THis is an incorrect assumption, the friction is a function of the weight and width of the tires. Added width to the tires means more perisitic friction loss due to contact with the road surface. This can result in lower mileage.

 

What jeff said is correct. 5 pounds to the each wheel is a HUGE ammount of weight to the car. Its rotational and unsprung weight. That 20 pounds is probably equal to 60 pounds of extra weight inside the car. You car is probably .3 seconds slower than it was before as well, if not more.

-Sean

 

I see, so if we're not talking acceleration, just rolling resistance:

From a friction standpoint a 225 tire increased to 245 is ~9% increase in friction? As far is friction is concerned, added weight is hardly noticeable because each tire already has 900+ lbs between it and the ground. But, what has more friction, aerodynamic resistance, or tire resistance? Because let's say if the ratio between air and tire resitance is 1:1, then total resitance has increased only ~4%. Uh, this is crazy.



Not ranting at all, but I know I've heard that a million times. Are there any studies or data to show that the difference is that great? Has someone done back to back time trials? Are there any formulas to calculate differences between different weight/diameter wheels? I just don't feel any difference compared to stock. And I DID notice a big difference in acceleration from my old 05 Coupe (~14.2 car) to my new car (~13.9 car) so at least I know my butt dyno isn't numb.

 

I have no idea what the new sedans run, but 13.9 with the m45 wheels seems pretty damn low.

Anyways, if i remember unsprung weight has a 1 to 8 factor. 1 pound of unsprung weight is equal to 8 pounds of sprung weight. I have not yet taken my car to the track to test this theory, but maybe this thursday i will. Then again weather conditions come into play as well so it is hard to tell. With my OEM 17" rims my best run was a 13.8 i have lost about 3 to 4 pounds per wheel when i put my 19" rims on and thats factoring the tires as well. So if i do the math thats equal to about 96 pounds of regular weight i lost. Which would be something around .2 seconds off the 1/4 mile.

-Sean

 

Side note: 100 lbs = 10hp = .1 second (1/4 mile) roughly.


All I know is that heavier wheels and tires hurt fuel economy, acceleration, etc. more than just adding that weight to the trunk.

But 4mpg seems too much.

 

Maybe this sounds dumb, but when you change the size of your wheels/tires, it affects the speedometer and odometer, so perhaps that is the difference?

 

I could get WAY involved with this, but according to my calculations, with the rated PSI on the tire, you should lose about 4.6-4.7% mpg. This is due to the "sag" of the tire at the point of contact. Where the car weighs down, there is a larger contact point and the tire sags under the weight., this is why a rolling ball stops (eventually) even if air resistance in negligible.

Dont make me get into it.

Kevin

 

Not if the tires are sized correctly, the diameters should be the same as stock. This might attribute a slight error, but can't be the sole reason.

 

i did not have a noticeable change in mpg when i went to 20's....

But, when i dropped the fujita's in it dropped 3mpg lol.

can't keep the foot out of the throttle now

 

Increased rotating mass = reduced fuel economy

 

Or just put your stock wheels/tires back on and compare.

 

To try something new, I put in ~42psi with zero increase in mileage, only a major decrease in ride quality.

Tires are same diameter as stock.

I'd put my stock wheels back on to compare, but I'm trying to sell them.

"But 4mpg seems too much." Agreed, that's why I'm trying to figure this out.

 

Hmmm. My understanding is that at equal pressure, the contact patch for a wider tire is the same size (area) as a narrower tire, but the patch is a different shape (i.e. wider). The shape of the contact patch can impact how the tread contacts the road, which could in turn affect rolling resistance.

The tread and compound material determine rolling resistance, not tire width. Big wide slicks have almost no rolling resistance, but narrower snows still have a lot. Mileage with slicks is better than with snows, regardless of width.

my 2 pennies ;-)

 

Something else to try: Calculate the fuel economy yourself. Fill up tank, drive on highway, measure distance, fill up again, divide the two numbers.

Heat, humidity, wind, speed, etc. can also affect fuel economy.

But, I still can't see how you'd be getting as low as 20mpg. I'm assuming you're resetting your average fuel economy when you're on the highway, right?