I know I should be able to figure out the math here, but I'm a little stuck. I have a Canadian 2008 Prius, bought used. The summer tires are 205 55 r16. I just put on winter tires that are old (manufactured in 2004), but have been stored for about 7 years. They have lots of tread (used for 2 or 3 winters), and are mounted on rims. The winter tires are Nokian Hakkapeliitta Q. This fall I have been averaging just over 4.8 l/100 km. Part way through my first tank of gas on the winter tires I'm averaging 4.7 l /100 km. The mornings have been cold lately (between 3 and -3 C) so I wasn't expecting great milage to begin with. I thought I'd get worse milage, but so far I haven't. So there are 2 possible explanations: a) I guess the 15" tires are slightly smaller than the stock tires, and that the current summer tires I have are slightly bigger....however, I don't know what tires the car originally came with. b) Maybe these Nokians are really good? So it has occurred to me that maybe my summer tires have caused me to underestimate my gas consumption...(to over report my L/100 km rate). Can someone explain the math to me?
What are the actual diameter differences? Find a website that estimates diameters or revs per mile for the two different sizes. Are your summer tires low rolling resistance rated? It's possible that the Nokians are simply better in this department. [Edit: Those consumption rates are awfully close to each other. Colder temps, wind, rain, any of these can make for a bigger difference than that.]
The 205 55 r16 is 78.1 inches in circumference; the 185 65 r15 is 76.8 inches. (-1.7%). Diameters: 24.9 inches compared to 24.5 inches. (-1.6%)
Tire size difference makes actual consumption same. Although actual tire size is going to be a bit different. Many winter tires actually have lower rolling resistance than normal summer tires. Some old summer tire test (Tekniikan Maailma about 10 years ago) had used Nokian Hakkepeliitta Q to test against new summer tires and Hakkapeliitta Q had smallest rolling resistance of all tires tested! Temperatures around 0c or 32F don’t make big difference in fuel consumption if you have pre heated car and/or trip of about 10km or 6miles or longer.
The snow tires are narrower. All things being equal, that'll help with rolling resistance. Then with snows having smaller outside diameter, it's changing the rate at which the odometer will accumulate, so the car will think you're going a bit further, comparing to the regular tires. I have similar experience. Our OEM's are 215/45R17 Michelin Pilot. Our snows are 195/65R15 Michelin XIce. Again, narrower and slightly larger OD snows. Plus the snows are rated "LRR", and the Pilots are not. With all these factors, there's little or no mpg diff between the two.
Well this is good then. On my last car (Ford Focus) the winter tires made a big difference - I think I went up at least 1, maybe 2 l/100 km...I was expecting the same this year.
Add three more to your list: c) wide tires (205) are less fuel efficient than narrow tires (185); d) new tires generally have greater rolling resistance when new than when broken in. Not knowing the comparative distances traveled by your summer and winter sets, this could be a factor; e) Even when broken in, different tire models have greatly different rolling resistances. Not knowing the model of your summer set, it could be an inefficient model. Most likely, the fuel economy difference you see is a mixture of several of these factors. Width difference will be significant. Circumference doesn't really change real fuel consumption, but shifts the odometer to create an illusion of such.
The summer tires are Falken Sincera touring. I'm not sure how long they have been on for but they have a lot of tread left.
As far as I can tell that's not a LRR tire. That doesn't make it a bad tire--just not very efficient.
