Would going from
Factory Cruze 16" steels which have an overall diameter of 664mm
Factory Sonic 15" alloys which have an overall diameter of 634mm
cause me any real problems? I'd have the smaller tires on for 30k miles or so.
If you're changing the diameter to the tire tread, that's going to upset the speedometer and odometer. They're both calibrated for a specific size. Your car would have to be re-programmed for the new size.
Well, to get a true picture of things, you have to know the revolutions per mile of each fitment, which is based on the tires mounted on each wheel. Then you calculate the delta, or difference, between the two. If the percentage of difference is significant, say more than 3 or 4 percent, then it may be enough to mess up the calculations of the various processors. It's vehicle dependent. You only provided the wheel diameters, which are 4.5% different, but you need to factor with the tires. I believe only a Chevy dealer, or a shop with the appropriate computer programming, can reprogram for a different revolutions per mile.
Ha, for eleven years of my life, been fooling around with two buck tires available at full service gas stations, blow outs were common. Around here at least, the tire size has to meet the label requirements on the inside of the drivers door.
Your life depends on having good tires that also affect traction, handling and braking. Vehicle manufacturers are liable for this, if you change, will be accepting the liability.