Old Car, Modern Tire Tech - 2007 Dodge Charger SRT8 Long-Term Road Test

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Old Car, Modern Tire Tech - 2007 Dodge Charger SRT8 Long-Term Road Test

All four tires were suddenly low but luckily this 2007 Dodge Charger was ahead of its time.

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daryleason

This sort of leads to a question that I've always had about people that change tire and/or rim sizes. So when a car is designed, they'll specify the tire pressure, based on the rim size, tire size, suspension, vehicle weight, camber, castor, etc. So, if you change your tire or rim sizes, how do you calculate the proper tire inflation? Lets assume that a base line (factory) recommended tire size is supposed to be inflated to 32 PSI. Go with a larger tire, but the same rim, it would be possible that the 32 PSI would be under-inflated. Go with a larger rim, but stock tire height, and you could be over-inflated. Anyone have a reliable method for determining what the best PSI for an aftermarket tire or rim combo would be?

As a follow up question, changing tire size also could change the camber and castor, could it not? Is there a way to calculate the proper toe-in, etc?

s197gt

the short answer is there is no one perfect tire pressure. everything about suspension, tires, and wheels is about compromise and priorities. find what works for you.

the manufacturer suggests its settings based on even more than what you listed. comfort, mpg, tire wear, etc...

what first got me thinking about it was the various pressure settings bmw listed for our vehicle depending on the number of occupants, what type of driving, etc...

i realized that different pressures work better for different circumstances. this is something race teams have known since the beginning of racing. tire pressures are constantly adjusted for track conditions. some times if you go off-roading you might want to "air down".

i don't think changing tire/wheel diameter or width affects most alignment settings. if it does it would be so small that (unless you are already on the ragged edge of manufacturer specs) it really wouldn't matter.

and you can go outside manufacturer specs if you know what you are doing; you just better do your homework first.

this guys says it can (he doesn't mention diameter):

"...fitting a car with a wheel or tire combo that has a different offset or width will affect camber, toe, and caster angles in the suspension. ...

Even where the alignment is corrected, there can be significant changes in the scrub radius of the vehicle and unfortunately, in almost all production cars, scrub radius cannot be directly adjusted."

http://www.tuneruniversity.com/blog/2011/04/wheel-tech-part-ii-size-matters/

i think the important take from his articles (and something i've done) is to stay as close to the manufacturer specs/tolerances as possible and you don't have to worry. i've used a wheel that is well known in the mustang community and my alignment was adjusted after lowering and is within manufacturer spec (though at the very edge of negative camber).

daryleason said:

So, if you change your tire or rim sizes, how do you calculate the proper tire inflation?

As a follow up question, changing tire size also could change the camber and castor, could it not? Is there a way to calculate the proper toe-in, etc?