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Tire Plus Sizing Guide by Cody Graham

Definition

Plus sizing is using larger diameter wheels with a tire that has less sidewall which keeps the speedometer relatively accurate, as well as other factors such as torque that is transferred to the ground, handling ability, clearances for suspension, as well as others. More on this later. Reasons for plus sizing are many. Many people plus size just because it looks good. This is of course an opinion. 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. Of course, just changing to a different wheel and tire can save weight, keeping the same size, but 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. It will handle better, the suspension will work better, and it will accelerate faster, and stop quicker.

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. Width - in millimeters (mm), Aspect Ratio - percentage (%), and diameter - in inches (in.). Other things noted on a tire are the speed rating, treadwear rating, traction rating, temperature rating, and load capacity as well as maximum inflation pressure. We won't bother with those, as we are concerned with sizing right now. For 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 tires overall diameter, we would take the sidewall height times 2, then add the wheel diameter of 16. 25.74 inches diameter is our result. So, any tire/wheel combo we would be interested in would be similar in diameter to this. Other common sizing for 3000GT and Stealth cars are 245/45R-17 and 245/40R-18. For the 17" wheel, after the math, we get a wheel/tire diameter of 25.68 inches, and the 18" wheel/tire gives us a result of 25.71 inches. As you can see, any of these are within 6 hundredths of an inch. Tires will wear more than that over their lifetime, so the small differences are of no consequence. Basically, this says that any of the 3000GT / Stealth wheel/tire combos will work on any other GT / Stealth no different than the original tires/wheels that were on that car. (brake clearances are a slightly different issue, so make sure the wheels will clear the calipers before purchasing or mounting just any wheel to an AWD model - non-turbo models generally do not have this problem) When plus sizing, you do not have to stay with the same width tire, but you do need to keep in mind that width has an effect on the aspect ratio that you should use. This is not really plus sizing related, but when going to a wider tire, and keeping the rim size the same, you will need a tire with a smaller aspect ratio to keep the diameter the same or close.

Common sizing for 3000GT and Stealth Cars
Tire Size Rolling Diameter
Factory Sizing: 
???/??R-15  
225/55R-16 25.74 in.
245/45R-17 25.68 in.
245/40R-18 25.72 in.
   
Optional Sizing: 
245/50R-16 25.65 in.
265/40R-17 25.35 in.
265/35R-18 25.3 in.
245/35R-19 25.75 in.
   
Theoretical:  
225/35R-20 26.2 in.

*Wheel offset can be found on my FAQ page.

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 (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

Side Wall Height
E = A * B (resulting in mm)  OR
E = (A * B)/25.4mm (resulting in inches)

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

Tire Circumference
F = D * Pi

Where:
   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)

Reasons To Keep Diameter The Same

Speedometer error is the number one reason to keep the tire diameter the same. Mitsubishi lists -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 in tact. 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 from manufacturer to manufacturer. 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 vicious 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.

Thanks to Cody Graham for writing this document.

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