This FAQ is intended to provide some basic information about aftermarket wheels. It will attempt to provide basic information about wheels and tires, using the 3rd Generation RX7 as an example, and what to look for in an aftermarket wheel and tire combination. No slight against other car models is intended, and I'd be glad to add any additional information about other types. Please send corrections and/or additions to Ed Hahn, ehahn@mitre.org .
These links will take you to pages with specific information for your car. You may want to read the information on this page first, however.
1) What should I know about tires?
The stock tires on some models of RX7/Miatas are known as "all season" tires, while other models have dedicated "summer" performance tires. While all-season tires are certainly adequate for going around town, and even some spirited driving, they were designed for year-around usage (a.k.a. Mud & Snow [M+S]). In other words, some compromises were made to allow for adequate snow performance. While this is good for most applications, some people would prefer dedicated snow tires in the winter, while others would prefer better dry & light rain performance, or both.
A typical tire from the factory has the size P225/50VR-16. In English, this translates as:
P Passenger Tire 225 Tread Width in mm 50 Aspect Ratio (see below) V Speed Designator (see below) R Radial Construction 16 Rim Size in inchesNOTE: Currently, a move seems to be afoot to give tire dimensions in a slightly different order, with the speed rating moved after a "load rating": e.g. P225/50R-16 91V. The "91" is a load rating, which unfortunately doesn't translate directly into a weight. Not all manufacturers are providing their dimensions in this alternate form yet, however.
2) Tread width and rim size seem fairly straightforward. What do the aspect ratio and speed rating mean?
The aspect ratio is a way of describing the sidewall height, or the height which the tread sits off of the rim. Rather than give a simple length measurement, it is expressed as a percentage of the tire section width. In the above example, the "50" means the sidewall is 50% of the maximum width of the tire, or 112.5mm (about 4.4 inches). A lower aspect ratio means a lower "profile" tire, which generally means a stiffer sidewall and firmer cornering, sometimes at the expense of ride comfort. People often speak of tires with the same aspect ratios as a "series" of tire, for example, "50-series tires" means tires with a 50 aspect ratio.
Performance tires are also given a speed rating to give people some idea of how the tires were constructed and how far they can be pushed. The table below translates speed rating to design maximum mph.
Q 100 mph H 130 mph R 106 mph V 149 mph S 112 mph Z over 149 mph T 118 mph W 168 mph U 124 mph Y 186 mphNote that while Z implies performance in excess of 149 mph, two new categories (W & Y) have been recently added to more fully describe their capabilities. In these cases, a "Z" might precede the "R" in the tire size itself, while the "W" or "Y" would appear after the load rating. (e.g. 235/45ZR-17 88W).
3) I probably won't ever get near 149 mph. Why should I buy tires that are rated that high?
A tire that is able to take a higher speed is designed and constructed differently than more pedestrian tires. It has lower rotating mass (i.e. moment of inertia) and stiffer sidewall construction, which translates to better acceleration, braking, handling, and cornering, even at low speeds.
Furthermore, since tires turning at 140+ mph tend to generate lots of heat, V & Z rated tires tend to be more forgiving if you occasionally overload or underinflate your tires.
4) What should I know about wheels?
The stock alloy wheels on 3rd Generation RX7s are 16x8JJ. In English: 16 inch diameter, 8 inch rim width, with the JJ designating the wheel contour (i.e. the expected tire bead and rim shape).
While there are many details about the wheel contour, according to Luis Haddock, most P-metric radials (typical tires, rather than TRX or other rarely-seen variants) will fit a wheel with a J, JJ, or JK wheel contour. (In other word, as long as you are using 13-17 inch tires, you can use J, JJ, or JK wheels. Bottom line - don't sweat it.)
The offset of the 3rd Gen RX7 is +50mm, with a 5-lug, 114.3mm radius circle bolt pattern. The centerbore (hub) is 67.06mm in diameter.
5) Whoa. What's offset?
Imagine your wheel as a round two-layer cake, with the street side of the wheel corresponding to the top surface of the cake. If the layers are exactly the same height, the center layer of frosting on the cake would be at the centerline of the wheel. The offset describes how far from this centerline the plane at which vehicle hub and wheel touch each other, with positive toward the street side.
Naturally, if the offset is non-zero, there is an uneven load on the wheel bearing, which was designed into the wheel bearing. Choosing a correct offset is very important, as the bearings on the car are designed for a specific load. The rule of thumb is that an aftermarket wheel offset shouldn't more than 20% of the stock value, which for 3rd Gen RX7s gives a range of 40mm to 60mm or so.
(Incidentally, the tire width does make some difference in choosing a correct offset. If we put significantly wider rubber on our cars, ideally, we would want to add in a little additional offset (in proportion to the increase in tire width). For example, if we were moving from 225mm to 275mm rubber, this corresponds to an increase of 22%. In this case, we would want to add the same percentage to the offset value. However, other constraints, such as wheel well size and suspension clearance, often prevent one from getting wheels with the ideal offset. This means that the wheel hub will be loaded somewhat differently from the factory design, and thus should be periodically inspected for signs of premature failure.)
6) And Centerbore is?
The centerbore is the size of the hole in the center of the wheel mounting face, which the plastic cap with the Mazda logo sits in. It is very important that an aftermarket wheel have the same centerbore as the stock wheel, as the matching flange on the vehicle hub is sized exactly the same to ensure that most of the load is taken by the axle and not by the mounting bolt studs.
If you don't have the correct centerbore, your wheel will either not fit at all on the car, or will vibrate noticeably.
7) What's wrong with the stock wheels, and why should I consider aftermarket wheels?
The stock wheel on the 3rd Gen RX7 is in fact a very good wheel for a stocker, as it is fairly wide and is quite light (15.4 lbs each). However, there are three reasons to consider aftermarket wheels: 1) you want more performance, 2) you want better or more distinctive styling, and/or 3) you have money to burn. (I will keep my remarks focused on the 1st reason.)
For best performance, I've uncovered two rules of thumb for matching up wheel and tire widths in the course of research: 1) the tire width and wheel width should be about the same, and 2) in no case should the wheel width be less than 70% of the tire width . Doing the numbers for a stock 3rd Gen RX7, 8 inch wheels are about 90% of the width of a 225mm tire, which conforms to both rules of thumb.
8) Can I put wider tires on the stock wheels?
As for putting wider tires on the stock wheels proceed with caution. While a wider tire will probably give a bit better performance, the wheel width may cause "mushing" at the cornering limits as the sidewalls get pinched down to fit the rim. BTW, a magazine article a few years back did a report comparing 1) stock wheels and tires, 2) stock wheels and wider tires, and 3) wider wheels and wider tires, using a VW Scirocco. Naturally, combination (3) did the best, but the most improvement in lateral g's happened when the wider wheels were put on, not when the wider tires were put on the stock wheels. If you do go with wider tires, keep the 70% rule in mind.
9) Can I put on really wide tires if I get wider wheels?
Other 3rd Generation owners have successfully put on 255 front / 275 rear on their cars without body/suspension work, but sometimes with some minor clearance problems (i.e. rubbing at extreme steering angles and some vertical bump-stop problems.) I'm soliciting other figures for maximum width for other generation RX7s and Miatas; if there is a consensus on the max practical width, please send it to me (along with the wheel offset used).
10) How can I find a tire size which will keep my speedometer and odometer reasonably accurate?
As for choosing a tire size, the Toyota MR2 list has put a page to help determine compatible tire widths and aspect ratios (it works for all car types). Any wheel diameter + tire diameter within 5% of stock should be fine. Check it out!
11) What should I be looking for in a performance or snow tire?
In general, tires can be designed for 1) dry traction, 2) wet/snowy traction, 3) noise and ride comfort, 4) treadwear, and 5) cost. It is rare to see a tire which can be optimized for more than two of these constraints. All-season (M+S) performance tires will tend to provide better wet/snowy traction, usually with some compromises at the limit in dry traction. Snow tires typically take this to an even further extreme. Summer tires are usually optimized for dry traction, sometimes at the expense of all of the other factors. Drag racing or autocrossing tires can be extremely dangerous in cold weather or rain! (BTW, I've ignored the DOT Treadwear / Temperature / Traction ratings intentionally, as they do not make good comparison benchmarks between brands.)
For snow tires, look for lots of sipes, or cuts molded or machined into the treadblocks on the tread. Also, narrow tires tend to perform better in the snow as they can "dig into" the snow rather than "floating" along on top. If you only get the occasional snow which doesn't accumulate, you probably don't even need dedicated snow tires. Luis Haddock (of the Talon list) has tested several snow tires personally, and recommends tires in this order (best to worst):
For street performance, look for large tread blocks without sipes, but with enough circumferential and radial grooves to allow for adequate rain performance. For maximum performance, the tire should be as light weight as possible, but this comes somewhat at the expense of impact durability. Do not use racing slicks on the street, as they can be very dangerous in an unexpected rain, or in cold conditions.
There are a lot of good street performance tires out there right now. The rule of thumb for street seems to be that your wallet can be your guide, as the cheapest high performance tire for sale now is still much better than the most expensive one of 5-10 years ago. Luis Haddock believes that it would be very difficult to out-drive many of the tires on the street. Some more popular models (in no particular order) are:
Modifyd suggests getting a pair of Hoosier slicks for max performance, as they heat up nicely and stick very well once warmed up.
Brands used by Talon list members include:
For autocrossing / road racing, look for a very stiff sidewall, with a sticky compound. Large treadblocks and/or circumferential tread areas. Again, light weight is very important, perhaps most important in this application.
People recommend:
Again, there seem to be a million of these tires out there these days, but here's a small list of tires which people have had positive experience with:
Bottom line - get the lightest wheel you can find for maximum performance. When combined with a light weight tire, this combination leads to the best vehicle dynamics, as there is less momentum (angular and linear) to overcome when accelerating, braking, or changing directions. Forged aluminum (which is formed by heat and pressure) can be constructed to yield a higher strength wheel while remaining lighter than cast wheels, but you will pay for the benefit.
Note that for 3rd Gen RX7s, one really has to go to forged wheels to get a better performing wheel.
Since there seem to be a million different wheel makers and individual models, I'm not going to suggest anything specifically, since it's a matter of personal taste. Fikse forged wheels seem to be popular among local RX7 owners. I would suggest talking to someone like the Tire Rack who will guarantee that a given wheel and tire combination will fit (although they will not deviate from standard "Plus" fitments). They generally have multi-page color ads in C/D, R/T, and MT each month.
In choosing a wheel diameter, keep in mind that a larger diameter wheel will likely improve handling and cornering at the expense of ride comfort. Tires start to become very expensive beyond 16 inches, as well.
For the street, keep in mind that you will meet the occasional curb, pothole, or road hazard. It might be worth it to trade off a little extra weight for a more durable wheel.
For the drag strip, again, since you hopefully won't be needing much cornering performance, the best wheel/tire combination might actually be a smaller wheel and larger tire, as the tire generally weighs less than the wheel.
For autocrossing / road racing, you need to have a very strong wheel for cornering, while maintaining as light a weight as possible. As for reducing weight, note that Road and Track for their autocrossing project 1st Gen RX7 went from the stock 15" wheel to a 13" wheel (!) with Toyo Proxes tires, in the name of saving weight. On the other hand, it's usually in racing that you see the largest wheels / lowest profile tire combinations. In short, YMMV.
Finally, make sure that whatever wheel you choose meets the offset and centerbore guidelines in sections 5) & 6) above to ensure you don't break your wheel bearings prematurely.
14) How can I choose a wheel style looking at the itty-bitty pictures from the car magazine ads?
I've actually found a reasonably good way. Find a digitized picture on computer of an RX7 or Miata from the side (or scan one in yourself); it should be a true side view for best results. Cut out the little wheel pictures from a couple of magazines (so you have two of each). Rescale the picture so that it roughly corresponds to the size of the wheel. Put tape on the back of the wheel and stick it on your computer monitor at the appropriate location. Voila!
Actually, this gave me a much better idea of how a wheel style would have looked on my previous car; I was considering another style, but decided it didn't look as good as I thought it might. The Tire Rack has a demo application of the Miata and 3rd Gen Rx7 showing various wheels electronically pasted on the car.
15) This FAQ sucked! Where can I get better information?
Luis Haddock recommends two books: 1) Carroll Smith, "Prepare to Win", and 2) Jim Horner, "High Performance Wheel and Tire Handbook". For you hardcore enthusiasts with racing applications, these are a good source for detailed information about wheels and tires.
Conclusion / Acknowledgments
I hope you enjoyed this FAQ, and hopefully learned something as well. (I certainly did researching the topic.) If you have any corrections and/or additions, send them to me at: ehahn@mitre.org .
Thanks to Nathan Freedenberg, John Duff, Todd Day and the Eagle Talon List, the MR2 List, Jim McKenna, Pat (Modifyd), Bill Lawrence, Eddie Hernandez, Luis Haddock, Larry at the Tire Rack, various Car & Driver, Motor Trend, and Road & Track magazine articles, and Porsche 944 Turbo racer Emil Blassic for their help and insights in assembling this FAQ.