To all who may know differently please correct me......I finally hooked the max with a set of 17' rims. Im told that the performance and ride comfort would be affected. No real difference in ride noticed, aceleration is a little more sluggish, BUT NO ONE said anything about improved performance at higher speeds!! I checked my Rpms at 80mph with my factory wheels and it was slightly over 3,000. Now at the same speed it reads about 2,600. Am I experiencing the infamous Mod Placebo effect here? I just find it hard to believe that with all the discussions on the topic of Larger Wheels that such an important detail is overlooked......well Maybe I missed it. All input greatly appreciated.

 

It depends. What size tires are you running? 235-45-17 is pretty close to the stock size. It's pretty hard to get the outside diameter exactly correct becuase even different maker's 235-45-17 are different than each other.
If you are running a tire that has a different outside diameter than stock, your speedo will be affected. Also the overall gearing will be different, again changing your speedo and rpm readings.

 

235-45-17 are the size im using. I never did understand what all those numbers mean. The brand is Yokohoma (Avid 4's I believe). So do you think that My Speedometer is just off? I was told on this forum that that wouldn't be too big a problem so long as I stay under 18'. Please advise

 

I think even if the wheel + tire combination is same as stock in diameter, if it is also wider... it will cause some effect as well,

go to http://www.miata.net/garage/tirecalc.html

you can match the stock diameter with the aftermarket set up you have and see how much it can throw your speedometer off...

 

Aaaaaah.....whats the specs on a a stock 15' rim? :]

 

Remember the % error will increase with speed. Check with a friend in a stock car. Cruise at different speeds and have him check his speedometer v.s. your reading +/- 3mph should be ok. Not all speedomoters are correct. You can always get your speedometer readjusted.

 

just look at the mile markers on the interstate and time how long it takes to go exactly one mile.

 

Capitone, stock, 205/65-15 for GXE and GLE, 215/60-15 for SE. The percentage error for 235/45-17 is 0.7%, meaning the speedometer is off about 1 MPH at 60. The mile marker idea is a great one to see how accurate your speedo is.

 

Hey Thanks! Medicsonic. Thats not to bad a difference. So even though the car may work harder when accelerating, the engine is working much easier at higher speeds......

 

if the speedo is off, the odo will be off too, right?

 

just drive by one of those police things that they put on the road and check what the speed is

 

Yes your Odometer will be off as well. I have 18" and I just got new tires today! 235/40/18 My percent change is only ~.36% which equates to 3 revolutions per mile. To me that is acceptable. BTW the other thing no one really talks about with new, bigger wheels is the increased tire prices. 17" may not be as much of a problem as 18", but it cost me $741.50 for 4 Nitto NT555s WITH a discount. It's great to have a bunch of Z rated tires to choose from until you have to pay.
-hype