Tire Size vs Economy

No, because the overall diameter of wheel+tire is roughly the same: the tires for the 17" wheels are not as tall as the tires for the 15" wheels. If this were not so you would have speedo and odometer errors.

I think you mean "increase fuel consumption:.

The 17" tires and rims tend to be wider than the 15", perhaps because they're more "performance" oriented. So they have a bit more wind resistance. Also, I believe the weights of the tires/wheel combinations tend to be greater for the larger rims. This affects rotational inertia, which is also affected by the distribution of weight between wheel and tire: for the same overall weight, a lighter rim and heavier tire will have more rotational inertia than a heavier rim and lighter tire because more of the weight is further from the rotational axis (the axle).

But I don't believe these are huge effects on the mpg.

 

Cheap rims are heavy rims. If you try to find light weight 17" rims, prices will skyrocket.

weight (mass) in unsprung rotating parts is always bad.

[ame="http://www.youtube.com/watch?v=-3FDthcLieE"]The Science of Wheel Performance: Rotational Inertia - YouTube[/ame]
Moment of Inertia Demonstration with Rulers - YouTube
Auto Racing
The Effects of Rotational Inertia on Automotive Acceleration

 

I agree with insight owner up to the point that those weight and distribution factors don't impact fuel economy much. I don't disagree really but we are still trying to understand why the mpg impact is so great when upsizing wheels on a Prius. The usual drop is 4-6 mpg on average. Weight usually increases from approximately 32lbs per wheel (includes tire) to 36lbs or much higher in the case of the OEM Prius and Scion 17" wheels which brings total weight to approximately 44 lbs (including tire).

Weight plays a part as does increased tire width and tire design. These issues combine to drop mpg and make hypermiling more difficult so the loss in mpg can be quite high for hypermilers.

 

weight of tire, weight of rim, weight of air inside the tire (larger tire, more air), friction of the larger contact patch, airflow effects (coefficient of drag).

I went over the weight per tire in another thread and F8L and a few others discussed the weight of differing rim choices.

Recently I had the opportunity to drive behind my Prius during heavy traffic and had much time at slightly differing angles driving about 10 MPH to observe. Mine is a 2005 Prius with Yokohama Avid Envigor 185/65/15. Looking at the car the small wheel wheel guards in front of and behind the tire were almost the exact width of my tire. A wider tire would have stuck out into the airflow increasing drag.

Compared to the OEM Integrity tires my Avid Envigor are 2 pounds per tire heavier, 0.1" wider section width but another less efficient tire the General Altimax HP is the same weight as the Integrity and same width. Weight and size aren't everything, how the tire is made and what it is made of matters as well.

Another comparison would be to compare tires of the same brand/model in differing rim sizes. Can't do that with all tires as they don't often make the same tire diameter in every possible rim size but you can pick a tire that has more options like the Yokohama Avid ENVigor to show the concept

195/65R15 20 lbs. 7.8" Section Width and 5.8" tread width 25" diameter

going up to 16" rim but closest match diameter gets us to

225/50R16 25 lbs. 9.1" Section width and 7.5" tread width 24.8" diameter

going up again to 17" rim but closest match diameter gets us to

205/50R17 25 lbs. 8.3" Section width and 6.9" tread width 25.1" diameter

but many tires aren't made in that size forcing you into a 225/45/17 or other larger width tire. In this line the 225 option is

225/45R17 25 lbs. 8.8" section width and 7.7" tread width 25.1" diameter

So you can see the width can vary quite a bit without changing how the car rolls. Thinner tires that don't weigh as much save fuel but just knowing the rim sizes don't tell the whole story.

There are dozens of factors as to why different rims mean different tires which mean differing weights, widths, materials, etcetera. It all adds up and not always in the most obvious ways.

 

As others have already pointed out, don't confuse tire diameter with wheel (rim) diameter. The sizes you mention are all rim sizes, which are usually linked to tire widths, aspect ratios, and other factors I won't address here.

Absent those other factors, larger tire diameter will improve fuel efficiency in my old non-hybrid for two reasons that don't apply to the Prius:

(1) It has an Otto cycle engine, which suffers considerable partial-throttle 'pumping loss'. Power is controlled by the throttle valve, which creates a vacuum in the intake manifold. The lighter the load -- and highway cruising is a light load -- the stronger the vacuum, and the greater the fraction of the engine's power is consumed creating this vacuum.

Most hybrids have Atkinson cycle engines, which very sharply reduce this loss mechanism.

(2) It's manual transmission is geared short for performance, not tall for economy. It spins a much higher RPM than necessary for highway cruising (because Americans love torque and acceleration and hate downshifting for hills), which makes the pumping loss in #1 even worse.

The Prius eCVT is not hampered by fixed mechanical gearing, so it can optimize its effective ratio to match the immediate conditions.

In the nonhybrid, a taller tire effectively makes the gear ratio 'taller', moving it towards better economy (at the cost of having to downshift earlier when climbing hills) by reducing engine pumping loss.

But Prius has already minimized this loss by other means. That leaves the various other factors that others have discussed.

 

The man said "Absent those other factors, larger tire diameter will improve fuel efficiency in my old non-hybrid for two reasons that don't apply to the Prius: ..."

 

I thought it was pretty understood..

Part of it the rubber selection (if you switch to 17" you'd get a performance shoes, right?) but for most it is the tire height reduction. Most of energy lost by tire is in sidewalls, and the sidewalls on shorter tire have to flex more to absorb car weight.

The MPG loss in Prius is more pronounced b/c of better aerodynamics and higher efficiency. At 45MPH ~50% losses in Prius are due to tires. With respect to efficiency, 5% from 60MPG is 3, and 5% from 1.3, so it looks bigger.

 

Im not so sure about your sidewall flex vs. energy idea. Energy is lost during sidewall flex so a smaller sidewall should waste less energy.

Tire size does tend to lean towards performance as you go larger in diameter but in the case of my tires, I would not classify them as a performance tire. They are designed for luxury, longevity and wet driving yet are LRR.

 

Otto Cycle is the standard four stroke engine, where the compression and expansion strokes are the same length. Atkinson Cycle uses a longer expansion stroke, allowing for more energy extraction at the expense of reduced torque. In the context of this discussion it is not very important. What is important is pumping loss. Gasoline engines normally use a throttle plate to control power output. At low power levels this plate is nearly closed, meaning that all air flowing through the engine is being pulled through a very small opening. It's like trying to run while breathing through a soda straw.

The hybrid system on the Prius allows the engine to avoid inefficient low power operation. At low power levels the Prius uses the battery.

Your Prius has an automatic transmission. It is indeed a fixed ratio single speed transmission, but the two motor-generators provide an electrical path in parallel to the mechanical path. This electrical path provides for essentially infinite gear ratios, which is why it is called an "electrical CVT" or eCVT.

Tom

 

Unfortunately it has to bend to higher degree and that's were the additional losses come from. Imagine taking a 2" long stick and deflecting end 1" and then cutting it half and doing the same.

Other reason is that 17" tires are a little bit wider, wider contact patch, more belt to bent at contact patch. But that probably doesn't have as much impact since the contact area for 15" and 17" tire is the same.

 

Interesting, but the loss from sidewall bending is at least partially negated by the fact that the wider tires contact patch is wider but also shorter so the sidewall doesn't have to bend as much.

 

Road & TRack magazine did actual tests on the effects of different wheel sizes on a VW Golf. They tried 15 thru 19 inch wheels and noted the effects on fuel economy and acceleration.

I can't post the link because I am a newbie, but Google "Effects of Upsized Wheels" and you will locate it.

As others have stated, the effects are due to rotational inertia as well as the larger cross-section of the tire.

 

Effects of Upsized Wheels and Tires Tested - Tech Dept. - Auto Reviews - Car and Driver

yeah but then if sidewall doesn't bend much then suspension will have to..

Interesting that empirical data from article show that taller/narrower tires get better MPG. I have to disagree on their assessment that weight is to blame.


also the impact of rotational mass is x2. So the 14lbs heavier wheel will have an impact of putting extra 112lbs (14*2*4=112). If you look at overall weight of the Golf+driver and acceleration numbers, it just does not add up.

 

Fuel injected engines still have throttle plates - that's how you control the amount of air entering the engine. The fuel injection system uses a mass flow sensor to determine the amount of air getting in, and then injects the appropriate amount of fuel for that amount of air.

There are some very new gas engine designs that use variable intake valve timing to control the amount of air getting into the engine; these do not need throttle plates. I am not aware of any production cars using this technology, but perhaps it might be in some of the newer high end models.

The type of fuel injection you reference is used in diesel engines. Diesel engines pull the same amount of air for each cycle, but inject a variable amount of fuel to control power. In a gasoline engine this would lead to varying fuel ratios, with predicable bad results.

Tom

 

to my knowledge not the one sold in US yet. I think Fiat twin-air, Mazda SkyActiv (coming in 2012 Mazda3) and some boutique BMW are the ones in production w/o throttle plate, besides the diesels..

 

Why does the suspension or sidewall need to flex? If the sidewall is shorter and stiffer and the suspension is also stiffer then the energy loss should be reduced and what energy is lost is transfered to the drivers butt. lol