So why are people saying you lose a few mpg with the 17" wheels? Is it actually true the 17" wheel V gets worse mpg than the 15" wheel II-IV? People who upgraded to aftermarket 17-18" wheels from 15" have reported losing a few mpg (around 3 mpg). However, the rims they chose, despite weighing much less than the factory 17" (24.2 lbs), are usually open spokes, which create more drag/turbulence in the wheel wells and this leads to mpg loss. The factory 17" have wide flush spokes and are designed with aerodynamics in mind (those of you who remove the OEM covers on your 15" may be taking a mpg hit as well). Moreover, I highly suspect that the computer is programmed differently in Vs and II-IVs to fit their respective wheel size characteristics and maximize mpg (although I have no evidence for this).
EPA rules allow the same estimate to be used across models that have the same engine. EU rules don't and so, on our weaker test that produces higher results, the 17" wheeled cars get 70.6 mpg (Imperial gallon, 20% larger than US gallon) and the 15" cars get 72.4 mpg. The mpg figures are back-calculated from the rounded metric figures: 3.9 L/100km for the 15", 4.0 L/100km for 17" (rounded to ?nearest 0.1 L/100km). 2.5% more fuel consumption.
"EPA also says your mileage may vary." Mileage figure was not adjusted because it falls within a certain percentage of the standard/base vehicle? Just my 2 cents...
You are incorrect regarding the computer tuning being different. The final, outside radius of the 17" wheels and tires is virtually the same as that of the 15" wheels and tires. (lower profile by about 2 inches, wider rim by about 2 inches...) However, the part that I was unable to get my head around is why it made a difference in Fuel Economy. Then it was mentioned, in other posts about 'Unsprung weight' versus sprung weight (you can google this). Depending upon who's math you believe, you will add an equivalient of 8-20 pounds to the cars apparent weight foir each pound of unsprung weight. This means effectively that the car must overcome the momentum of the spinning wheels more (as more weight is spinning), causing less efficeint movement, when compared to a car with less unsprung weight. Based on this, it would seem that is we could find a good 17" tire with similar aero qualities to the 15" stock wheel, we could conceievably add those to the car with no mpg effect.
I haven't seen anyone say they lost mpg in a Gen III when adding larger wheels...as far as I know only a couple people have done it. Do you have a link? In Gen II I have seen people report losses of mpg with aftermarket wheels, but the larger stock wheels weighed a lot less then and were only 16". I think the aerodynamics will not have anywhere close to an impact as the weight does- I haven't seen anybody report a loss in mpg from removing the wheel cover in either generation.
When I drive my dads Prius III the car picks up and goes a bit easier than in my Prius V. Definitely I am able to get higher MPG's when driving my dads car compared to mine on the same roads, same route here where I live, going to work etc. I guess it is the added weight as well as the added resistance from the 17" tires, but when driving one and then the other from day to day it is quite obvious in just the feel of the engine/motor pulling the car...
Dakine50 took a 3 mpg hit when moving from 15" to 18", despite the aftermarket 18" being 18.3 lbs (vs. 24.2 lbs for factory 17") and I was wondering why. He attributed the worse mpg hit to the design of the aftermarket wheel (open spokes) vs. the more aerodynamics, flush design of the factory 17", as well as the larger rotational mass. If we trust the numbers posted by Mike Dimmick, the hit going to factory 17" should be only 1.8 mpg.
I think one other thing we might be missing is that "every motor is not identical" to the next. Some motors may have tighter tolerances than other motors. Where are the engineers? Or am I just talking BS? Isn't that why we always hear of some people getting 50-60 MPG when others are getting 38-42 MPG? And yes I know driving style and terrain have alot to do with it.
And a Diet Pepsi shows zero calories on the label because the FDA lets you round down. Anything below 5 gets reported as zero.
I have 3k miles on my Prius V with 17" wheels and on trips I am getting 57.5 to 58.7 mpg with cruise control set to 3 mph over the speed limit. I am satisfied.
Acutally the wheel is 2" less in diameter, but since the rubber is on "both" sides (top and bottom) of the rim, the tire itself is only an inch nominally shorter in sidewall going from the 15" wheel to the 17" wheel. I don't believe the rim is 2" difference in widths. The tire width is around 8/10th's of an inch wider. As for the weight of the wheel, while I do believe this will impact FE, I don't think it has (in this specific case) as significant of an impact as the tire width. When initially accelerating, I don't believe the weight of the wheel plays that much more of a part in the FE than the overall weight of the vehicle. I may be wrong, and I know it impacts it some, but I just don't believe it's that much. If it did then I would also assume that the heavier wheel might actually help FE once moving as the centrifical force would be larger and it would take a larger frictional force to slow the vehicle down(??? just a thought). However the heavier wheels and unsprung weight will impact handling and suspension componet life.
I would like to add two more points: 1. A wider tire has a larger frontal area and thus more aerodynamic drag. 2. The wider 17" tire is perhaps more optimized for good handling and grip instead of low rolling resistance. But I don't know if this is the case for the 17" tire mounted on the Prius.
