I've noticed recently that when some one purchases a bigger wheel neither the person selling or the person buying has any idea of the proper up grade to match the circumferance of the original wheel assembly. A good example of this is with my 1997 Rav4. I was purchasing bigger wheels and decided to look into some rubber that would compliment the wheel. The guy I was dealing with suggested a few tires that would, supposedly, not change the speedometer reading. He told me that there was an acceptable 15 percent change +/- in the speedo reading, it wouldn't be much of a change... so he said. Upon researching more upon those, supposed, facts I found that in fact there was a bigger change than he told me. Example; My original tire size is 215/70-16, the new wheel I chose was an 18" wheel and the tire size he "SUGGESTED" for this new size was a 235/45-18... The total circumferance was smaller than the original wheel assembly and caused my speedometer to register faster than I actually was going... Fuel consumption was horrendous and my engine whined! The point is that you need to do some research on the wheel and tire assembly that you plan on purchasing or you'll end up getting a speeding ticket if the assembly is bigger than you really want it to be. I.E; Bigger wheel assembly= less revs at the wheel= slower speedo reading than actual... smaller wheel assembly= more revs at the wheel= faster speedo reading than actual. I found out after some research that the correct and even size to my O.E. tire was a 225/50-18! And wouldn't you know it! This new size is the same size as the assembly being used on the new Skyline... go figure!!! Who's had some nightmare issues because of things like this? Let me hear you!!!  

It happens everyday. Put a tire in your tire rack and all of a sudden you're a discount tire expert. Tires and wheels are very touchy things. Mess with them and they'll mess with you. The important things besides fitting under your car and good looks is can it carry the weight of your car.

To see how it fits Bad Boy Tires has teamed up with Miata Garage and we link their tire size calculator for your fitment questions. Although it doesn't take into effect the weight just the size. Check it out as it's a great tool. http://www.badboytires.com or go directly to the links page and click on tire size calculator at Bad Boy Tires - Tire Size Calculator. We hope this helps because it does us everyday.  

I  have never gone into discount or Tire Rack and not known what size I was getting.

I rock 245/45/17's for the Stock Wheels, and 245/40/18 for the SSR GT1's. Never had any issues with tires or sizing. Like I said, I make sure I know what I am getting before I bother going in. I would only expect people who don't know what should be on the vehicle, or are too lazy to google "tire size calculator" to be victims to this.

A lot of tire places have some pretty incompetent people. I'm not saying all do, but I've run into my fair share at places. I'm a big do it yourself fan, and the only things I leave to shops are machine work ( IE: Balancing for engine, etc. ) and paint work.

The only way I trust that it is done right, is if I do it.  

Black,

Relative to your stock wheel/tire combo, the initial one you had where you had "engine whining" was 5.8% different. As in instead of 60mph, it would say 63.4.

You mean to tell me a difference of 3.4 mph made your engine whine and fuel consumption all happen?

Doing a little math will go a long way. If you want even better than 225/50/18, get some 235/50/18. You're rolling diameter will only be 2.16% different, meaning you will see 61.3 on the speedo, instead of 60. That's a negligible difference.  

After some research, BOOsted, that is what I came up with as well. That was a long while ago for me. Toyota and Ford have a slightly complicated math equation that does the same as tire calc and I verify with that as well. I only posted this to get people not in the know to do more research for themselves before getting in too deep. I don't get those issues any more since I educated my self with my rav4. I do get along very well! But thank you for your input!