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Do you think that if you bought the 18" or 19" wheels from the factory the speedometer calibration would be adjusted? Or is the difference too small to worry about?
 
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The speedo is calibrated from the factory based on your wheel option. 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.

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.
 

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I also have the 19" wheels and purchased a set of OEM GMC 18" for winter.

I did'nt go with the OEM tire size of the 18" as they are smaller...

OEM 19" : 235/55/R19
Winter 18" : 235/60/R18

Now, my 19" and my 18" are the same overall size so no correction needed to the speedometer calibration.

mmarges : on your 17" wheels you shoud have opted for 235/65/R17 so you would had no speed error.
 

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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. :(
 
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SuperMat64 said:
I also have the 19" wheels and purchased a set of OEM GMC 18" for winter.

I did'nt go with the OEM tire size of the 18" as they are smaller...

OEM 19" : 235/55/R19
Winter 18" : 235/60/R18

Now, my 19" and my 18" are the same overall size so no correction needed to the speedometer calibration.

mmarges : on your 17" wheels you shoud have opted for 235/65/R17 so you would had no speed error.
Yes, I was thinking of the 235/65/17 to make the exact size but didn't for a couple reasons.
1. With winters a narrower tire is better. The analogy of the skinny sleigh skies cutting through the snow as opposed to the wider taboggan riding on top of the snow.
2. The wider, yet still taller tires at 17" are getting into pickup truck realm, not car/CUV realm. Therefore I found this size less available across tire manufacurers models and also more expensive being a less pouplar size....at least in my region a less popular size. The 225/65/17 seem to be a very popular and cheaper. This is also the OEM size and the speedo being out a few % hasn't really changed anything.

In fact, instead of travelling faster than what my speedo says to keep up...I can kind of forget about it and instead potentially have those few km/hr difference save me from a speeding ticket or at least save me a little on the gas mileage by really driving a tad slower from what the speedo says.
 

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Any noticeable change in fuel economy between tire sizes?
 

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jdubs1211 said:
Any noticeable change in fuel economy between tire sizes?
If you change the OEM tire diameter then you will receive better or worse mpg depending on which tire wheel combo you switch with.[/color]
 

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MFG's can't have speedos that read 'slow'; reading a slower speed than actual. The reality of it is that most OEM speedometers read fast a few percent.
 

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POLRCUB said:
If you change the OEM tire diameter then you will receive better or worse mpg depending on which tire wheel combo you switch with.[/color]
Any estimate as to how much?
 
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