OE tire size mix up: '04 Malibu Classic?!?

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
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What is the difference in the circumference of the two tire sizes?
You gotta realize that nothing is exact. The variance that you are seeing is not that much and probably within industry standards. A mid '80s Honda that I once drove read 5 mph high at freeway speeds. As I recall the owners manual said that this was on purpose to help to keep the driver from going over the speed limit. I wonder if this is why people drive slow in the fast lane: they think that they are going faster that they are.
For reference, my 95 GA shows 2 mph over actual speed.

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Scott Buchanan wrote:

What size tires do you run on the GA and what was the original size? I think the base version around that time had something like 195/70/14, a seemingly weird size, (especially for a semi-sports car which would normally take lower-profile, 55 or 60-series, tires), that I don't think is used as OE on any new car these days.
You are right that the variance on my Classic is not 'that' big....but I'd still like to figure out why it's there.
Regards, Eric M
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I don't recall of the top of my head, but I did get the same size that is stated on the tag on the door.
Modern speedometers work by counting pulses from the speed sensor. If these pulses don't mathematically work exactly into the circumference of the tire there will be some error. And that is not taking into the tolerance of the electronics of the speedometer.
Would your speedometer be closer if it had the other tire size that you mentioned?

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Scott Buchanan wrote:

I think so, but I'm not about to spend the $$$ to buy new tires to find out. Next time I need tires I will consider it.
Personally I like the slightly narrower, higher 205/65 tires for ride quality and winter traction. The small tradeoff in handling and cornering I can live with, at least in a car like the Classic. If I ever win the lottery I'll probably get a sports car to take out in the summer on the side roads; in that case I'll go for wide low sticky tires. I'm not holding my breath.
If your tires are the designated size per the door sticker, the speedometer/odometer should not be that far off. Strange....
Regards, Eric M
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You're being just a bit fussy. That 1 or 2% error is very close. Knowing GM they probably saved money by mounting 65% tires.
Usually the ODO is a bit fast, but I once experienced it 8% slow on a Chrysler Horizon sports model with 60% tires; 70% profile was on the std. car model. When the crappy Firestone original tires ended their useful life very early I mounted a set of 70% tires that I calculated would correct my speedometer error. It did and I no longer feared getting caught in speed traps. Of course if your ODO is low, you car accumulates miles slower, better for resale.
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Just Facts wrote:

Yep; that's my theory. Perhaps it is a bit "fussy" but I think the least they could do is recalibrate the speedometer if they change tire sizes.

8% is too big a difference; it can mess with your braking, or so say the alleged experts. Of course if you're going to run 60-series tires on a car that was designed for 70, (or vice versa), you have to change the width of the tires you use to compensate. For instance 225/60/16 is almost the same as 205/65/16, (0.5% off), while 205/60/16 is a pretty big difference. (3.6%)
Hmm, that's not a good example....Here's a better one: There's more than a 6% difference between 205/60/15 and 205/70/15.
Not everyone takes this into account when buying tires. I know people who in general know more about cars than I do who still refer to tires solely by wheel size, i.e. "16-inch tires," "15-inch tires," etc.
It goes back to the old days in the '70s and even into the '80s, when most passenger car tires were high profile and---by today''s standards---relatively narrow. Back then you could, for the most part, pull into a garage or tire shop, ask for good "15-inch tires" and run whatever they had in stock.
Not any more: The trend now is for bigger wheels and wider, lower-profile tires, even on vehicles no one will ever think of as "sporty." i.e. the new Ford Crown Vic and Chevy Impala come with 225/60 tires.
Anyway, thanks for the advice.
Regards, Eric M

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