These are rims and tires plus sizing changes:
1. Plus-Zero: This maintains the original wheel rim diameter, but increases tire section width by 10mm, and drops the aspect ratio by five points.
For example, if the vehicle was originally equipped with 205/55R15 tires, you could keep the same rims and move to a 215/50R15 tire. This provides a slight increase in tread contact patch, along with a slightly shorter sidewall for quicker steering response and less lateral lean, while maintaining the original overall tire diameter.
If you are concerned with the budget, this move will save bucks, since it does not require wheel replacement.
2. Plus-One: Now you can begin to use larger rims.
A Plus-One move requires an increase of 10mm in section width, a reduction of aspect ratio of 10 points, and an increase in wheel rim diameter by one inch. If the original setup featured 185/65R14 tires (with overall diameter of 23.47 inches), this would move you into a set of 195/55R15 tires. (In theory, if that size is not available, you could move to a set of 205/50R15 tires, which may have an overall diameter of 23.07 inches).
3. Plus-Two: A Plus-Two move would require a 20mm increase in section width, a 20-point drop in aspect ratio and a two-inch increase in rim diameter, as compared to the original package.
Example: Moving from 185/65R14 would place you into a size 205/45R16 (since that size is, indeed, available). This is an example of where the theoretical math works out.
4. Plus-Three and beyond: OK, now you are moving into some serious stuff. As in Plus-One or Plus-Two upgrades, you move up yet another inch in rims diameter (in this case, increasing the original wheel diameter by three inches). You still need to maintain the original overall tire diameter and you need to maintain sufficient load index.
However, determining the tire section width and aspect ratio gets a bit trickier, since the "rule of thumb" used in determining Plus-One and Plus-Two fitments is basically tossed out of the window.
When considering a Plus-Three or greater upgrade, you can't blindly add a set number of millimeters to section width and drop aspect ratio by a set number of points. Beyond Plus-One and Plus-Two, larger rim diameters and the limitation of practically available tire sizes make the old standard formula non-practical.
In other words, if you want a Plus-Three, you don't automatically look for a tire that is 30mm wider in section width and 30 points lower in aspect ratio, because that specific tire size may not exist.
Instead, whenever moving to a Plus-Three or beyond, simply review the manufacturer's tire sizing chart to find a tire that:
1. matches the new rim diameter,
2. maintains the desired overall OE tire diameter, and
3. matches or goes beyond the OE tire load index. As an example of a Plus-Three upgrade, moving from 195/65R15 (with an overall diameter of 25.0 inches) might require choosing a 225/30R18 or a 215/40R18 (with an overall diameter of 24.8 inches).
As an example of a Plus-Five move, let's say that you're dealing with a 2005 Caddy Escalade originally fitted with 265/70R17 tires (31.5 inch overall tire diameter). While in theory a Plus-Five change would require a 315/20R17, that size may not exist, so moving to a 275/45R22 that features a 31.8 inch overall diameter would fill the bill.
