Who says larger wheels = less MPG

That's a dealer installed option. They're not going to change the EPA sticker for that. Unless the 18" wheel weighs equal or less than the 15 or 16" stock wheels there will be a hit on MPG.

 

Granted, wider tires probably weigh more, weight isn't all there is to it. The tire's contact patch and rolling resistance will also factor in.

 

They don't change the EPA mileage sticker even if larger and wider wheels and tires are done at the factory as an option or part of a higher line model of the car.

Since the Touring Prius with its 16" wheels and 195/55/16 tires came out Toyota has been reporting the exact same EPA mileage figures for the Touring as those for the Standard Prius with its 15" wheels and 185/65/15 tires. But when Consumer Reports recently road tested the Touring Prius, it said that it got 2 fewer MPGs than the Standard Prius.

 

I say they do! In most cases you are not going to get away without seeing a reduced MPG when swapping out to a larger wheel.

I'm running a fairly lightweight setup right now and it is killing my milage and gliding ability. I've documented the changes 3 times now and each time I am disappointed with the MPG reduction.

Like stated above, your reasoning is incorrect. An 18" wheel is almost guaranteed to reduce your mpg with all other factors being equal (weather conditions, fuel, and driving style).

Furthermore, why do you keep making references and links to this company. Do you work for them? Are you trying to plug for free advertising?

 

You may get better mileage but unless you tell the Prius computer that you go farther on a single rotation of the wheel, it will actually calculate a worse mileage because of the reduced mechanical advantage.

 

You are right. I confused you with someone else. I apologize. I stand by the rest of my post though.

 

I can't believe this thread was carried this long.

there are probably more variables but these are the main thing
1) Inertia (rolling resistance)
2) Contact
3) Weight

Keep in my that inertia grows in the third power. So even with lower weight... 15 will outperform 18 inch.

Also i think there's a post bout how the car is calculating the wrong rotation due to larger diameter wheel... if the 18" is a low pro and have the same overall diameter it shouldn't matter...

either way... Larger wheel = less gas (it's physics so let's not argue)
even if hold volume and density constant

 

Here is the pic for lazy people like me.

 

ummm... i did college physics many many moons ago so the memory is pretty dim, but pretty sure that as the diameter increases more torque is required to achieve equal acceleration... so unless you GREATLY change your driving habits, your mileage will go down

 

It is overall diameter (tire edge to tire edge). Also, it is rotational mass that impacts the hit on mpgs. You can have a 17 inch wheel with a tire that has the same diameter, width, weight and rolling resistance as the stock 15 inch wheel & tire combination. With those parameters being equal, there will be no change in mpgs. Start changing things and mpgs will suffer.

 

Aye but the problem is you cannot find a 17" or larger tire with a lower rolling resistance, weight, and efficiency-based sidewall construction (to reduce tread and sidewall deformation resulting in loss of energy via heat).

 

True that. I was being theoretical. I have OTH found a wheel & tire combination in 15 inches that give great handling at no mpg penalty....

 

I have a Camaro that achieves better miles per gallon with 28" tires as opposed to the 26" stock tires. It is like putting numerically lower gears in the car. The size of the wheel is just the wheel, but the overall tire size is what can make the most impact.

 

1. The 18" wheels are not "stock". They are dealer added. They are not offered by Toyota USA (or any other division of Toyota) for application on a Prius.

2. Fuel economy is affected by rotational mass (among other things). The overall diameter of a Prius wheel/tire combo should be kept at the stock 15"/16" wheel/tire value. An 18" wheel is heavier than a 15" wheel. An 18" tire is also heavier than a 15" tire, even when you keep the overall diameter the same. A heavier wheel/tire combo will give you poorer mileage. Those on this forum who have "blinged up" their cars with larger wheels/tires (both 17" and 18" have been reported) report much poorer mileage.

Please, you also should do the research before posting such things as fact!

Now, it may be possible to go from a stock 15" wheel to the Volk Racing 16" wheel and keep the rotational mass the same. You'd have to do some research to see if you could get a 16" tire that is light enough to not increase the overall weight of the wheel/tire combo (the Volk 16" eco wheel is much lighter than the stock 15" one).

 

These are very good points that you raise... On my car I have lightweight Centerline 15" rims (stock was 16") coupled with the taller (and lighter)
tires. I also have an aluminum driveshaft to further reduce unwanted rotating mass.

 

Not to mention that even if you could find a larger wheel the same weight as a smaller one, the larger will will have more mass further away from the center of the wheel which also means higher rotational mass.

 

Now you're complicating the argument with facts!

Add to that the fact an 18" very low profile tire will be heavier than a 15" or 16" "moderate profile" tire, putting still more weight out at the edge of the wheel/tire combo.

But some really want to be losers, because it looks "cool".

 

The key term here would be "centrifugal inertia." Even if total weight, rolling resistance, and contact patch size are equal, the energy required to move the wheel from a halt (or accelerate in any fashion) will be greater than that of a wheel with a smaller diameter. Besides taking a hit to your fuel economy (and you will) there is a performance sacrifice as well - which makes me wonder why so many performance cars come fitted with large wheels.