Tires: Tire size spec 215/65R16 vs 215/60R16, tirerack com, tire rack

Question
Are the 215 width and 65 profile measured in millimeters?  How much smaller tread diameter is a 215/60R16 tire vs the mfg spec 215/65R16.  I suspect we will get slightly poorer fuel economy because the tire has to spin a higher rpm to make the same speed/and more rotations for the same distance.  I just can't figure out how much.  Thank you for lending your expertise.
Alan

Answer
Alan,

If I am reading your question correctly, you are thinking about replacing a P215/65R16 with a P215/60R16.  This is not a good idea.  Aside from the things you've mentioned, there is the issue of load carrying capacity - and that size change would result in lower capacity - and therefore more unsafe.

But if you need to read up a bit more, spend some time at Tire Rack's web site. Look for their tire tech area.

http://www.tirerack.com/tires/tiretech/tiretech.jsp