mmarges said:
The speedo is calibrated from the factory based on your wheel option.
Good to know.
mmarges said:
I just removed my 19" Chrome Clad factory upgraded wheels and put on my dedicated set of snows which are the equinox 17" factory wheels I got on ebay. I mounted Michelin X-ice 225/65/17 which is the proper factory size for 17" wheels.
The speedo is now out by 3-4% and will increase by an additional 3-4 % as the tread wears down over the years.
I don't notice any difference in performance such as accelleration and stuff but the speed does read about 103-104km/hours when I'm actually doing a real speed of 100km/hr as shown on my Garmin GPS. When I hit about 130km real speed on the GPS my speedo shows about 140km/hr...the spread seems to get larger the faster you go.
Yes, the spread will get larger as you go faster, since the error is in terms of %--according to the chart at the top of this post, the circumference of the 17" is 2.3% smaller than the 19", so it's turning faster to be at the same true speed. If the speedometer were perfect at 100 kmh with the 19's (do you know how close it was?), it should read 102 when the GPS says 100.
mmarges said:
I wonder how all this is tied together for the fuel economy? Would the system actually be tricked in to thinking I'm getting better mileage. In other words...X amount of gas was used...I really traveled 100km but the computer and speedometer thinks I've traveled 104km.
The system will think you've traveled 2.3% farther than you actually did, since the wheel has to turn more to cover the same distance. And your gas mileage will artificially be 2.3% high. Actually in your case, assuming your GPS is accurate, your gas mileage will appear to be 4% too high.
And think about it...your warranty will run out 4% sooner. Instead of 36,000 miles, you'd only get 34,560.
