My sister has an Impreza Outback that should have stock tires sized 205/55-16. She bought the car slightly used a couple of weeks ago and I never noticed the tires were actually 225/60-16's. According to her trip odometer she is getting about 15MPG on the highway. She should be getting at least 22MPG (27MPG according to Subaru) or so. Do you think the bigger tires are screwing up the trip odometer and she is actually getting the MPG she should or do you think the bigger tires are affecting her MPG?

The tires are brand new so I am guessing the previous owner got them for more ground clearance since it is from washington state.

The tires are making the speedodometer read about 7% slower according to this:

http://www.1010tires.com/TireSizeCal...?action=submit

 

A larger diameter tire is going to have to turn less times to travel a mile, so if there was any change in MPG, I would think it would be the other way around, but I'm not so sure it would work that simply as you would also have to factor in the additional fuel consumed to move a larger wheel/tire.

It is probably causing her speedometer to be off though.

 

yeah, I had heard a larger tire would actually benefit MPG but the car has had been dealer maintained and has a new air filter, etc.

 

I don't think the tires should make that big of a change in MPG, however 225's are a bit wider than 205's which cause great rolling resistance and can definetly hurt fuel economy.

 

Yeah, I don't think that would make a difference. The only time I hear wheel size affecting gas mileage, it's usually because of rotating mass, not directly on size.

 

Rolling resistance does make a difference when going to a wider tire. Tire size itself can also make a difference. Let me explain. In your sisters cast she has a larger circumference tire than stock on the car. This throws the speedo out and at highway speeds makes a difference on MPG. Not knowing the make of tire on her car I can just ballpark how much the speedo is out, but here goes...
At an indicated 60 MPH on the speedo she is actually doing 64.2 MPH. (at an indicated 65 MPH on the speedo she would actually be going 69.6 MPH) Since the faster you drive the move gas you consume, this would also contribute to lower than expected MPG.

Here is a link to a calculator, check it out: http://swatt.j.porter.name/tech-cent...ire-calculator

 

Wow, the calculator I linked said her speedo would read faster than she is actually traveling with the bigger tires. The tires are Fusion HRI's I believe 225/60-16.

 

After checking your calculator maybe I just read the first calculator wrong. Good point on the faster speed equalling less MPG.

 

Thanks for the help guys.

 

Larger tires will result in what appears to be reduced MPG. Consider this...Your odometer calculates distance traveled based on wheel/tire rpm. Because of your larger tires, you're actually travelling approximately 7% further per revolution, therefore if your odometer indicates 100 mi., you've actually gone 107 miles. You should be able to multiply your odometer/trip meter reading by 1.07 to get the true miles traveled and them divide that by the no. of gallons.

I have this same issue on my truck. Original tires were P215/75/R15. Now I'm running 31/10.50/15. This results in my speedometer and odometer both indicating 10% lower than reality. In other words when my speedometer indicates 60, I'm really doing 66.