Is wheel size a significant variable in efficiency?

Size does matter.

My Gen 3 Prius had 17" wheels and I averaged about 4MPG less than other Gen 3 Priuses. And when i got my Prime, which has 15" wheels, I average a little better than most in hybrid mode, so it's not my driving style that made the difference. I like the 15" ride better as well and the handling is only slightly less than the 17's.

I don't know if the 16's would be that much different though. rim weight and tire profile are the keys, so if it's close, it probably wouldn't make as much difference.

 

What tires you put ON the different size rims likely is more important than the overall size.

And unless maybe you keep the car for 300K miles......the slight fuel savings probably won't offset the cost of the tires and rims over the life of the car.

 

Yeah we get better mpg through winter with 195/65R15 Michelin X-Ice on Corolla steel rims, than rest of year with Michelin Primacy MXM4 215/45R17 on OEM 3rd gen alloys. The latter rims weigh 24 lbs apiece too.

 

Mathew, it works the other way. For the same car weight and inflation pressure, the tire contact patch is about the same. A narrower tire has a longer contract patch requiring more deformation of the tire as it rolls around and contacts the flat pavement. This results in more movement in the rubber, more heat (not an excessive heat problem), and more fuel consumption. The wider tire, while it has more wind resistance, has the shorter, wider contact patch for less deformation and less fuel consumed.

As noted above, the type of tire matters more than a small change in the size. While there is no industry standard for labeling a tire as energy saving, the tire models so listed by the top tire makers do save a bit of fuel. Tread wear, traction, and rolling resistance are the tire design triangle. If one is increased, one or two others are decreased. Newer tire designs can improve things, but there is no free lunch. And, the fuel savings might be less than the cost due to increased tire wear. Lowest overall cost might be with the tires with the longest tread life but not labeled as low rolling resistance. Here's more from a retired tire engineer: Barry's Tire Tech

 

It is the weight. Adding or removing it to the wheels has a bigger impact than to the trunk. The US Camry Hybrid LE has smaller steel wheels to help it get that 50mpg rating.

Smaller wheels and narrow tires generally mean a lower weight than the larger ones with wide tires. The larger wheels also have the weight closer to the outside diameter, which adds to the negative impact.

 

Until 2016 I used to have a Gen III Prius. In my country, there are strict rules about the wheels you can fit on your car, they have to be listed in the type specifications sheet, an official document that you have to keep in your car gloves tray and be able to show to the police officer in case of road checks.
Now, in my Prius TSS only 17" wheels with 215/45W tyres were listed
The OEM tyres where Bridgestone Potenza, and I beared their hard reactions on the road bumps for a while, until I replaced the Bridge-Stones with a set of low-rolling resistance Toyo Nanoenergy tyres.
Although of the same size, the Toyos demonstrated to be a lot softer on the road bumps, and a lot quieter too.. but above all the fuel consumption dropped by 10-15% in respect to the Bridge-Stones... I have to admit that the braking distance on wet increased as well, but one cannot have both "full barrell and drunk wife"... it is a law of nature.

After the Gen III Prius I bough a Prius Prime, once again with only one type of wheels listed in its TSS, and they are the 15" 195/65H.

End of story: although the fuel economy and road comfort are OK, the Prime handling qualities are not as much as good as I would like to be, when compared to my old Gen III Prius which had less sophisticated suspension too.

So, fuel consumption wise, tyres size and footprint are important elements, sure, but tyres compound is a critical element IMO.

 

Two important notes:

(*) On Prius, the recommended tire inflation pressures are not the same, the trims with larger wheels - wider tread, come with lower inflation pressure labels.

(*) The Toyota engineers at the Gen3 introduction event (attended by a number of specially invited PriusChat folk) indicated that the trim with larger wheels - wider tread had lower MPG, an observation backed up by numerous PC members since.

 

Its funny but I have not owned a car with such small tires since my vintage 1965 Volvo 122S which almost has the same size wheels and tires. Anyway I usually replace or modify my cars tires and wheels first. I have not done this with the Prius Prime and kept it stock and have stock size snow tires. This car is about efficiency and everyday comfort and I trust the engineers have maximized it with the 195/65-15 size. If you want sportiness change it but if that is your concern the Prius is not the car most would choose.

 

Have you read this thread? It explains all kinds of sizes and looks of aftermarket wheels.
Aftermarket Wheel Fitment Guidelines | PriusChat

There are many PP owners who prefer the looks and some handling improvement the larger wheels give. For me, Prius is my daily drive, and I have no intention of spending money on new rims and tires to decrease efficiency, although I did buy very cheap 15" wheels solely for the purpose of putting snow tires for winter. But, it's your car, do as you please.