Mustang Wheels And Tires FAQ

Posted by on January 14, 2009 - more

Many people ask questions about their wheels and tires. Overall there are a lot of things that one should know when buying one or the other for their vehicle. This is especially important when people are changing out their rims completely. The wheels and tires you choose will drastically change how your car looks handles. This informational article was written to get the most out about this subject as possible. I will be updating it as new information becomes available.

Wheel and Tire Sections
1. What the Numbers on Tires Mean
2. Tire Maintenance Information
3. Suggestions for Lowered Cars
4. Rim Fitment Overview Information
5. Rim Backspacing Information
6. Rim Offset Information
8. 1999-2004 Fitment Information
7. Frequently Asked Questions (FAQ)

What the Numbers on Tires Mean

Well first lets pick a tire size for an example. A 245/45-17 tire is the OEM GT mustang tire size for their 17 by 8 inch rims.

The 245 number represents the width of the tire in millimeters, every 24.5 millimeters is equal to 1 inch for reference.

The 45 number represents the sidewall height of the tire. The actual height is not 45 millimeters, rather it is found by taking the width and multiplying it by the sidewall number * .01. You see the 45 is a percentage not a size, that being said a 245/45/17′s sidewall height is 45% of its width, and the width in this case is 245mm.

Calculation Examples:

45% * 245 = the sidewall height for this given tire, or .45 * 245 = 110.25mm, The final number, in this case 17, is the size rim that the tire will fit.

Tire Maintenance

Rotate your tires EVERY 6 thousand miles, but if you run a 275mm or wider tire or experience a lot of noise from your tires you should defiantly rotate the tires every 3 thousand miles. Make sure to maintain the correct air pressure in your tires at all times, this can increase your gas millage and life of the tires along with ride quality. A 275/40-17 wears good at 32-34psi from my personal experience. Most tires probably wear pretty good in this range.

Have your wheels/tires balanced if you experience shaking in your steering wheel, for example you may experience shaking only in a given mph range. You only need to have this done if you experience problems. Don’t let some business try to get you to pay for this if you don’t have a problem.

Have your car checked for Alignment every 6 to 12 thousand miles. If your car stays in alignment you can go longer but you should defiantly consider keeping the car in a good alignment state. Keep track of previous alignments with print outs so that you can alter specifications or just see how far out the specifications change from one alignment to the next. Just because they are the factory specifications does not mean they are the best for your car, in your area, under your driving habits.

If you car is lower than stock make sure the shop that aligns your car can get it to stock specifications or what ever specifications you like to use.. If they can’t they may not tell you so be sure to ask for a print out and how far the car is from stock specs. The print outs can be used for future reference to alter alignment specifications for better tire wear.

Ask for a print out of the alignment specs since afterwords you may forget what they did possibly and you can use the print outs as a reference to change some settings for better tire wear. This being said date the print out or you may not know which was the last one you had done.

When you clean your car take a bit to look over your tires and make sure everything is good with them such as no nails in them, they do not look deflated, or anything out of the ordinary.

Suggestions for Lowered Cars

When you get your car aligned after you let your springs settle for 250-500 miles~ see if they can set the car back to the stock specifications, if they can not invest in some caster/camber plates so you can correct the adjustment. Generally speaking you will need Caster/Camber plates after a drop that is more than one inch. This varies car to car so there is no exact rule. When you lower your car the strut and shock shaft will be pushed into the strut/shock body more overall. This can cause them to wear out earlier than you expect.

If you have a set of Caster/Camber plates that allows you to raise the strut shaft upwards to offset the lowering of the car pushing them inwards this will help if not solve this problem. With most lowering springs the spring rate will be stiffer. This means less pressure on your struts and shocks. This alone can make them last longer as they will not be working quite as hard. There comes a point though when the spring is to stiff or to soft for the struts/shocks. Make sure you know of this so you can purchase a correctly valved set of shocks and struts with you springs if you wish.

Maximum Motorsport’s 4 bolt Caster Camber Plates raise the strut travel up a bit so you can run stock struts and shocks on a lowered vehicle (around 1.5 inches of drop) and they also have great quality of CC plates.

Try the install of springs and caster/camber plates your self as it is very easy. If you like working on cars and have a good assortment of tools and a hydrolic jack you should be able to have the swap done in a day or less with the help from a friend. A good set of Caster/Camber plates may be needed to fix alignment issues, so don’t drive around to long out of stock spec. Also don’t buy cheap items as you get what you pay for.

Rim Fitment Overview Information

Rim Fitment Info

Backspacing = Rim Width + Wheel Offset
Frontspacing = Rim Width – Backspacing
Offset = Wheel Centerline – Backspacing * -1
25.4mm = 1 inch

Rim Backspacing Information

Backspacing is the distance from the back edge of the wheel’s back lip to the mounting surface such that the measurement is perpendicular to the mounting surface back side where the rim meets the rotor.

To determine the wheel backspace:
1. Position the wheel with the back side facing upwards.
2. Lay something flat across the back of the wheel. Try to make the item go across the middle of the rim.
3. Measure the distance from the flat item you have sitting across the rim to the wheel’s mounting surface (where the rim meets the rotor). This will be the backspacing for your given rim.

Backspacing helps to determine how much of the rim will be under the car. If you have an eight inch wide rim and five inches is the given rims backspacing then five of the total eight inches will be from the mounting surface back into your fender well. Backspacing can increase or decrease how far in or out your rims sit on your car. Increasing or decreasing your rim’s backspacing can make your wheels/tires less likely or more likely to rub on something.

Rim Offset Information

A wheel’s offset is calculated by the distance from the rim’s mounting surface to the center line of the wheel. There are three potential offset types.

Zero offset – The rim’s mounting surface is even with the center line of the wheel.

Positive offset – The rim mounting surface is toward the front of the wheel (the front being the side of the rim that faces outward). Positive offset wheels are almost always found on front wheel drive vehicles.

Negative offset – The rim’s mounting surface is toward the back of the rim’s center line. Wheels that are large in size are typically a negative offset rim.

How to calculate your rim’s offset – A rim’s offset can be found by measuring the rims width. Next, find the rims backspacing and subtract it from the width of the rim. Finally, divide the answer you got from above by two.

1999-2004 Fitment Information

Front Wheel Fitment:
Your frontspacing should be around 3.00 inches. Ford’s OEM rims seem to range mostly between 3.07-2.89 frontspacing. As you lower your car your front wheels tuck in more and more due to the McPherson strut design they use. Keep this in mind. The widest wheels you should use on the front of your vehicle should be 9 inches. You may still experience some rubbing. The 2003/2004 Cobra use rack limiters to prevent rubbing. This is not a major issue though.

Rear Wheel Fitment:
If you want the maximum frontspacing of your rims without having to roll your rear fenders then you should have 3.5 inches of frontspacing. If you want fender flush then you want to have 4.0 inches of frontspacing. You will probably have to roll your fenders for this.

If you have an IRS (Independent Rear) equipped vehicle you may can get by with a little more as the tire tuck in as they come up much like the front wheels will. The rim will not be flush with the fenders, but after you install tires they should be very close to flush. Some need 4.25 to become perfectly flush, but most want a little but under the fender for appearance. So 4.25 is the maximum unless for a normal sidewall tire.

An 11 inch rear rim with a 7 inch backspacing is about the limit for the size of rims you can run on the rear. You may have to either flip or eliminate your quad shocks if you have them for this. A seven inch backspacing is pretty much all you can get out of it. If you have an IRS (Independent Rear) vehicle then you will need to look into shaving your IRS rear frame bolts or installing button head bolts to clear over a 6 inch backspacing rim. Having your car lower than stock or larger than 17 inch rims may gain you a little extra room here.

Frequently Asked Questions

Q. Will a 17 by 9 rim fit on the front and rear of my 94-04 mustang?
A. Yes, a 17 by 9 rim will fit perfectly fine as long as its backspacing and offset are set correctly, the backspacing for a Ford 17 by 9 mustang rim is 5.95-6.11 inches.

Q.Can I run a more narrow tire on my rims than suggested?
A. Yes, but you must realize that if you stray to much from the sizes listed you will not have enough tire to protect your rims from curb damage. One curbing will usually ruin a rim and can be very costly to even consider repairing a rim if it not ruined. It is much better to spend a little extra money and save your rims from being messed up. Also, the correct size tire looks MUCH better on a rim, a narrow tire looks very tacky and stretched out.

Q. Is it ok to stagger rim and tire sizes, such as run a 17 by 9 up front and a 17 by 10 out back?
A. Yes, this will work great but you must remember you wont be able to rotate your tires and this will cause the front to get more bad wear if your alignment specs are not kept in specified tolerances. If you do this I recommend keeping a close eye on the front tires for bad wear and adjust with your alignment shop accordingly. Finally, check the tire maintenance section for tips on getting the most from your tires.

Q. Will upgrading from a smaller size rim, such as a 15 inch rim, to a larger size rim, such as a 17 inch rim, cause my speedometer to register to fast or to slow?
A. No, as long as you use the correct size tire there will be no difference in the over all diamter of your wheels and your speedometer will register correctly.

Q. What can I do to correct my speedometer if I want to run other than the correct size tire?
A. You can possibly get a speed calibration unit from Mustang Parts Online Restoration and Performance Superstore – Catalogs, Steeda Autosports | Mustang Performance Parts | Official High End Performance Parts, and other such sources. If a speed calibration unit will not work for your given year/transmission combination you will have to buy a chip to alter the speedometer to read correctly. Defiantly check to determine if your year vehicle can benefit from a speed calibration unit, which alters the transmission’s VSS sensor input/output to the computer.

Q. Will rims or tires from a 2005+ Mustang work on my 94-04 Mustang?
A. They will bolt up but the issue with this is the backspacing. The rims sit deeper in
the fender well so when you place them on a pre 2005 Mustang they will set to far in. They will work, but
expect some rubbing. You will need a spacer to correct this. If you go the opposite from a 99-04 to a 05+ the wheels will also work but they will sit out to far. There is no easy cure for this one other than having the mounting surface taken down. This may or may not be possible based on the rims design.

Q. Where do I need to place wheel weights at when balancing?
A. I personally would not place them on the outside of the rim due to it looks bad and scratches your rim. I would either place them on the back lip or use stick on weights inside the rim. The problem with weights on the rim lip is that they can catch on things and be pulled off. If you have the correct size tire for your application this should not be an issue as the edge of the tire will protect the rim lip.

Q. How wide of a rim/tire can I fit on the back of my Mustang?
A. Every year change may differ a bit. On 79-93 Fox Mustangs an 8 inch wide rim fits fine with the standard offset (94-04 offset). If you wish to go with a 9 inch wide rim it will work but expect some rubbing and depending on the fender gap and the lowering of the vehicle you may rub on your fender lip. For 94-04 with a live rear axle you can fit up to 11 inch rims with out to much of an issue on the rear and 9 inch wide rims on the front. If you have an IRS equipped car you may have to modify the IRS lower frame bolts or run spacers to clear a wider tire. It varies vehicle to vehicle since Ford’s tolerances vary. Sometimes it comes down to trying things to see what works.

Q. Whats the big deal with C/C plates, what do they even do?
A. If you take a moment to look at the suspension setup on your Mustang you will notice that as the tire comes closer to the fender (the spring compresses) the tip of the tire tips inward and the bottom of the tire tips outward. This is due to the pivot of the front A Arm’s mounting type and the top mounting type for the strut shaft. The same thing happens when you lower the car, its in a constant lower state. You need to pull the strut shaft mounting location outwards to offset the tip in that the tire has. This is the function of Caster/Camber plates. You may also adjust your Caster (front to back) with most good aftermarket C/C plates. Once again with Ford’s specifications some vehicles may need C/C plates and some may not. No one alignment specification is best for all vehicles as driving habbits and the roads you drive on can vary greatly.

Q. Will wheels/tires effect my gas mileage?
A. Yes, they can. Rotational mass is about 10 times greater than dead weight. That means that every pound of dead weight is equal to ten pounds of rotating weight or close to it. Its not a super drastic change but it is noticeable. Overall do not expect a huge decrease overall, maybe a half to one mile per gallon depending on what you upgrade from and to.

Q. Should I upgrade my stock brake system with new wheels/tires?
A. It is always a good idea if you can, its not needed though. The best upgrades for the money would be a set of Cobra brakes and rotors for your GT or V6. If you already have a Cobra then you should be ok unless you have a good bit of power. Then consider some Brembo brake calipers from a 00 Cobra R.

Q. I have bad road noise, what is the problem?
A. Loud road noise or vibrations from your tires can be cause by tire thread separation. This can be due to many factors. Generally speaking, unless the tire itself is known to have these issues, this is caused from poor upkeep and lack of rotating. The only real thing you can do here is rotate the tires to the back from the front as generally the vibration is much less noticeable if the tires are on the rear of the vehicle. If this fails or the tires wont wear normally when placed on the rear of the vehicle you can have the tires cut down to smooth them again. Make sure this is the problem first as tires being out of balance can cause many problems, the same with warped rims or tires that have internal issues.

Q. When I turn my wheels I hear rubbing, what is the problem?
A. The problem is the tire is hitting something of course. There are a few things you can do. If the backspacing is not right you can use spacers to increase it. You can use rack limiters to keep the tire from turning to far if it only happens at a full wheel turn. The 2003 and 2004 cobra for example had to have a place on the chassis notched for its 17×9 rims as well as a steering rack limiter to keep the tire from turning quite as far as it would cause some slight rubbing. All 03-04 Mustangs have the chassis modification so fear not if you have a GT or V6 in this year set.

Q. Can a tire be plugged to fix a leak from a nail or screw?
A. Generally yes, it all depends on where the leak is at. I have had great success with tire plugs or patches. They are usually cheap to have done as well.

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  • http://profile.yahoo.com/4WOPAWNGTMGKQZXIMJOQSE2Y7Y NarindraG

    what choice of size tires do i have for a Ford Mustang pony package convertible 2010, V6 engine.

  • one_bad_tbird

    I have A 1997 Mustang gt with the 17 inch wheels. I would like to know if i could run 17X8 with a backspace of 20 up front and 19X9.5 with a backspace of 20 on the rear. Front tires would be 245/45/17 and rear would be 275/40/19. Would this be ok?

  • Dale Phillips

    have a 94 mustang gt that im lowering 1.6 in front and 2.0 in the rear on 17×9 in front and 17×10.5 in rear do I need wheel spacers and if so what size?

  • Enrique Gonzalez

    can I put 18×8 with a +35 offset wheels on a 02 mustang gt?