"Plus" Tire and Wheel Sizing

Plus tire and wheel sizing was developed in the seventies as a shorthand way to refer to wider wheels and larger tires in a way that that attempted to simplify choosing appropriately oversized replacements. The goal of Plus sizing is a change to wider rims and/or tires while retaining within about 1-2% the same overall diameter, which maintains original overall gearing and reasonable speedometer accuracy. Generally, the Plus sizes are ever so slightly shorter to give a gearing advantage that compensates slightly for the additional weight normally encountered with larger replacements. The proof is that load capacity will also be within 1-2%. All this is really theoretical, as tire and wheel makers haven't provided us every possible size permutation. The Plus concept was developed when 15" was the largest commonly available diameter. As a consequence, the concept doesn't play quite right since advent of larger diameters, but it does remain close enough to be useful.

Theoretically, Plus One means three things: 1" larger diameter rim 1 size lower aspect ratio 1 size wider section For Plus Two and Plus Three simply substitute "2" or "3" for "1" above. In addition, there is Plus Zero, which keeps the original rim diameter, but switches to a wider section and lower profile to result in the same overall diameter. OEM rims are commonly narrower than the minimum required width for a plus zero tire size. One profile size is always a tens digit difference, eg /60 -> /50 or /55 -> /45. One section size is not always 10 mm. In larger sizes, 20 mm is commonly one size, as manufacturers do not make tires in every 10 mm size increment.

"Plus" Sizing Theory Illustrated

	Stock	Plus 1	Plus 2	Plus 3
Illustrative example
RX-7 GS	13" x 5.5" Wheel 185/70-13 Tire A=23.2" B=185 mm	14" x 6.0" Wheel 195/60-14 Tire A=23.2" B=195 mm	15" x 7.0" Wheel 205/50-15 Tire A=23.1" B=205 mm	16" x 8.0" Wheel 225/40-16 Tire A=23.1" B=225 mm
RX-7 GSL-SE	14" x 5.5" Wheel 205/60-14 Tire A=23.8" B=205 mm	15" x 7.0" Wheel 215/50-15 Tire A=23.5" B=215 mm	16" x 8.0" Wheel 225/40-16 Tire A=23.1" B=225 mm
RX-7 2nd gen base	14" x 5.5" Wheel 185/70-14 Tire A=24.3" B=185 mm	15" x 6.0" Wheel 195/60-15 Tire A=24.3" B=195 mm	16" x 7.0" Wheel 205/50-16 Tire A=24.0" B=205 mm	17" x 8.0" Wheel 215/40-17 Tire A=23.7" B=215 mm
RX-7 GXL	15" x 6.0" Wheel 205/60-15 Tire A=24.7" B=205 mm	16" x 7.0" Wheel 215/50-16 Tire A=24.5" B=215 mm	17" x 8.0" Wheel 235/40-17 Tire A=24.3" B=235 mm

Notes

The presence of any size in the table is no guarantee that that size is, was, or might become available for purchase. Except for the stock sizes, the presence of any size in the table is no guarantee that that size will fit the car indicated. Fit selection depends on wheel selection, whether body or suspension modifications have been applied, and the degree to which possible rubbing might be acceptable to the user.

Other Important Tire & Wheel Concepts

Aspect Ratio

"Aspect ratio", "series", and "profile" are interchangable terms. This is simply a percentage that the height of the tire section represents to the width of the section. A "60" series tire 205 mm in overall width measures approximately 123 mm from tread to bead.

Load Capacity

Maximum load the tire is designed to carry at maximum permissible inflation pressure is its "load capacity". All you need to know to compare tire size equivalence is found in load capacity. Load capacity and maximum permissible inflation pressure are stamped into the sidewall of the tire near the bead as "MAX LOAD @ . . .".

The tire is an air container. The air carries the load. The tire controls the air and thus the car. An uninflated tire has zero load capacity. It will disintegrate used without air.

Pressure is related to the volume of air necessary to carry load. More pressure is required to allow a smaller physical volume to carry the same amount of air as a larger physical volume. The total number of air molecules in the compressed volume determines load capacity. Cheaper tire designs max at 35 or 36 PSI. Most higher rated tires max at 44, though some are rated for even more.

Calculating required tire capacity is not simple, and is best left to the engineers who design cars. Here's an example that makes some simplistic calculations (weight in pounds; capacity in pounds per square inch):

	Total Weight	Front Weight	Comments
79 GS 5-speed	2385		(no A/C)
Maximum load	340
Total	2725	1390	(51%)
Stomp brakes	NC	2044	(weight distribution shift to 75/25)
Stomp brakes	NC	2180	(weight distribution shift to 80/20)
Stomp brakes	NC	2316	(weight distribution shift to 85/15)
185/70-13 capacity	4540	2270

If you never have to stop on a downslope, you will probably never approach having 85% of your car's weight on the front, and maybe not even 80% otherwise. Nevertheless, the capacity numbers above represent maximum inflation pressure, which few of us maintain, leaving the real capacity of the tires at less than in the chart above. With A/C, you add more than 40 pounds, most of which is on the front tires. Put two adult males and some gear in the car and you easily exceed the 340 pounds maximum rated vehicle load, especially if you've added 20-30 pounds of stereo. Thus the chart clearly confirms that manufacturers, for cost reasons mostly, generally use the smallest tires appropriate for the application. Smaller tires are highly likely to be insufficient in capacity, regardless of how appropriate, or not, is their diameter or width.

In most locales you are probably free to do whatever you please with your own vehicle unless you carry people for hire or rent the car for profit. If you are in the rental, passenger hire, tire and wheel, or automotive performance modifications business, legal considerations should force you to adhere to this rule regarding replacement tire selection:

Tires should not be installed unless they have equal or greater load rating than stock, with a one or two percent tolerance considered equal. Put another way, in the absence of special circumstances, replacements should rarely be of significantly lower capacity than stock.

What racers use on the racetrack is one special circumstance. Race cars don't carry passengers or luggage, and typically run at significantly less weight than stock, with spare tire, A/C & tire jack removed at the least. They rarely have to contend with low shoulders, railroad tracks, curbs, or potholes either, any of which can more easily damage an overstressed tire than one running safely within design limits.

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