Hi all.

I drive a 2004 Chevy (Malibu) Classic. Recently I've started to wonder
if something is screwy with the OE tire size of 205/65/15.

Specifically, I wonder if it throws off the speedometer, which might be
calibrated using 215/60/15 tires. That was the standard equipment tire
size on the 2003 Malibu, of which the Malibu is pretty much a clone,
apart from the smaller engine.

The reason I say this is that I recently returned from a long road trip
to visit family. This time I clocked the mileage from here (northeast
PA) to Ohio using the trip odometer. It registered 364.9....and it's
supposed to be a shade over 373, per MapQuest. That gave me a jolt when
I got there.

Even if MapQuest is wrong, on the way home I checked the trip odometer
against the mile markers on the side of I-80. What should have been 307
miles in PA on I-80 was 300-301. The overall mileage again was 364-365
per the trip odometer.

This means the speedo is too slow by a little more than 2%, if my math
is right. Once I studied possible causes online, I suspected the
potential tire mixup and logged onto an online tire size calculator,
which says that using 205/65 rather than 215/60 on 15-inch wheels
yields a error ratio of 1.348% too slow. The speedo would read 59.2
when the car was going 60, per the calculator.

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

That still does not account for the whole difference between the
odometer and reality, but it goes a long way.

Then I checked the speedometer against the "your speed is X" flashing
signs we see on the sides of highways around here. Sure enough, when I
looked close I saw that my speedo read 58 while the sign flashed 59.
Just to be sure I checked again and got the same 1 mph too slow result.
(The precise speeds were slightly lower.)

Finally, it occurred to me this might one of the reasons for the
disappointing fuel economy the Classic has delivered in the 8 months
I've had it. I've gotten nowhere near the EPA readings of 24//34, as
I've posted here a few times.

Even adding an extra 1.5 or 2% to the number of miles driven does not
make up the difference in gas mileage, but again it's something.

Has anyone else who owns a Classic noticed that their
odometer/speedometer is off?

My theory is that Chevy decided 205/65 was a better tire size to sell
to fleet buyers, I guess because the ride is slightly smoother, and
also that size is slightly cheaper in most cases. However, the Classic
speedometer is still calibrated as though the car would take 215/60, if
my theory proves true.

I must admit to a complete lack of technical know-how as to the way
speedometers are calibrated. If that's not the answer, though, it means
something is screwy with my speedo and odometer.

If I could afford it, I'd buy a set of cheapo 215/60/15 tires and take
them on a long highway trip to test my theory. Since that's not too
practical, finanially or otherwise, I'm interested in all comments and
opinions on this.

Thanks.

Regards,
Eric M