This was asked in another thread on another forum, and frankly, this site seems to give better answers. So Here is the question: Hello Those running larger than oem tires; does anyone switch out the spare to match? Would there be any reason for concern of putting on the spare even tho its temporarily smaller diameter? If you do run a spare, put it on the front. Do not put it on the rear. This is especially important of you have the G80 or any other locker or limited slip (This was my quote, not trying to be bias, just get a fair honest answer) I'm going to contradict this... If you have a spare you need to put on, and are willing to swap around two tires (if needed), it should go on the rear. The simple reason is its more important to steer and stop as safe as possible then it is to try to accelerate. If that small spare were to blow, or if you have bad tires you are making last as long as possible, the good ones go in front. If you ever have a blowout, its easier to control if the blowout is in the rear. THE BIGGEST REASON, everyone is taught to use two hands on the wheel, is because if you have a blowout: and are just chilling, you won't be able to control the vehicle if it happens in the front. Even if you have a good smaller spare on the front, its still going to pull to that side when you brake. Even an open diff is hell on the spider gears. So I wouldn't go far or fast. The spiders aren't meant to be turning fast all the time. Granted they aren't turning as fast as a one tire fire burnout, but still not great for them. Ok, I could see that. I read up, and the G80 gets 100% lockup at 100rpMinute difference (I also read 120, but 100rpm is from eaton.com). I was curious and 265/70/17 VS 265/70/18 has a difference of 19.37rpMile. I feel like this should be an easy answer, (and I'm blaming this on being tired). For the sake of argument, lets say you are going straight, and gentle enough to never cause your tires to spin. Put the smaller spare on the left side, and decided to only make right turns, so that your spare is the only one that would be turning more than straight (or figure out what your turning radius/angle is, and calculate the difference in)... . You would have to go in excess of 300mph before your G80 differential would lock up on those two 265/70 size tire in a 17" vs an 18", (19.37 rpMILE difference. (60 minutes per mile) means that you have to go more than 5 miles per minute (with no spin) to lock up the diff at 100 rpMINUTE...… One of the factory size tires (I just picked a 2006 Silverado for the hell of it), is 275/55/20. I just picked what I think could be a donut size 225/70/17, ( a 4.5" tire size difference). The difference is 103.85 RPMile. 100 (max rpMinute) / 103.85 (the RPMile = .96292 etc..% of revolutions per minute. then multiply that percentage times 60minutes = 57.775 MPH... would be your max speed before your diff would lock up with what I'm thinking are real size tires and a donut, (275/55/20 VS 225/70/17, 4.5" difference). To me this seems like a simple formula that if you are running a smaller size spare tire, and a g80, would be easy to figure out to just keep that information in the back of your mind for whatever size tires you got. Or am I missing something simple? I know not everyone can fit their full size spare underneath, and some people don't want to spend several hundred or more on a new wheel to match, and a couple hundred or more on a new tire, when they may never use it, and it may do nothing more than to add weight. So in the purpose to best understand what options we have...…………………. What you got?