- Joined
- Sep 12, 2023
- Messages
- 1,084
- Location
- Charlotte, NC
- Vehicle Details
- '95 Thunderbird with '18 TF 5.3L - SVO Engine
Trunk Monkey said:
Tires, Rims & the MN12
What do those numbers mean? Tires size is measured by three numbers which will be displayed on the side of the tire and one letter representing the speed rating. A stock MN12 (except for the SC) uses a tire size of 205/70/15. The first number, 205, is the width of the tire in millimeters. The second number, 70, is the aspect ratio or height of the sidewall, in percentage. Here the tire's sidewall is 70% of the width. The last number, 15, is the size rim it mounts on. Some tires will have an "R" before the last number. This simply means it is a radial tire. Somewhere around the rim size you will find a letter representing the speed rating. Sometimes it is with the R such as HR but mostly it will be alone after the rims size. It is important not to push a ire past it's speed rating as it will become unstable and will balloon and can burst. Some people may ask why you need a tire rated for such a high speed. The answer is that as you get a higher rated tire you almost always get a better handling tire as well. So it's not about top speed, it's about control at lower speeds. Here is the speeds the letters represent:
P = 93 mph Q = 99 mph S = 112 mph T = 118 mph U = 124 mph
H = 130 mph V = 149 mph Z = 149+ mph W = 168 mph Y = 186 mph
Changing tire size on stock rims Tire size plays an important role in the performance of your car and choosing the correct size is important. By simply changing to a wider and better performing tire you can get a drastic improvement from your car. In general, a wider tire will give you better traction and handling while in the snow a narrower tire will give the best results. Here we will just focus on performance. More important than width is tire circumference and when changing tire sizes you should try too keep as close to the stock circumference as possible. If you vary in this area not only will your speedometer be inaccurate but a smaller tire will actually make the car slower! And of course when changing tire size make sure it will not only fit on your car but your rim. The stock rim size is 15" by 6.5". A common question is how wide of a tire can I fit on my car. Physically you can fit a tire 245mm wide on the front of an MN12 and even wider in back but that is too wide for a stock rim. The width of a tire you can fit on a rim is subjective to the aspect ratio due to tire flex, and with keeping circumference in check the widest for a stock rim is 235mm with a 60% aspect ratio. Some people have mounted wider tires but either the aspect ratio was greater than 60 (making for too large of a circumference) or they were simple unsafe.
When changing tire size you need to calculate tire circumference for both the stock and planned tire and try to get them as close as possible. Circumference is calculated by the multiplying 2 times pi times the radius of the tire. This can be tricky in that the tire is measured both in millimeters and inches. First find the tire width, 205 for a stock tire. Now calculate the sidewall high by multiplying the width and the aspect ratio. Here it is 205 x .7. This gives you a sidewall height of 143.5mm. Now convert the sidewall height to inches by dividing by 25.4 to get 5.65". Now calculate the tire height by adding the two sidewalls to the rim size to get 26.3". Lastly, multiply by pi, or 3.14, to get the circumference of 82.58". You want to follow the same equation for the planned tire size to try to get the circumference as close as possible. I would recommend within 5% of the stock size.
To make things simpler, the best tire size to upgrade to is 235/60/15. This tire has a circumference 99% the size of a stock tire while adding the benefit of a wider tire.
Changing rim size and style Changing rim style and size is a great way to add more style to your car and improve your handling. A larger rim which requires a tire with a shorter sidewall will not flex as much as a stock rim and tire combination. This will result in a firmer ride and improved grip. The first thing to look for in a rim is the bolt pattern to make sure it will fit your car. The MN12 uses a bolt pattern of 4.25" x 5.
Note: the Mustang uses a bolt pattern of 4.5" x 5 and these rims will not fit on an MN12 without swapping on Mustangs hubsas well which can be done.
If you find a rim you really like that is only offered in 4.5" x 5 it may be easier to contact the store or manufacturer and have them custom drill you a set. Yes, they do do this for you but may take 4+ weeks to have done.
The second thing to look for in a rim is proper backspacing. Backspacing is the distance from the hub to the inside edge of the rim. Improper backspacing will drastically effect the cars handling besides making the rims either stick out too far from the car or sit too deep in the wheel well. To find the proper backspacing you want to add half of the difference in rim width to the stock backspacing. A stock rim as mentioned above is 6.5" and if you are switching to a 9" wide rim the difference is 2.5". Half of that difference is 1.25" and when added to the stock backspacing of 5.57" your ideal backspacing is 6.82". It is most likely impossible to find a rim with your ideal backspacing, but try to aim as close as possible to it as you can.
The third thing to look for in a rim, which is commonly overlooked, is hub bore. Contrary to common belief, the rim does not rest on the car by the studs but rather it sits snugly on the hub. The studs and nuts just hold the wheel onto the hub but proper hub fitment is achieved by proper rim bore. If the bore is too large the rim will not rest on the hub and the weight of the car will rest solely on the studs eventually causing them to snap. Also, without the rim properly centered on the hub it will cause a bad vibration and the handling will feel mushy as the tire rotates off center. Most aftermarket rims have a larger hub bore than stock rims and therefore hub centric rings are needed. These small rings mount inside the hub bore of the new rims and their outer diameter is that of the rim bore while their inner is that of the hub. This lets your new wheel mount properly to your MN12.
The last thing to look for in changing rims is if they are too wide for your car. A 9" rim with the proper backspacing will fit fine on our cars up front with just a slight protrusion from the wheel well, but not enough to rub. In back a 9" wide rim with proper backspacing will fit but will rub on the wheel well lip if the lip is not removed. To remove the lip I simply cut slits in it every 4" with a rotary tool and bent the sections up into the wheel well with a hammer. It is recommended that you paint any exposed metal to avoid rusting when you do this. Removing the lip in this manner allows just enough room to fit a 9" rim with proper backspacing.
Picking the right tire size for your new rim This process is basically the same as if you were changing the size on a stock rim except most likely your new rim is wider than stock. As before, it is important to keep the circumference equal to stock. And again, it is important to make sure the tires will fit on your car without rubbing. With proper backspacing 245mm is the widest you can fit in front AND in back when you remove the wheel well lip. If you try to go wider in front they will rub on the upper control arm and in back on the wheel well lip when you go over a bump however the back may not rub over bumps if your car is not lowered. If you were planning on a wider tire my suggestion would be to instead of going wider, get a better performing tire. Chances are a better tire in a smaller width will out perform a lesser tire in a wider size and cost the same.
Mounting wheels It is important to make sure your lugs are on tightly but not too tight. Over-tightening can damage the wheel and will lead to rotor warpage. To make sure your lugs are properly tightened always use a torque wrench and torque the lugs down to 90-100 lb-ft. A decent wrench may cost $80 or more but it's a small price to pay compared to having to replace your brakes or worse. After mounting your wheels be sure to re-torque them after 100 miles as the lugs will work loose after initial driving. Another point to pay attention to is hub balancing to the wheel. The end of one stud will be painted yellow and on a stock rim, that stud is to be aligned to the lug hole on the rim next to the valve stem. This balancing trick will prevent vibrations at higher speeds, usually over 60 mph. On the Cobra rims you will notice the valve stem is directly between two lug holes. I found that the best balancing is achieved by mounting the yellow stud directly across from the valve stem rather than either of the two closer holes.
Where to get tires & rims Basically any shop that has the right size rim at the right price will work.
Trunk Monkey said:
I highly recommend Custom Wheels Market. They have the standard selection of rims for our cars including 280 series Cobra R & Cobra R Motorsport, Y2K Cobra R & Cobra R Motorsport, and the Bullitt style. All come in either silver, polished or chrome and their prices are the best around and their rims have the proper backspacing and they sell the proper hub centric rings too. Their help and service was perfect and my rims shipped the next day. They even offered to take them to a local shop to have them custom powder coated before they shipped them to me! Check out their site at http://www.wheelsmarket.com or call them at (909) 433-9009 10am-4pm Pacific Standard Time.
For powder coating I took them to Craig at P&M Finishing in Waterbury, CT. They did a great job color matching them to my car and sandblasting and powder coating them for me. You can call them at (800) 927-8285.
My 18" x 9" 280 series Cobra R replicas with Y-rated Bridgestone 245/45/18 S-03 on a 1.5" dropped suspension
