Tire size effect on calculated vs. real fuel economy..

Find a highway with mile markers.

Drive a number of miles. The more, the better. Keep track of how many full miles you drive.

When you start your trip, reset your odometer. Keep track of how many miles you drive.

Compare the actual miles to the count from your car. Create a conversion factor.

My old one was to multipy actual miles times 1.0646 when figuring in miles for maintenance and mpgs.

Alternately, you could find the rotations per mile of your old tires and your new tires and do it that way.

tirerack.com.

 

Your choice of tires should affect fuel economy in the following ways:

1. Rolling resistance, which is probably the only factor here that'll make a measurable difference.

2. Effective gear ratio - revs per mile. This is where taller tires benefit most conventional cars, which are geared shorter than most fuel economy minded drivers prefer. But with a CVT, you can do 1000 RPM at highway speed anyway. I don't think you'll notice much whether it's 43, 45, or 47 mph at which you can shut off the engine.

3. Aerodynamics. Tires are aerodynamically very ugly. Very blocky. Skinny tires are better.

4. Aerodynamics. If the tire affects the ride height, it will affect the air travelling under the car.

5. Weight, and rotating mass. Extra inertia hurts every time you brake.

As to which of your tires are more efficient... you haven't got enough data to calculate it. You should grab a GPS, find a quiet road, and do some coast-down testing. Put it in N and let off the parking brake at a spot with a gentle downhill, and see which tire lets you roll the farthest. Time how quickly you lose speed from 55 mph in neutral, and see which tire makes you the most aerodynamic.

 

Tire manufacturers routinely do tests on various vehicle models to determine the best combination of mpg. cornering, handling, and comfort.
Generally speaking mpg depends pretty much on tire diameter and weight. Mpg goes down with increasing weight and diameter. While larger diamers have less rolling resistance, the weight is at a greater dstance from the hub center and that increased torque to turn the wheeloften overrides the reduction in the rolling resistance. If you drive far at constant speed the bigger tire is better.
Because he vehicle weight, gearing and engine varies so greatly its impossible to write a formukla relating the mpg vx tire size. We've all tried; the closest anyone has come relating the torque to rotate the wheel/tire fr comparison but rarely did we get a relationship between mpg and tire size. In short, here are the rules for changing wheels/tires
1. when up sizing (to a wider, heavier tire) reduce the diameter of the tire to try to keep the torque the same. Remember when you go shorter the mpg may appear to increase because the wheel rotates more per actual mile traveled. You can get good revs per mile data oin tire rack.com under specs.
2. Narrower section tires generally have less weight and are better in snow especially if they are a bit bigger diameter as they ';push' thru snow better.
3. Larger diameter tires do achieve better mpg if you drive at constant speeds for long,
4. tread designs and performance characteristics matter; choosing a summer tire for winter duty is a certain loser..
5. using tires past the minimum tread depth is also a loser when it rains.
That help?

 

Hey just how high should you go with tire pressure? Like if the tire is rated for a maximum of 44 psi, and I fill it to 44 psi, and then later in the week it gets warmer and sunny outside is the tire going to explode when I take the car out on the road?

 

The warming that occurs from the weather will not raise pressure anywhere near the point that the tire will burst. The law of diminishing returns starts to take effect though when you raise your tire pressure much higher than listed max sidewall pressure.

 

Other han the tread 'stickiness' the things that influence mpg based on tires are the diameter of the tires and their weight. ( all season tires are stickier than summer tires by a bunch at 32F)
When the diameter decreases the tire rotates more per mile driven and thus yields higher miles than larger diameter tires and must be taken in condsideration doing mpg. While the diameter comparision with the OEM tire id good accuracy means using the rotation of the tires per mile driven from the manufacturer. TireRack tire specs include that data.
The other significant factor is the weight of the tire; unless driving at a constant speed the effort to increase or decrease the rotation of the wheels and tires are direct causes for increased - or decreased - mpg. Those commuting at constant speeds will benefit from larger diameter tires regardless of weight within reason will benfit. Those driving traffic will benefit more from decreased tire weight with at least OEM tire diameters. Tire rack also has the tire weight data in their specs.
Having been involved in many tire manufactuer rests of vehicles we never found a good formula for comparing mpg with tire size other than weight and diameter and those aren't that precise, just guides.

 

...what is your guidance on pressure? Obviously lots of Prii owners like to keep it high.

 

There is an improvement in mpg with increasing pressure until the contact patch is reduced enough to lose traction. Tire contact patch slippage causes the tire to not only slide sideways easier but also slip on the diameter, both of which reduce mpg.
The experiments we have tried with several prissy owners indicates about 40 psig is likely the best pressure for any tire with a spe diameter of 24 inches. As you increase stire section width and decrease tire diameter the pressure can reach 45 psig. We did try 50 psig but didn't like the increased understeer that occurred.
For most using 175/65x15 we like 35 to 40 psig; for 195/55x15's we like 40 psig. For 205/50x16 we like 45 psig even if it cuts mpg.

 

Gen2 think we have 185/65R15 would that be same as 175/65x15...35 to 40psig?

 

I had winter tires and steel rims put on mine within the first 1000km (tis the season!) so hard to give real numbers since now that I'm almost at 5000km I've noticed fuel economy has improved even with the steel rims+winter tires (X-Ice 3)
BUT I definitely have "poor" gas mileage. I'm getting average 4.8-5.0L/100KM (around 50MPG) but just before putting on the winter tires +steel rims I was around 4.2L/100km and that was before the better economy had kicked in. My Highlander Hybrid didn't really get into its good fuel economy till around 5000km.
I'm looking forward to spring and getting the stock tires put back on to see. But even with the X-Ice3 and steel rims its still not bad. I think if I hadn't learned Hybrid driving habits before getting the wee Prius C I would not have such high fuel economy.