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18 inch tires vs 20 inch tires comparison


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For reasons unknown to me, the 20 inch wheels on my 2020 Sierra SLT were changed by the previous owner to 18 inch wheels.  Factory tires were 275/60R20 and I am now running 265/65R18 tires.  If I am comparing correctly, the differance of 2 inches in diameter is making the speedometer show around 3 MPH more than the actual speed, and I am guessing I am putting on more miles than actually driven.  Using a tire size calculator, it looks like 285/65R18 tires would drop this difference to about 3/4 MPH more than actual and width would increase about 1/2 an inch.  295/65R18 tires would make the actual speed about 1/4 MPH more than the speedometer shows, but width would increase just over 3/4 of an inch.

Short of buying 20 inch wheels and tires, which is not in the budget, has anyone tried increasing the size of their existing tires, are there any disadvantages to the wider sidewall height and the wider width causing any clearance issues?  I would probably prefer to go with the 285/65R18, as there appears to be more choices to pick from but the 295/65R18 get me closest to the correct speedometer reading.  My existing 18 inch tires have less than 10,000 miles on them, so might get some trade in value.  My other choice is to leave them as is, and just know I can drive faster than speedometer shows, but don't like adding miles I'm not driving.  Any suggestions.

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the other way around this is to get a speedometer calibrator. Multiple companies make it and it's easy to install. I jumped from the 265/65/18 up to a 33" tire and used the below to keep MPH and everything else working correctly.

 

Speedometer Calibrator | Chevy/GMC 1500/2500HD (19-22) | Rough Country

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18 hours ago, cabledude said:

For reasons unknown to me, the 20 inch wheels on my 2020 Sierra SLT were changed by the previous owner to 18 inch wheels.  Factory tires were 275/60R20 and I am now running 265/65R18 tires.  If I am comparing correctly, the differance of 2 inches in diameter is making the speedometer show around 3 MPH more than the actual speed, and I am guessing I am putting on more miles than actually driven.  Using a tire size calculator, it looks like 285/65R18 tires would drop this difference to about 3/4 MPH more than actual and width would increase about 1/2 an inch.  295/65R18 tires would make the actual speed about 1/4 MPH more than the speedometer shows, but width would increase just over 3/4 of an inch.

Short of buying 20 inch wheels and tires, which is not in the budget, has anyone tried increasing the size of their existing tires, are there any disadvantages to the wider sidewall height and the wider width causing any clearance issues?  I would probably prefer to go with the 285/65R18, as there appears to be more choices to pick from but the 295/65R18 get me closest to the correct speedometer reading.  My existing 18 inch tires have less than 10,000 miles on them, so might get some trade in value.  My other choice is to leave them as is, and just know I can drive faster than speedometer shows, but don't like adding miles I'm not driving.  Any suggestions.

Your speedometer ratios are very close to my calculations:

 

(1)275/65/20 is approximately 32.99" diameter(OEM Tire)

(2)265/65/18 is approximately 31.56" diameter

(3)285/65/18 is approximately 32.59" diameter

 

The difference between (1) and (2) is approximately 1.43"(not 2" as indicated above), or a ratio difference of approximate .95665 or 1.0453 depending on how you account for your ratio, giving you a variation in speed of approximately 3.1 mph when your speedometer is reading 70 mph.  So by putting your speedometer at 70 mph, you are effectively traveling approximately 66.9 mph.

 

Going to (1) from (3), the difference is approximately .4", or a ratio difference of approximate  .9879 or 1.012 depending on how you account for your ratio, giving you a variation in speed of approximately .85 mph when your speedometer is reading 70 mph.  So by putting your speedometer at 70 mph, you are effectively traveling approximately 69.15 mph.

 

Neither of these options are bad, but the good news is there is a 3rd option that is best!

 

The best option for you to stick near the factory diameter while utilizing the current 18" wheel is to look at the 275/70/18 tire.  It has a diameter of approximately 33.16" and will most closely matchup with the factory programming for the 32.99" diameter OEM tire.  There are also a TON of tires offered in this size.

 

Good Luck!

 

 

 

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