Tire size

I just put new tires on my 2002 V70 XC. I have noticed almost 2 Mpg better mileage with the new tires.
The old tires were 215/55/16 and the new tires are 205/55/16. Does the size difference throw off the speedometer enough that that is the reason for the mileage increase, or is it just that new tires can make a difference in mileage? I wouldn't think there would be enough difference in those sizes to really matter. Also, should odometer be recalculated for different tires, and how is that done?

Ummm...yes! Here's a comparison using 1010tire.com

http://www.1010tires.com/Tools/Tire-...R16/205-55R16#.

Stock is 215/65R16, the tires you currently have installed have a smaller outside diameter meaning it will rotate more times per mile the then stock tire. The computer measures distance traveled (miles) from number of wheel revolutions. A smaller diameter tire will register as a shorter mile to the computer, you follow me?

Smaller diameter tire will also throw off your spedometer by reading faster, and the odometer will add more miles faster.

WAY too small a tire
This is not the old days where you swapped out an external gear for the speedo, you can't change the speedo to reflect the new tire size

Stock size, and thus speedo calibration, is based on a 215/65-16 tire size. By running a 205/55-16, your rolling diameter is smaller by about 8%. That is, at an actual speed of 60 mph your speedo is going to read about 65mph (dependent upon whatever error is included in the calibration and there is some error put in at the factory). The odo is also off by 8% indicating that you have traveled farther than you actually have. Now with all this said and as far as your instrumentation is concerned, the car will still think it is spinning the motor at x rpm at y vehicle speed over z distance and therefore burning a certain volume of fuel over that distance regardless of what that actual distance is. In other words, if you are driving at an indicated 60mph on either set of tires, the engine will still be turning the same revs and, in theory, still burning the same volume of fuel, if you follow me on my thinking. None of this, however, takes into account any differences in tire construction and tread compounding, which IMHO is probably the biggest factor in mileage differences at a given speed, or aero effects (if you are driving an indicated 60mph your true air speed and aero resistance is less) having a small effect on mileage.

So why are you running a 205/55-16 on an XC? Typically if you don't want to run the stock size, most folks run a 225/60 or 235/60 so that their speedo is closer in calibration with a wider tire. You've gone to a narrower and shorter tire which has no advantage over a stock size no matter how you cut it plus it probably looks a bit strange. Unless you got a slamming deal, I also suspect that any difference in price would have been small.

Cheers,

Bill

I changed the tire size simply for cost and convenience. The smaller tires were about $150 less for the set. This was a comparison on what was in stock, and I didn't want to come back and waste another day.

Not the best and brightest of ideas
While that is the stock size for a NON XC wagon, those cars are substantially lower with stiffer springs and struts/shocks.
An XC with it's soft TALL springs also needs a taller, thinner tire

Okay, thanks for the replies. Live and learn I guess. I had his a huge pothole and blew out two tires, so had to do something right then, and try to save a little money. I normally do more research on my tires before I purchase, but blowing out two tires prematurely through a wrench into that plan. I guess the upside is that I did go on the low-end as far as mileage warranty so my mistake won't last as long as you normally would.

You can also go with 235/60R16 which is exactly the same size (well almost by 0.36% or a 1/3 of a percent :D ) as 215/65R16.

When I first got my XC70, wasn't a whole lot of options in the 215 size. Now there is almost as many in the 235. The 235 size provides a wider grip and better lateral stability. Mind you, last set I had General UHP Grabbers at 235-60R16 - a whoooole lotta fun and they lasted 6 years because I also have snow tires.

It's funny, before you mentioned that they were too small, I didn't even notice. Now that it's been pointed out, they look way to small. Oh well, these are only 40,000 mile tires so in less than a year I can get the right ones.

I will soon need to replace the rear tires on my 2004 XC70. I currently have stock 215/65R16s installed all around. Can I swap the rears to 235/60R16 and use the same wheels?