Icncrease MPG with taller tire

<div class='quotetop'>QUOTE(ken1784 @ Sep 18 2006, 11:20 PM) [snapback]321495[/snapback]</div>

Thank you for another superb reference Ken. How do you find all these? Are you an automotive engineer? According to this reference we should count ourselves lucky, as this regulation doesn't allow for a speedometer that might possibly underestimate the speed at all, but if it can overestimate the speed by more than 10%.

Regarding the odometer, I used highway mile markers and my GPS's trip record to cross check the odometer on a 20 mile stretch of highway. According to the mile markers and the GPS, the odometer read too high by 0.2 miles or 1%. This was on nearly new 185/65HR15 Dunlop Sport A2 Plus tires holding 40 psi in front. As tires wear and their radius decreases by 2%, I expect the odometer would over-read by 3% on worn tires.

Results will vary with different tires in the same size and different tire pressures and vehicle loads. This highway stretch had some gentle inclines which might have caused an error in the GPS measurements assuming it wasn't considering minor altitude changes. The mile markers and the GPS were in agreement.

 

An interesting discussion. In the bad old days, taller tires did help gas mileage, because it basically altered the final gearing, and cars of the day were typically turning high rpms at crusing speed. So, if you sacrificed some acceleration to enable you to cruise at 65 at 3200 RPM instead of 3500 RPM, then it was worth it. With modern cars, taller tires in most cases are probably getting you into a point of negative returns since the axle ratio was probably selected with an overdrive transmission in mind etc. And with a Prius, I'd tend to think they've optimized all those factors already.

Anyway, it seems like compensating for the ODO reading and finding your true MPG should be fairly simple if you have installed taller tires. Just find the percentage that the ODO is off, and factor that back in. If your tire size is such that 1 mile observered on the ODO is actually 1.04 miles traveled, then just multiply your MPG by 1.04 to get actual MPG. I've actually considered taller tires, mainly because I drive on some pretty rough roads with rim-eating pot holes on occasion. The speedo/odo being off I could mentally adjust for, but I'd really hate to stomp too hard on mileage. Anyone actually running taller tires?

 

<div class='quotetop'>QUOTE(Beryl Octet @ Sep 21 2006, 10:34 AM) [snapback]322802[/snapback]</div>

I don't think you'll find any 185/70-15 tires. I'm running taller and slightly wider 195/65HR15 ContiProContact tires. Many who haven't spoken up elsewhere in PriusChat are running slightly taller and wider 205/60-15 tires. FreshAirGuy's 215/50-17 tires are much taller (radius not sidewall) and much wider on bigger rims.

 

<div class='quotetop'>QUOTE(MechaJohn @ Sep 20 2006, 01:31 PM) [snapback]322431[/snapback]</div>

I'm not the original poster, but I thought I'd add my two. I had a similar physics discussion a long time ago with some engineer friends about using compact a spare tire.

I think, from the physics angle, we're sort of missing the point. First of all, in a frictionless environment, at a constant speed, the car will require no power to keep its speed constant.

There are a few major factors slowing down the car in the real world:

1. air resistance
2. rolling friction on the tires
3. friction in the wheel subsystems as the car rolls forward

To keep the speed constant, we need to supply enough power to offset the energy lost above. The amount of energy you need is related to MPG. The more power you need over a given distance, the worse your MPG will be.

Anyway, by changing the tire profile, we affect the above characteristics.

Air resistance is going to be about the same at a given speed, regardless of the tire height. The car might get a little extra height, but I doubt it will change the car's wind characteristics much.

By changing the tires, we are modifying the last two factors. Bigger tires tend to deform more, and I suspect that the rolling friction on higher profile tires will be greater the "normal" profile tires. Note that different tires of the same size have different characteristics just due to the rubber. Oversizing the tires just adds another variable.

However, as someone else pointed out, the bigger tires will provide a higher gear ratio and cause the wheels to rotate more slowly. The loss of energy due to friction in the wheel subsystems will be decreased.

My gut feeling is that the increase in rolling friction is greater than the benefits from getting the wheels to spin less, but I don't have anything more than anecdotal evidence to back that up.

We would have to do a similar analysis to see how the power efficiency of the car is affected when the tires are changed. Unfortunately, though, I'm out of time. I think I'll do another edit/post later.

Chris

 

<div class='quotetop'>QUOTE(Chris Hayashida @ Sep 21 2006, 01:49 PM) [snapback]322917[/snapback]</div>

I'd think a a larger taller tire, inflated at the likely pressures a Prius driver here would inflate them to wouldn't lose much to "deformity", i.e., energy lost as heat due to sidewall flexing. If that 185/70 15 did exist, I'd expect it to have less friction than the stock tire because the contact patch is smaller becuase of the width, assuming similar tread and rubber compounds of course. Hmm. They do make a 195/70 15, wonder if that would fit on the stock wheel?

 

<div class='quotetop'>QUOTE(Chris Hayashida @ Sep 21 2006, 01:49 PM) [snapback]322917[/snapback]</div>

I don't know of any careful tests on cars, but I did read some on cycling tires. The conventional wisdom was that skinnier road tires offer lower rolling resistance than wider road tires. Careful testing revealed that the benefit of the skinny tires was all in the lower air resistance and lower mass. The wider tires, with larger contact patches, deformed less and had lower measured rolling resistance in most cases than the skinnier tires, especially if inflated to the same pressure. I don't believe that this has clear implications for the Prius, but still thought it's interesting.

I do believe that the aerodynamic effect of tire height is relevant. The exposed front and back of the tires are the least aerodynamic portion of the car. Many test vehicles will lower a car even an inch to improve the aerodynamics. On the modified top speed prius they used custom tires (and rims) that were very narrow and very tall. They also modified the suspension and wheel wells so that the car would ride lower than stock, even with taller tires. (They also taped body panel seams and covered the grille on one test run.)

 

<div class='quotetop'>QUOTE(theorist @ Sep 21 2006, 10:21 PM) [snapback]322762[/snapback]</div>

I usually get these materials on forums, but I sometimes forget the url. Google is my friend too.
I'm a computer engineer and an amateur in the automobile industry. I just would like to let everyone know the correct information.

Ken@Japan

 

<div class='quotetop'>QUOTE(theorist @ Sep 21 2006, 03:13 PM) [snapback]323107[/snapback]</div>

Interesting. I'm not sure what to make of it, though. I would not have guessed that wider tires had a lower rolling resistance. I guess I could see how it might, since the weight would be spread out over a larger area. However, I still think that tires of the same width with a higher aspect ratio would deform more and have a greater rolling resistance.

<div class='quotetop'>QUOTE(theorist @ Sep 21 2006, 03:13 PM) [snapback]323107[/snapback]</div>

I agree that the height of the car does have an impact on aerodynamics, but I find it hard to believe that it will make much difference at the speeds we are discussing. For top speed tests, it makes sense that the Prius should be as aerodynamic as possible. Aerodynamic drag is related to the square of the speed of the car, so you would see a greater effect due to aerodynamic improvements at higher speeds.

However, switching from P185/65R15s to P185/70R15s is a difference of about four-tenths of an inch.

(185 mm * .65 / 25 mm/in) + 15 = 19.7"
(185 mm * .7 / 25 mm/in) + 15 = 20.4"

It's a difference of about .7", so the center of the tire is higher by less than .4". I can't tell how much of a difference that would make.

I'm beginning to think that the only way to tell is to measure. Anyone want to take one for the team and invest in an extra set of tires and rims?

Chris

 

Aren't the Prius wheels intentionally smaller than conventional wheels because that improves milage?

If anything it seems the Prius would be tuned to get the best possible milage for the wheel size it ships with. Although hybrids like it in general might stand to benefit from smaller or bigger wheels, the way the Prius is tuned you can probably only do worse.

 

<div class='quotetop'>QUOTE(SoopahMan @ Sep 22 2006, 07:56 PM) [snapback]323296[/snapback]</div>

It is not a good judgement just comparing with tire sizes.
We have to think about it as a total system.
- Final drive ratio
- ICE rpm at max. torque
- ICE rpm at max. power
- Torque split ratio of the PSD
and so on...

Ken@Japan

 

actually the concept of a taller tire is a good option to explore. normally the drawback to this would be the increased torque required to turn a bigger tire that might lead to less efficiency in a normal transmission, especially one that was tuned to run the stock size.

with the CVT on the Prius however, that would become a non issue. therefore on situations where gravity was helping, a larger tire would seem to maintain speeds better since, gravity or no, intertia would be greater. (iow, slower to gain the speed but also slower to lose the speed over a small circumference of rubber)

 

If everything else stays the same, a taller tire will provide better economy, until a point where the vehicle does not have the power to stay in an efficient powerband zone.

I have 28" tall tires on my camaro, which originally came with 26" tires. The taller tire provide better miles per gallon. They are also lighter than the original tires. However the compound in the tires is stickier which increases rolling resistance.

I went to the taller tire to slow the car down as it was covering 1/4 mile quicker than 12 seconds and since I race the car on the weekends and the quickest I am allowed to dial in is 12.00 seconds in street class, I needed to slow it down. As a side benefit my gas mileage increased by over two miles per gallon.

The only reason i know this is because I know the mileage from the track to my house. I have no speedometer (unplugged) and no fuel gauge so I fill up before i leave for the track and top it off at the same convenience store when i arrive in New Hampshire.

My suspicion is the Prius would not have enough usable torque to handle a tire much taller than it comes with and the taller sidewall would adversely affect the already stressed handling characteristics of the vehicle in stock configuration. I think Toyota did an admirable job in putting together the package and would tend to leave it alone.

 

<div class='quotetop'>QUOTE(Chris Hayashida @ Sep 22 2006, 04:47 AM) [snapback]323273[/snapback]</div>

Hi Chris,

Wouldn't it be (2 *(185 * .65 / 25.4)) + 15 = 24.47 ? And the 185/70 result would be 25.20. Or a .73 inch difference in diameter ?

 

<div class='quotetop'>QUOTE(tnthub @ Sep 22 2006, 05:45 PM) [snapback]323566[/snapback]</div>

Hi Tnthub,

Sure the Prius would have the torque to handle the bigger tire. Lets see we are talking a change in wheel torque of 25.20/24.47 or 1.03 - 3 percent increase.

Maybe what you meant to say is that the Prius motor being a fixed ratio to the wheel and thus fixed in maximum torque it can provide. But the Prius engine being variable ratio, that engine and generator would run at higher rpms to deliver the increased torque needed at the wheels? The generator at a higher speed would generate more power for the motor, so less power will come from the battery during accelleration, as the control is only going to let so much current to the motor. And the engine at higher RPM through a higher ratio, would provide more power to the wheels during accelleration.

Not sure if the cruise fuel economy would get any better, but it sounds like the Prius might accellerate quicker with larger diameter tires, as the engine would run at higher RPM for a given speed and use less battery. Which would result in short term cruise economy that was better, as less battery power would be needed to be replaced.

There is a program put out by the NREL, that runs on MATLAB that would be able to figure this out. If you have Matlab (which is like $5K) the free NREL ADVISOR program that plugs into NREL has Prius models. I imagine a tweak of tire diameter in the model will answer all these questions.

 

<div class='quotetop'>QUOTE(donee @ Sep 22 2006, 08:10 PM) [snapback]323663[/snapback]</div>

Does anyone know if these simulations are compatible with Octave, the GNU freeware analog of Matlab?

 

I've had my Prius option #3 for a few weeks and I make the same 90-mile highway, moderately hilly, drive three times each week. On those drives I have been achieving 50-50.4 mpg with the OEM tires. About a week ago I checked the tire pressures for the first time. Each tire had only 29 PSI. I pumped each to 38 psi. Since then I did the same 90 mile trips three more times. For those three trips I achieved 50.1-50.4 mpg. No mpg gain after adding 9 pounds to each tire. I think the effect of psi on mpg is seriously overrated.

 

<div class='quotetop'>QUOTE(pogo @ Sep 14 2006, 05:16 PM) [snapback]319697[/snapback]</div>

Actually I had caught the error but had not gotten back here to admit it. You are right about 4% of course. More importantly there seems to be a dramtic drop in MPG. I had been averging 45 MPG and my two refills since I changed wheels and tires are about 40 MPG. The tires weigh more and the larger wheels weigh more. The 11% drop in MPG is no fun at all. The improved handling and great chrome wheel looks are not cheap.