Icncrease MPG with taller tire

At 65 MPH, my speedometer in 1 MPH or less off from My GPS. The GPS updated once per second and, as far as I know, the accuracy is dead on. At the time the test was made, I had 3100 miles on the orginal tires. Some of this error is based on the round off on the speedometer, which shows whole numbers, while the GPS shows speeds in tenths (ie, 64.5, 64.6, etc).

 

<div class='quotetop'>QUOTE(syclone @ Sep 13 2006, 01:16 PM) [snapback]319096[/snapback]</div>

Good to know syclone. I checked mine the same way, except I had 16,000 miles on the OEM tires and checked it at 70 MPH. It was about 2 MPH fast. I then replaced the 855 rev/mile OEM tires with some that were 847 revs/mile. The speedometer was a bit less than 1 MPH fast with the new tires. I didn't bother to check the odometer but I wish I had. I would guess the difference between my test and yours is mostly due to tire wear on my old tires and maybe a little bit due to the extra 5 MPH, I noticed the error increases as you go faster as would be expected.

 

<div class='quotetop'>QUOTE(Kaos1 @ Apr 13 2006, 06:58 PM) [snapback]239470[/snapback]</div>

The bigger tires probably weigh more. And yes, it does take more energy to move them. Either to accelerate or decelerate. If I was to guess, the bigger tires would be less efficient than whats on the car from the factory. Just my opinion.

 

<div class='quotetop'>QUOTE(Tom_06 @ Apr 5 2006, 07:50 AM) [snapback]235193[/snapback]</div>

Using the Maita calculator there is 4% speed difference at 60 MPH if you change 185 65 15 tires for 215 50 17 tires and wheels like I did. Because of the lower profile the actual diameter of the entire wheel is only 1 inch greater. The 4% means that I am going 62.4 MPH when the speedometer says 60. The circumference of the larger tires is 3in. larger than the originals. The larger wheels therefore turn 32 times less (revs) in a mile. That means the odometer is off 96 inches per mile. Since there are 63360 inches in a mile, that is not off by much (.001%).

 

<div class='quotetop'>QUOTE(FreshAirGuy @ Sep 13 2006, 08:39 PM) [snapback]319301[/snapback]</div>

I'm not sure where the 96 comes from. Your 25.46 inch tire will make about 32 fewer turns/mile than the original tire. Meaning when the odometer thinks you've done a mile you've really done a mile plus 32 X 79.96 inches -- an error of 2559 inches. 2559/63360 is (surprise) 4%. (proving once more that there is order in the universe and the math really does work )

The bigger issue with the idea is what others have toucned on. The extra weight of the wheels aside, it is simply not possible to alter the amount of energy required to move your car by changing the size of your tires.
(Physics works too.)

Outside of the theoretical, frictionless world of pure physics, however, it should certainly be possible to cause an adverse change due to tire weight, available torque, etc.

Edit: I guess I see your error now. You were approximating pi with 3. An okay thing to do, but the problem is you had to consider the entire circumference of the tire in your error calculation, not just the 3 inch change.

 

I think Pogo is right about the first order physics issues (of course). The thing the tires will do is change the effective gear ratio between the engine and the car's movement. If the Prius had a manual , fixed ratio transmission, the engine would run slower with the larger tires. So, the optimum load RPM would occur at a higher speed. But at the higher speed there would be more air-drag. Sounds like a tradeoff, where the lower speed fuel economy would get worse, and the higher speed fuel economy would get sligthly better until wind drag dominated and pulled it down.

But the Prius has the ECVT. Its probable the ECVT would just change effective gear ratio to get the engine back up the optimum speed and torque. Expectation : no change in fuel economy (Based on actual miles traveled, versus fuel consumed). But the displayed fuel economy (wheel measured miles traveled versus fuel consumed) might show a difference.

 

benighted,

Where do you get your information? Why would an auto manufacturer design a car to misrepresent the actual speed?

All,

I would imagine that fuel economy would be worse with larger tires. The reason is that the motor and ICE would have to generate more torque to propel the car the same speed.

Torque is force x distance. The force to propel a car at a given velocity can be assumed to be constant (ignoring changes in road conditions, headwind, etc.). The distance is the radius of the wheel. Larger wheel=larger distance=increased torque.

Also, as others have noted, with larger wheels your calculated mileage and MFD would both be wrong, because the odometer would be wrong. More distance traveled on each revolution of the wheel means that speed would actually be higher than the speedometer shows and mileage would be greater than the odometer shows.

Actually, it might be a way, albeit sneaky and underhanded, to have low mileage on your odometer for resale purposes.

 

<div class='quotetop'>QUOTE(MechaJohn @ Sep 16 2006, 06:44 AM) [snapback]320262[/snapback]</div>

So, in your theory, smaller tires improves the fuel economy.
Do you know any good reson that Prius does not have smaller tires such as 13 inch or smaller?

Ken@Japan

 

<div class='quotetop'>QUOTE(MechaJohn @ Sep 15 2006, 02:44 PM) [snapback]320262[/snapback]</div>

Although nothing above is incorrect, you've reached the wrong conclusion.

As you say, the force required to propel the car at a given velocity is constant (all else be stiputated to be held constant) This means exactly that the energy required to propel the car at this velocity for a fixed distance is constant. Although the larger wheels will require greater torque for a given angular velocity, for a fixed car velocity the larger wheels will turn slower. Power being the product of torque and angular velocity will be constant for a given linear velocity of the car, and energy expended being the product of power and time will be constant. If you're talking about a constant angular velocity for the two different wheels, then the linear velocity will be higher for the larger wheel and the time required to cover the distance proportionally less. Power input goes up, time goes down, energy consumption remains constant.
Bottom line -- All else being held constant, you cannot alter the energy required to move the car by changing the size of the wheels.

 

I chose a size 195/65HR15 ContinentalProContact for a few reasons. The car was nearly new when I had these tires installed and I didn't use the same tires in a smaller size, but it seems it seems that there wasn't a large effect on fuel consumption.

The speedometer accuracy improved somewhat as I detailed in other threads.

It's only a quarter inch difference in the rolling radius and sidewall height so I didn't expect much of a difference. I chose the larger size for several reasons

• larger contact patch for better dry traction
• taller sidewall for a smoother ride with high pressures over Boston potholes and roads
• high inflation and tall sidewall as an alternative to a shorter sidewall and lower pressure
• corrected odometer and speedometer inaccuracy
• less unsprung weight for potentially better handling and fuel efficiency -- for some reason the ContiProContact weighs less in the 195/65HR15 size than the 185/65HR15 size.
• possibly longer treadwear from more tread

<div class='quotetop'>QUOTE(FreshAirGuy @ Sep 13 2006, 11:39 PM) [snapback]319301[/snapback]</div>

Do the 215 50 17 scrub? I thought they might rub on the wheelwells or struts when turning, going over bumps, or carrying heavy loads. The 195/65-15 don't, but the 215/50-17 are so much larger.

 

<div class='quotetop'>QUOTE(theorist @ Sep 19 2006, 12:55 AM) [snapback]321153[/snapback]</div>

The odometer is most accurate with the standard 185/65R15 tires, but speedometer is different story.

Ken@Japan

 

<div class='quotetop'>QUOTE(ken1784 @ Sep 18 2006, 06:11 PM) [snapback]321424[/snapback]</div>

Elaborate please. I'm sure they're driven by the same input. But how they process that input can be different. What do you know? Inquiring minds want to know.

 

<div class='quotetop'>QUOTE(pogo @ Sep 19 2006, 11:06 AM) [snapback]321456[/snapback]</div>

If you took your own data using handy GPS, highway mile post or whatever, you'll see the odometer is accurate within about 0.5% error with standard 185/65R15 tires.
OTOH, the speedometer value is intentionally biased a few % to meet the ECE regulation No.39.
http://www.unece.org/trans/main/wp29/wp29regs/39rv1e.pdf

Ken@Japan

 

<div class='quotetop'>QUOTE(pogo @ Sep 16 2006, 12:51 AM) [snapback]320444[/snapback]</div>

Pogo, you're right about the power, I didn't think it through from a power standpoint. Since wheel radius is in the numerator in the torque equation and in the denominator for calculating angular velocity, it cancels itself out. However, since engines produce different horsepower at different rpms, I think my conclusion still holds true. That is the car wouldn't be as efficient. The optimal efficient rpm of the engine would occur at a higher vehicle velocity which requires more energy to maintain.

Your thoughts?

 

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

My thoughts are that I'm not smart enough to say. The math and the physics are clear, but in the practical application you're
dealing with a number of effects that you can neither control nor easily quantify. My gut (which is substantial) tells me that the effects of a 1 inch greater tire diameter are probably small compared to the other variables that drive your FE from trip to trip and tank to tank. I doubt you would ever successfully isolate it. (If it were a significant enough difference to detect easily, the question would have been answered by now.)



<div class='quotetop'>QUOTE(ken1784 @ Sep 18 2006, 08:20 PM) [snapback]321495[/snapback]</div>

Ken, thanks for the link. Interesting. BTW, do you know what the nominal bias is in Kph? Is it a constant percentage bias for kph vs. mph?

 

<div class='quotetop'>QUOTE(pogo @ Sep 16 2006, 12:51 AM) [snapback]320444[/snapback]</div>

I'm late to the discussion, but I would guess that it there would be no real change in a car like the Prius.

In a conventional car with an automatic or manual transmission/transaxle, larger tires would increase the final drive ratio (decrease it numerically) which would have the effect of moving up in the speed range the transmission's shift points.

That would tend to yield poorer acceleration from standstill but more efficient cruising.

I suspect that, with the Prius, however, it will always attempt to find an optimal combination of the heat engine and the electric motor, which would negate any negative or positive effects of increasing the overall circumference of the tires/wheels.

 

<div class='quotetop'>QUOTE(pogo @ Sep 21 2006, 07:22 AM) [snapback]322505[/snapback]</div>

Japanese 2004-2005(first) 6 km/h bias and 2005(later)-2006 3 km/h bias.
Camry hybrid owners are observing 3 mph bias.

Ken@Japan

 

<div class='quotetop'>QUOTE(ken1784 @ Sep 20 2006, 05:48 PM) [snapback]322579[/snapback]</div>

For part two of my question -- are you saying that it's a constant offset regardless of speed?

 

<div class='quotetop'>QUOTE(pogo @ Sep 21 2006, 11:15 AM) [snapback]322630[/snapback]</div>

6km/h bias means we see 66 km/h at actual 60 km/h and 106 km/h at 100 km/h.

Ken@Japan