I really am surprised that only going up one width in size affected your milage that much. How many tanks did you go through to make that determination?
I switched to 195/65x14's, also the BFG's, and my overall tire height remained the same (NOTE the 14" size). I have about 7k mile on the new ones so far and gas milage has not changed at all. Like you said though, the ride quality has been a big plus.

Walt

 

I chuckle at the fact that some folks seem to think that their speedometer and odometers are dead accurate with the stock tires. Fact is, the speedometer reads about 5% high (57MPH at 60MPH indicated) and the odometer reads about 4% low on my stock-tired xB, when compared to a GPS over several hundred miles.

The other shaky procedure is assuming that filling the tank brim full will result in the same amount of gas in the tank each time. My experience over 22,000 miles of driving is that "full" can vary by a gallon or so with slight changes in vehicle orientation. The only way to get accurate mileage data is over several tankfuls where this variation is minimized.

George

 

seriously a 10mm wider tire is not going to make a difference in gas mileage or handling.....

 

I would expect more difference between brands than between such similar sizes. Tires with high treadlife and low temperature ratings should get better mileage, although they will probably not be optimum for handling.

George

 

With all other things equal (weight, hardness, tread, etc) if the OVERALL diameter of the wheel and tire is increased, you are requiring more torque at the axle to do the same amount of work at the ground ( T = Fd), so theoretically you should notice a slight decline in mileage in this scenario. While cruising down the road, you are requiring the engine to turn more slowly, but during all accelleration (when you use the most gas.. and where you spend a majority of the time in daily driving) you are requiring more work. So on long road trips you may not see much difference, but I would expect it to become more evident in average mileage.

And I have been asked before, so I will clarify:

T = torque at the axle
F = Force applied to groud (tangential to tire)
d = radius of wheel/tire combo

So if T = Fd, then F = T/d , so if F stays the same, and d increases, so must T

 

would you say that the comfort is improved over stock? i was considering downsizing to 14" lightweight rims with 185/70-14 goodyear comfortreds. i would think that the smaller footprint and the lower rotational mass would help the mileage a little as well..

 

But, work equals angular displacement times torque

W=[theta]*[tau]

If you make theta smaller by increasing the radius, then increase tau to maintain the same force at the periphery of the wheel, then you will get the same work out of the system as you would with the stock setup. Conservation of energy and all that.

 

What he said. I don't know where you're getting better mileage from. Larger tires only have more rolling resistance and more weight. That's why the tires on a small car like the Scion are small to begin with.

 

What he said. The amount of work done is the same. The only thing you can change is efficiency. Like I said, bigger wheels/tires are less efficient than smaller ones.

 

It takes a larger amount of torque to get the wheel up to speed. That is in-disputable. Once there, a larger wheel has an advantage, which I stated in my original post. However, (again stated in my original post) in most city driving you spend more time accellerating and decellerating. During the accelleration time you are needing to create more torque to speed up the wheel (given the same value of accel. between wheel sizes). So, unless you start driving like granny when you speed up, you are using more fuel than with the smaller wheel. On long road trips you will see the scales tip back a bit.

So, W = theta*tau,

So if the diameter of the wheel increases, the angle through which the wheel travels for a given tangential travel on the ground decreases. If torque stays the same, the amount of work taken to travel a given distance is decreased.

But, in order to accelerate the vehicle at the same rate of speed, you must apply more torque to the axle to get things going.

So we can see that it is a balancing act between accelleration and long term rolling. So, if you want off the line performance, or are driving in the city most of your time, you will see an advantage with a smaller wheel.

This topic has been discussed by physicists, gear heads and even bicycle racers for quite some time. It all comes down to what you are doing most with the vehicle and what you are trying to gain. As with anything you cannot look at one effect and not the others.

 

I can dispute it in that the effect on mileage and acceleration of the increase in moment of inertia is negligible. You're talking a few pounds in 2500! The real effect comes because of differences in rolling resistance which varies by size and design of tire.

 

I do agree that the rolling resistance is likely the largest factor. One thing you all did not mention is the impact on aerodynamic drag from the larger tires.......The car's further from the pavement with more frontal area.

 

The lightweight Perrin crank pulley (stock diameter) is almost 4 lbs lighter than the stock pulley. However, its affect is equal to removing 100lbs of weight from the vehicle. Rotational mass does make a difference.

Most upsized wheels are heavier than stock. Assume each tire/wheel is 5-10 lbs heavier. If the engine and trans were on a stand and the wheels were thought of as JUST flywheels, it definitely would take more energy to accelerate the heavier wheels. Conversely, it would take more energy to decelerate the wheels.

A lot of people are noticeing the diffence heavier wheels are making in gas mileage. I wonder how much faster their brakes are wearing.

 

But we are not talking a few pounds out of 2500 when we are discussing torque effect. We are talking more like 5 - 10 lbs out of 160. The increase in torque needed to provide the same acceleration value for larger wheels is directly proportional to the increase in wheel size. Again, when in the city, this part of driving (Acceleration) is much more prevalent.

 

MORE INFO:

I was also surprised about the gas mileage change to. For accuracy, I went though 4 tank-fulls doing the same kind of commutes to work, traveling the same roads that I always do. I let it run down till the gas-light came on and noted that the distance traveled and the amount of gas replaced was about the same at each fill up.

Also, before I put the larger tires on, I not only measured the height and width difference, but also checked the new tires weight. I noted that the larger tires were 3.4 lbs heavier (each) that stock.

The thing that bothered me the most was the sluggish acceleration at take off. Long ago when I was doing research, I found a tire size calculator on the net that also calculated gear ratios, but did not think that the change would be enough to make a significant difference.

The clincher was the strange vibration at high speed (65+) that occurred in the front end. I had the xB taken back to the tire-store and had the tires re-balanced, and the alignment checked, both of which were fine, I even had the tires rotated, but the vibration still persisted.

Finally, I switched from the larger 195/60/15 back to the OEM 185/60/15 size, but I decided to stay with the same “BF Goodrich traction T/A” brand.

Maybe the new tires were defective? Maybe the xB isn’t geared for slightly larger tires? Maybe the added weight was too much? Whatever the reason, the BF Goodrich tires are a lot better than stock as the box rides a little smoother, has lots of traction, and handles great.
Yup, I still love my xB.

 

Let's see some calculations that show that it takes _significantly_ more energy to accelerate a stock crank pulley or heavier custom wheels to speed, as compared to the energy needed to get the remaining mass of the car up to speed. I really doubt that the difference is significant.

What everyone seems to have missed is that those larger, heavier wheels also come with larger, stickier, lower profile tires that are not designed for low rolling resistance. More than one thing has changed, but everyone seems to concentrate on the weight of the wheels.

A Perrin pulley being worth 100 pounds of car? Compare the reduction in rotational inertia due to this pulley to the rotational inertia of the stock flywheel. It's tiny, because the flywheel is specifically designed to have a large rotational inertia. Lighten the flywheel and you'll get a significant effect, but the lighter pulley's effect is tiny compared to that of a light flywheel. Why have a lighter pulley instead of a lighter flywheel? Because the pulley can be changed a lot easier than the engine can be split from the transmission. Of course, by fiddling with the mass on either end of the crankshaft you are risking harmonic vibration problems,

 

^^ I wasnt reffering to heavier wheels, but I think you were aiming that at his post.

As far as the effects on torque, it is a proportional ratio. Whether we think of a car which is not in equilibrium (accellerating) or one that is in static equilibrium (cruising at a constant speed) the effect is there. Lets say you start with a wheel/tire combo with an 11 inch radius, and increase that to a 12 inch radius (9%). Now, lets assume you would like to drive the vehicle in the same manner as before. That 9% increase equates to a 9% increase of torque needed to keep the same performance capability (force applied to the ground in the direction of movement) . Now, that is not a small effect in the least.

Now this is offset by the lowered rolling resistance of the larger wheel, especially during constant speed driving (where rpm levels will be lowered due to larger wheels). But during all of those takeoffs you either are going to drive it much easier or work the engine harder.

So either way, considering all peices, this will not have a large effect on the performance, but it will have some effect. Is it enough to worry about (especially on a street driven vehicle)? Probably not, and the comfort of the larger wheel is worth it.

And as far as the lightweight pulleys, especially those with no harmonic balancer, the small increase in throttle response is not near worth the potential effect of removing balance from the crank IMO. They are balanced for a reason. There is a small advantage to them when you start looking at the rotational physics involved, but it is not a huge difference.

 

I'm not sure that the Scion even has a harmonic balancer on the stock pulley.

However the difference in moment of inertia for the entire rotating mass of the engine (pulley, crank, flywheel) with the light pulley is so tiny that any perceived difference is due to the lightening of the owner's wallet.

Having played with lightened flywheels on VWs, I've found that they can be a good change for a light car with a relatively powerful engine. Put one in a heavier, less powerful car and all it does is make the clutch harder to use. The Scion tend to fall into the latter category, as it has a modest power output.

George

 

Just to add more technical observation to the mix:
The weight of your wheel/tire combination DOES have a large affect on actual power and acceleration available at any speed. You can directly calculate how a weight increase (or decrease) affects the amount of horsepower that is wasted just getting the tires to turn (overcoming unsprung friction forces and rotational mass). The amount of horsepower left over is how much is actually left to accelerate the vehicle. At low RPMs, your engine isn't producing that much horsepower...so the wheel/tire weight has a large affect on "off the line" acceleration [the lighter, the better]. However, at high RPMs (assuming the wheels are already rotating), a larger diameter tire can effectively change your final gearing ratio which can give you better top end.

MOST IMPORTANT: Remember this vehicle just has a little 1.5L engine in it...so the tire/wheel combination can have a much more significant impact on the performance and effieciency of the engine overall.

 

I'm still waiting for some calculations to back up the assertion that heavier wheels would have a _measureable_ effect on acceleration or economy. Saying that there is a "large affect" doesn't show why such an effect would happen. "Seat of the pants" observations are subject to significant error.

Yes, I know how to do the calculations, but it would be nice if someone showed that they actually understand the physics involved in this situation.

George <--still boggling at the concept of "unsprung friction forces"