i apologize if this issue has been addressed already.

anyway you know how if you get bigger rims and tires it throws off your speedometer?
how could i get a speedometer that would show the right speed? or how could i correct it?

 

avoid the problem in the first place and use proper "plus" sizing with your wheel+tires.

 

i agree, if you do the tire upgrade the right way there should be no problems.

 

Precisely.

What they mean by proper plus sizing, is just because you increase the diameter of the wheel, you do not necessarily increase the overall diameter of both the wheel and tire. Sure, lower profile tires offer better handling performance, but they also are required to maintain the original combined diameter of the tire/wheel combination.

 

yupyup, to keep the same wheel height when getting bigger rims you need shorter side walls to compensate. thats why you see big rims with small side walls.

 

On a side note, aside from changing the speedometer accuracy, choosing an innacurate wheel/tire combination can also alter gear ratios, though usually to an insignificant degree. I intentionally trod my daily beater (96 prelude) with slightly undersized tires and it compliments the short gearing quite well. The difference was more noticable than expected.

Enlist the help of a professional before making any purchases, to ensure you maintain proper sizing.

 

A lot of people buy bigger wheels and tires to "fill" the wheel well. It's a simple matter to find the difference in diameter between tires. Just look up the stat's on line.

Example:
Brigestone S03
Stock size: 225/50/16 diameter 23.9"
Bigger size: 275/35/18 diameter 25.6"
The difference is approx 7%.

So if you buy these bigger tires, and your speedo says 65 mph, you are actually going about 7% faster..... 70 mph.

You don't need a new speedo to know your speed.

 

if you want you can get a digital/analogue stepper motor speedo wher you punch un wheel/tyre sizes and it does the rest.it can also do 1/4 mile 0-60,etc and has rpm and speed plus programmable shift lights with a recall data facility.cost bout £400 that would be bout $650 spa design here in england do it or stack etc.

 

Most upgrade wheel/tire combos are larger in diameter than the stock setup... thats just the way it is, "proper" or not. The speedo change isn't serious... a couple MPH. It does change the gearing slightly as well... if that really gets your panties in a bunch, you can always change to a 4:30 RZ rear end gear.

 

It will also throw your ABS out of wack, but the minimal tire change should not make a huge difference.

 

I find that difficult to believe. How about an explanation?

 

It will NOT.

I have virtually the worst 'offending" combo when it comes to diameters: 275 rears are 25.7" in diameter, and front 235s are 25.3, and i can tell you the ABS works fine, as it does for scores of other people with similar or other combos.

 

The ABS still works fine for those of us on grippy race tires too.

 

My mechanic has 275-40 - 17 in the rear and 235 -40-17 in front ,so3 s The speedo is now more close to the radar numbers than the stock tires were . So that worked on his car . The speedo sender unit shaft is offset from the mounting hole on the trans,so you can adjust the gear mesh . This means you can put in a bigger or smaller gear in as needed . My mechanic has made the gears from white nylon to replace the stock plastic one. You need to know if you need bigger or smaller . The unanswered question is how many teeth can you increase or decreace ?

 

Guess you already have the rims and tires but, here's a calculator.
http://www.miata.net/garage/tirecalc.html

 

nice link, demonstrates real well the whole upsizing deal.