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.
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