work turning the fat meats. You're not going much narrower with your proposed size, less than a half inch difference in width.

[GasSavers_Scott]

Hello Holy Cow, Am I not reading these tire numbers right, I thought 205 was the outer diameter, I thought by going to a 195 the outside diameter would be smaller. Is width, height, diameter changed by the calculation of 205/50/R16 vs. 195/60/ R15?

[theholycow]

Yes, the system is really weird. Here's how it works, using 205/50R16 as an example:
205 is the width in millimeters
50 is the sidewall height as a percentage of the width; so the height in this example is 102.5mm
16 is the rim size (of course)

That system really makes it a lot of work trying to get the diameter you want. Here's a couple good calculators to help:
http://miata.net/garage/tirecalc.html - this one is linked in my sig, is simple to use, and gives good details
http://wheelsmaster.com/rt_specs.jsp - this one presents you with a list of similar sizes

[GasSavers_Scott]

Believe it or not, it makes total sense, thats why the 205/55's on the front now are actually taller than the 50's and if I swap the 205/60's from the rear, I will actually make my gears taller, worsening the situation on hills. Thank you so much, off to the tire store I go.

[GasSavers_Scott]

Well now I went around measuring my tires and my neighbors, so I found out that a 205/60 16 is 25 inches tall, a 205/55 16 is 24 inches tall and the neighbors 185/65 14's are 22 inches tall, so it seems the tire diameter Im looking for is 23 inches tall. A 205/50 16 would be 23 inches tall, I would like to find that tire size in a 15 inch if possible. You should have seem me, Im calculating the width by height and then adding inches to milimeters on a yard stick, great fun.

[theholycow]

Yikes. That's a lot of work. Did you try the tire size calculators I posted? They do all the work for you and will give you diameter, effect on your gearing, etc; and they don't account for wear or manufacturing or measuring differences, which makes it easy to use them to compare sizes.

Did you measure them from the ground to the top, or from the front to the back?

205/50R16 (24 inches by the calculator) will probably be an expensive size. I think anything 23 inches diameter on a 16 inch rim will probably be expensive.

[GasSavers_Scott]

I measured them front to back, as to not get ground deformation from tire sag. I still have to look up the web sites.

[GasSavers_Scott]

Great findings from the web, Miata Net worked great, Wheelmaster kept taking me to Renegade Wheels. So here are my tire findings and what I plan to do about steelies and alloys.

These are the factory recommended tires for the Focus.

P185/65-14, 23.5 inches
P185/70-14, 24.2 inches
P195/60-15, 24.2 inches
P205/50-16, 24.1 inches

And what I have on there now:

P205/55-16, 24.9 inches, front
P205/60-16, 25.7 inches, rear

So Im 3 mph over on my speedo and the car is a dog on hills, incidentially the speedo sensor is magnetic off the diff in the trans, so its digital and not an old cable. I could go and just buy 2 new tires for the front and have it right on the gearing, course its $105 a tire, plus mounting and balancing. So if I go and get 4 tires over time, its $500.

Now if I get the 14'5 or the 15's its 46 to 65 dollars a tire, plus getting used rims, mounting and balancing, I come in at $260 for 14's and $336 for 15's.

The weight is also a factor, when I was at Wallmart pulling tires off the rack, measuring, checking tread design and compound, I felt the weight. The 205's are almost twice the weight of the 14's. So if a steelie rim comes it at 19 lbs. and a alloy comes in at 9 lbs. I literally have the same amount of weight per tire be it steel or alloy.

So now the tread theory of sidewall patch and all that stuff comes into play, will the wide hard sidewall tire be more fuel efficient or the medium skinny low priced radial? When I rented that Chevy Mailbu that could knock out 38 to 42 highway, it had 16's on it and it was much heavier than my Focus, so the wide hard sidewall theory worked on that.

Down to dollars and sense, its 260 vs. 500, Im actually leaning more to the P185/65-14's for the mechanical gearing advantage. I would be 1.5 mph under my speedo and the P185/65-14 is what is called for by Ford on their factory 14 inch rims.

So the convenience of throwing 2 tires on the front, or finding rims and tires and saving 164 to 240 bucks? Hmm!

[theholycow]

Having a big difference in tire size from front to rear will cause your ABS, if equipped, to false-alarm and activate all the time.

You can't compare to the Malibu, they are very different; however, I agree with your conclusion. The wider tire should help - when comparing the same rim size. I think that a taller tire on a smaller rim will be better even if it's narrower.

When you are talking about $240 difference, that pretty much blows everything else out of the water. Save the $240. If the efficiency question was a sure thing then you might factor it in, but with only vague theories available...save where you know you can significantly save.