[ae31744]

One question I have about running these tires with a larger sidewall is wouldn't you no longer be able to use the spare tire? Shouldn't the spare be less than 5% difference in diameter? All these sizes put it out of that range I believe.

[chayden2005]

Here's a basic reason to limit the time you run on a spare that isn't considered full size, in relation to your installed tires:

"The amount of power the internal computer sends to each wheel—based on what your AWD vehicle needs to go down the road—helps to manage how hard the transmission and transfer case are working. Inconsistent outside diameters caused by different tire sizes, tread patterns, or tread depths can force an AWD computer's readings to fluctuate, and various wheels will receive inefficient or incorrect power loads. As a result, the drivetrain (computer, transmission and transfer case) must constantly readjust, doing more work than necessary and eventually, may break down."

[ae31744]

Originally Posted by chayden2005

Here's a basic reason to limit the time you run on a spare that isn't considered full size, in relation to your installed tires:

"The amount of power the internal computer sends to each wheel—based on what your AWD vehicle needs to go down the road—helps to manage how hard the transmission and transfer case are working. Inconsistent outside diameters caused by different tire sizes, tread patterns, or tread depths can force an AWD computer's readings to fluctuate, and various wheels will receive inefficient or incorrect power loads. As a result, the drivetrain (computer, transmission and transfer case) must constantly readjust, doing more work than necessary and eventually, may break down."

Okay - so even running 275/45R21 all around you would be okay using the Modern Spare which is 155/85R18 (7.5% smaller) to get to a shop if you got a flat?

[JoeAyalaM]

Originally Posted by ae31744

Okay - so even running 275/45R21 all around you would be okay using the Modern Spare which is 155/85R18 (7.5% smaller) to get to a shop if you got a flat?

Since the spare tire use is minimal, yes it should be fine because the four main tires diameter is the same.