Prius 15's are better than aftermarket 17's! (Wrong wheels/tires can wreck your MPG)

[LocoJason]

I don't see anyone having asked about the offset of the wheels as well. If they were a wider wheel you obviously have a larger frontal area, but if the offset of the wheel is lower (as aftermarket wheels tend to be) the wheel will sit further out... it will alter the wake and cause a large disturbance in the airflow along the sides of the car.

I would almost be willing to ask a Prius owner to throw on some 1.5" wheel adapters with even the stock wheels and tires. I have no doubt this subtle difference would have a much more drastic effect on a vehicle like the Prius, which is highly optimized for aerodynamics (among other things).

[wyatt]

Old tires were BF Goodrich 215 45 ZR17's. They were much lower profile than what I would have picked if it was me doing the picking, but not quite "rubberbands" either. You might think they would be terrible, but they are 50PSI tires, so I had hope for them initially. Here's a picture. Sorry, you have to put up with me being in the picture, since I don't seem to be able to find another of those rims on my car.

The new tires are Bridgestone Ecopia 185 65 R15's.
Perhaps I will be able to get a picture of the new rims/tires on my car up tomorrow.

According to an online tire size calculator, the Ecopia's are 24.5" in diameter, and the 17's are 24.6". The difference is 0.6%.

[wyatt]

Originally Posted by MetroMPG

...also went to check your fuel log to see what it looked like. Not updated since 2010! Any chance you can add in some previous tanks & come back and post future ones so we can see how this plays out over a bit more time?

You can now check out the Fuel Log for Salsa Red, my 2004 Prius.

[rmay635703]

I wonder if 15" Prius rims (old 4 lug style) would have FE benefits on my Cobalt XFE?

Not sure if they ar elighter or the same.

[wyatt]

There are only 3 reasons why it would make a difference. 1) Mass 2) Aerodynamics 3) Rolling Resistance.
Are you sure the 4 lug Prius rims are 15" (I remember them being 14")? If so, are your current tires 15"? If so, you can get LRR tires, so that one's not a difference. As far as aerodynamics, you can always get some aluminum pizza pans (plus, looking at the old style of Prius rims, they don't look all that aero anyhow), so that leaves you with mass. Do you know what your rims weigh now? I have seen the mass of the Prius rims, but don't remember it offhand.

[vrmilionzx]

That's odd - I'm using a tire calculator online, and getting the same result, but it seems really....off. I checked using excel, and unless I'm entirely thick (very possible, I haven't had my morning coffee yet), I'm getting a difference in circumference of more like 5%.

Because I'm having a bit of an aneurism about this, if anyone wants to check my math and tell me where I screwed up, please do....

I converted to mm (so everything is in the same form of measurement), and used 25.4 mm in one inch, so:
Old Wheel Dia = 17x25.4 = 431.8mm
Old Tire Dia = 215mm x .45 (aspect ratio) = 96.75mm
Old combo total Dia = 528.55+96.75 = 528.55mm

New Wheel Dia = 15x25.4 = 381mm
New Tire Dia = 185mm x .65 (aspect ratio) = 120.25mm
New combo total Dia = 381+120.25 = 501.25mm

measuring the delta: (501.25-528.55)/528.55 = -0.05165 (or about 5.1%)

No need to calculate based on the circumference, since applying the same formula to both can be canceled out, but just in case you'd like to:
Circumference = 2 x 3.14 x R OR 3.14 x D
(so you'd have 1659.647mm on the old combo, 1573.925mm on the new, same -5.1% difference)

Reference for confirmation of Aspect ratio measurement being in mm: Tire code - Wikipedia, the free encyclopedia

I'm not trying to be a jerk, I'm just really trying to get how the tire calculator is so different. Obviously the other factors you (Wyatt) mentioned will play a role, so it's not JUST the size change, but it seems like a pretty significant change to me... And if I'm THAT wrong in my assumptions, I'd like to know before I make a tire/wheel combo changed based on those faulty assumptions...

Anyone?

Also, I think I have the same sweater hiding in a drawer somewhere...

NOTE: Sorry, had to edit, copy paste errors in numbers - should be correct now

[vrmilionzx]

Still solid gains, btw - impressive to say the least

[wyatt]

Originally Posted by vrmilionzx

I converted to mm (so everything is in the same form of measurement), and used 25.4 mm in one inch, so:
Old Wheel Dia = 17x25.4 = 431.8mm
Old Tire height = 215mm x .45 (aspect ratio) = 96.75mm
Old combo total Dia = 431.8+(2*96.75) = 625.3mm

New Wheel Dia = 15x25.4 = 381mm
New Tire height = 185mm x .65 (aspect ratio) = 120.25mm
New combo total Dia = 381+(2*120.25) = 621.5mm

Fixed that for ya.

[vrmilionzx]

Well crap - good catch, and thanks for setting me straight there. It makes sense that the sidewall counts twice, since the tire wraps around.

Sigh... mornings...

[eco_generator]

And you don't even need to do all the math. Tires with the same "size" can vary widely in overall diameter. They are listed on sites like Tire Rack or the manufacturer site. So you just look up the diameter, since it's not entirely correlated to the nominal sidewall size.