I'm running 255/55/18 tires and the tire calculator says if i'm going 60mph then i'm really going around 57mph on the odometer. So since the car is driving more miles then the odometer is reading, it makes it look like i'm getting horrible gas mileage. Does this make sense or am i confusing everyone? How can i find out what my real MPG is? add 3 miles to every 60 miles i drive?

Yes. Your "measured" MPG is 5% low - or about 1 MPG - because of the tire size changing the drive ratio.

For the record (note the title of the thread), rim size doesn't matter. Tire diameter is what matters.

If you own or can borrow a good GPS unit, take the car out on a straight, level highway, set the cruise, and see how the speedo and GPS align. As others have pointed out, published tire diameter figures aren't always quite accurate.

For the record (note the title of the thread), rim size doesn't matter. Tire diameter is what matters.

If you own or can borrow a good GPS unit, take the car out on a straight, level highway, set the cruise, and see how the speedo and GPS align. As others have pointed out, published tire diameter figures aren't always quite accurate.
i will try that out. thanks for the response guys!

For the record (note the title of the thread), rim size doesn't matter.

A larger rim also weighs more, therefore would it not need more gas to move them?

I was referring to speedometer accuracy, not mpg.

I reallt checked this out before I went bigger If the over all radius is the same it shouldnt mess with your Speedometer Now if the Tires are sticking out any yes it would effect MPG my 2cents

A larger rim also weighs more, therefore would it not need more gas to move them?

Yeap, I weighed the stock wheel and tire combo when I bought some 18"s. They weighed about 45 to 46 pounds each. I was able to find some 18" wheels and tires that weighed exactly the same as the stock ones so I am not losing that much gas mileage and performance wise.

what did you buy??

I asked a similiar question to see if people had looked into getting the speedo's recalibrated with the new tires. Nobody answered.

I went with 18's and lost about 1/2 MPG. The rims and tires were 4-5 lbs more than stock for each set. And with the wider tire I think that comes into play also ' cause there is more ground contact. I believe that the 18's were 2% smaller than stock in diam. for about a 3 mph difference, Tim

I'm running 255/55/18 tires and the tire calculator says if i'm going 60mph then i'm really going around 57mph on the odometer. So since the car is driving more miles then the odometer is reading, it makes it look like i'm getting horrible gas mileage. Does this make sense or am i confusing everyone? How can i find out what my real MPG is? add 3 miles to every 60 miles i drive?

To answer your question say you drove 300 miles by your odometer and filled with 15 gallons.

you actually drove: 300miles times 60 divided by 57 equals 315.8 miles

315.8 divided by 15 equals 21 mpg.

Just drove 450 mile to OHIO last night avg. 23.3 miles per. Its about 2 miles per better than in town and just hit the 10,000 mile mark too I was sad that I didnt see it looked down and it read 10,002 sighhhhhhhhh
Rooster OUT

Just drove 450 mile to OHIO last night avg. 23.3 miles per. Its about 2 miles per better than in town and just hit the 10,000 mile mark too I was sad that I didnt see it looked down and it read 10,002 sighhhhhhhhh
Rooster OUT

Is it just me was the joy of watching a mechanical odometer turn over more exciting?

oh yes Ive been waiting for it and I missed it But this was my first long trip in my Element so that makes up for it
OUT

A larger rim also weighs more, therefore would it not need more gas to move them?

The tire/wheel combo's weight will, at least in theory, make a difference on gas mileage. But of bigger importance is where the weight/mass is located in the combo.

Rotating mass is more important, with respect to fuel consumption, that non-rotating mass. Why? We have to turn the weight of the wheel to move the weight of the car. If a wheel is heavier it will take more energy to turn. I ride bicycles, and differences in wheels of different weight are very readily observable to an experienced rider. Mass concentration also has implications for deceleration, but I don't see those affecting fuel consumption.

As we put more weight at the outer edge of the wheel/tire, we make matters worse. However, more weight near the center of the wheel will have virtually no impact on fuel efficiency. Why? Leverage.

Example:
Tire/wheel pkg A: 45 lbs. with 25 lb wheel & 20 lb tire
Tire/wheel pkg B: 45 lbs. with 20 lb wheel & 25 lb tire

Tire/wheel pkg B, though weighing the same as pkg A, will, in theory, get a little worse gas milage because more of the mass is concentrated near the outer edge of the pkg where it has the most leverage to exert to maintain its state of inertia.

Having said all of this, I doubt there would be much if any measurable difference between A & B the way we all calculate our own gas mileage. Just too small a difference. I would also be surprised if a tire/wheel combo that weighed 5-10 lbs more than stock would make much of a measured difference if tire contact patch size remained constant. If we change tire sizes, let alone tire pressure, tire rubber composition, etc, I could buy into those making a small but measurable difference before weight doing so. Just my $.02. I suspect that observed changes in MPG after wheel tire changes are due primarily to changing to wider tires with a larger contact patch.

Keep your pressure up is my advice. On a side note, when finally needed I presently plan to buy the Kumho ECSTA ASX @ 235/60VR16 (http://www.tirerack.com/tires/tires.jsp?tireMake=Kumho&tireModel=ECSTA+ASX&partnum=36VR6EASX&vehicleSearch=true&fromCompare1=yes) due partially to the high max inflation of 51 PSI for this tire. I plan to run it around 44-46 PSI in part for MPG reasons.

The rotating mass should have some effect on city driving, but little effect on highway driving. Bring a flywheel up to speed takes energy, keeping it there just involves over coming friction. Then there's the matter of driving style. Accelerating faster because you now have lighter performance wheels and tires isn't going to help your mileage.

A wider tire doesn't necessarily have a larger contact patch. It will be wider, but that patch area depends on tire deformation, which in turn depends on vehicle weight and tire pressure. It is possible, though, that a wide short contact patch presents greater rolling resistance than a narrow long one of the same area.

If you put wider tires on, and then jack up the air pressure to reduce contact patch area, do you gain anything? At higher pressures you start lifting the shoulders off the ground, reducing the effective width of the tire.

If you want to choose a tire to improve gas mileage, compare what Honda puts on the Civic hybrid with the other Civics.

paulj

The rotating mass should have some effect on city driving, but little effect on highway driving. Bring a flywheel up to speed takes energy, keeping it there just involves over coming friction. Then there's the matter of driving style. Accelerating faster because you now have lighter performance wheels and tires isn't going to help your mileage.
Excellent point. I intended to draw this out, but ended the novela without same to keep anyone still reading awake.

If you put wider tires on, and then jack up the air pressure to reduce contact patch area, do you gain anything? At higher pressures you start lifting the shoulders off the ground, reducing the effective width of the tire.
I can only speak from my experience in that higher pressure results in better traction & more predictable handling. I don't think an auto tire contact patch changes that much with pressure changes within the recommended ranges of the tire, but I could be and have been completely wrong. Again, from experience, traction improves with pressure within range.