FAQ:  Tire Plus-Sizing


Article by Cody Graham

Definition

"Plus Sizing" is is the concept of using larger diameter wheels with a tire that has less sidewall.  The overall diameter of the wheel/tire combination (ideally) remains the same, which keeps the speedometer and odometer relatively accurate.  Maintaining the same diameter also insures that other dynamics are unaffected - torque that is transferred to the ground, clearances for suspension, and others.  More on this later.  The reasons for Plus Sizing are many. Many people Plus Size just because it looks good.  The combination of 'more wheel' and 'less tire' is appealing to many people.  Another common reason to plus size is to enable the car to handle better.  In general, with less tire, you get a better feel for the road, and a more sure-footed car.  The downside is, of course, a slightly harsher ride.

Weight can also be saved by plus sizing.  You can simply buy a lighter-weight wheel in the stock size, and change to a lighter tire, but if you're buying aftermarket wheels, why not "kill two birds with one stone"?  Look for lighter weight wheels in larger sizes, and you will generally find the smaller aspect ratios of larger tires will reduce the weight even further.  Reduction in weight at the wheels will benefit a car greatly.  Handing will be enhanced, since the suspension will work better, and it will accelerate faster, and stop quicker.

   Plus Size -  Putting on a combination of a larger wheel and a tire with a shorter sidewall than that which originally came with the vehicle.  Plus 1 would be a 1" larger wheel (ex.: 15" to 16" wheel).  Plus 2 would be a 2" larger wheel (ex.: 15" to 17" wheel).

   Aspect Ratio -  The sidewall height of the tire expressed as a percentage of its width.  If the tire size is P225/55R16, then 225 (millimeters) is the tire width, 55 (%) is the aspect ratio, and 16 (inches) is the wheel diameter.

   Offset -  Offset is the distance between the mounting surface of the wheel (which contacts the hub) and the center line of the wheel.  When wide tires are used, spacers are occasionally used to reduce the offset.  This is to prevent the tire from rubbing the wheel well at the extremes of steering travel.

The Ins and Outs of Plus Sizing

I am going to use one of the more common plus sizing related to 3000GT’s and Stealth’s.  Many owners whom have cars that originally came with 16" rims wish to upgrade to 17" or 18" rims.  Going from 16" to 17" is considered a Plus 1 and going from 16" to 18" would be considered a Plus 2.  Basically, all the Plus # is, is the difference between the smaller wheel and the larger wheel in inches.  (On a side note – I suppose there is minus sizing also, but not sure where that would really apply).  When a person substitutes a larger wheel on a car, new tires will need to be purchased also. A 16" tire will obviously not fit onto an 18" wheel.  Here is where tire sizing comes into play.

Tire Sizing

On the sidewall of every car tire produced is the tire size.  Generally, tires have three important size markings.

   1)  Width – in millimeters (mm)

   2)  Aspect Ratio – percentage (%)

   3)  Wheel Diameter – in inches (in.)

Let's use one of the stock wheel sizes for our cars as an example:  a tire noted as 225/55R-16 has a 225 mm section width, a 55% aspect ratio, and fits a 16" wheel.  With some quick math, we can easily figure the sidewall height as 55% times 225 mm = 123.75 mm, or approximately 4.87 inches (divide by 25.4 to get inches from mm).  Now, to figure a tire's overall diameter, we would take the sidewall height times 2, then add the wheel diameter of 16 inches.  This gives 25.74 inches diameter as our result.  Any other tire/wheel combo we might use would be similar in diameter to this - within a few fractions of an inch.  Here's a photo illustration showing overall tire diameter - it stays the same despite larger wheel sizes:

 
Notes about Aspect Ratio and the Contact Patch

The Aspect Ratio of a tire determines the shape of its contact patch (also know as its "footprint").  Low-profile performance tires have a short, wide contact patch, which makes for better handling, dry traction, and cornering stability.  Conversely, the longer, narrower contact patch of the stock wheel/tire combo has historically given better results in rain or snow.  The latest performance tires claim to have cured the traditional tendency to hydroplane in wet weather with deep tread grooves, at the same time as claiming to minimize "tracking" over road imperfections that was previously characteristic of the deep-groove design.  Here is a diagram of the differences in the shape of the contact patch made by stock and performance tires (not to scale):

Factory Sizing:

Optional Sizing:

Theoretical:

Of course, all stock sizes will work on ANY 3000GT or Stealth, assuming the wheel will clear the brakes on AWD models.  The sizing listed above will also work within acceptable speedometer accuracies (well, maybe not the 20” wheel).  There is no general way to tell exact speedometer error, because we are not entirely sure which wheel combo Mitsubishi really calibrated our speedometers with.  Did they recalibrate at any time?  Or were all their available options so close to being the same that they didn’t bother to recalibrate anything?

The Math Behind It All

Let's establish our terms:

A = tire width in mm
B = aspect ratio (as a percentage)
C = wheel diameter
D = Total wheel/tire diameter
E = Side Wall Height
F = Tire circumference
Pi = 3.14159 (rounded)

Side wall height:
E = A * B (in mm)
E = (A * B)/25.4mm (in inches)

Total Wheel/Tire Diameter:
D = ((A * B)/25.4mm)*2 + C

Tire Circumference:
F = D * pi

 
Reasons to Keep Diameter the Same

Speedometer error is the number one reason to keep the tire diameter the same.  Dodge & Mitsubishi list –5% to +10% speedometer error as acceptable.  I personally think even these percentages are extremely high, and it is ridiculous that if we are driving the speed limit as indicated on our speedometer (we’ll say 60 MPH), that in fact we could actually be traveling anywhere from 54 to 63 MPH.  So – we could be speeding and not know it, or we could be driving slower than we actually think.  Regardless, if you were to go with non-conforming size tires, you may be looking at even major speedometer differences.

Another reason to keep diameter the same is to keep the factory gearing as close to optimum as possible.  For instance – a car is geared a certain way for a reason.  Generally, automakers keep in mind torque applied to the ground and fuel economy.  If you change the diameter of the wheel and tire combo, you also change the amount of torque applied to the ground from your vehicles engine.  We’ll say that at 20 MPH, we are at 3500 rpm with factory tires in 1st gear.  So – we change to a smaller diameter wheel/tire combo, and then we now see that we are at 20 MPH (not indicated by the speedometer, because the speedometer is now inaccurate), but we are at maybe 4000 rpm.  This would allow our cars to accelerate through the gears faster (because they would effectively be shorter gears) without really accelerating in speed, although it may promote more wheelspin amongst other things.  This would also affect cruising speed on say the highway.  Instead of being at we’ll say 3000 rpm for 60 mph, now you may be at 3200 or 3500 rpm.  This can have an effect on gas mileage too.

Another reason to keep overall diameter the same is wheel well gap, fender and suspension clearance, and overall body clearance intact.  With a smaller combination, you will have more fender gap, more suspension clearance, and less ground clearance under the body of the car.  Now, if you go with a taller combination, you have to take into consideration that they may actually be too tall for the wheel and tire to actually fit without interfering with the suspension and fender/wheel well.

 
Other Info Regarding Tire Sizing

All tires are not made the same.  Ratings as far as treadwear, traction and such vary from manufacturer to manufacturer.  The actual tire measurements and “shape” of a tire will also vary between brands.  Some tires, such as the factory 18” tire (Yokohama A028), are more of a square shape, whereas the Nitto 555’s have a bulkier, yet more rounded tread corner.  This may come into play when fighting for space when trying to stuff wider tires under a car.  Also, the actual diameter of two tires from different tire manufacturers may also vary, so it is generally good to stay with one manufacturer and tire size, especially on a FWD or AWD car.  As a rule of thumb, it is generally good to keep identical sized tires on all 4 corners of a FWD car, and if anything – wider tires would go up front, as to reduce understeer.  On an AWD car, it is generally recommended to stay with all 4 tires equal no matter what.  Our AWD cars are prone to understeer more than anything, and wider tires in the rear will quickly make an AWD car understeer even worse.  Also, the 3/S AWD system is such that constant variances in the rolling diameter of the front versus the rear wheels will quickly heat up and possibly cause the center viscous coupling to fail.  I mentioned this earlier, but one thing to take caution about when choosing wheels for your AWD 3/S is to make sure they will clear the brake calipers.