yes the needle can be adjusted, but your speedo will still be off due to the greater circumference of the tire. you may be close to right at low speeds, but the faster you go the worse it gets. for what % bigger the circumference is thats the % your speedo will be off.

 

if i remember correctlywith 205 40 17 your speedo will be off about 2mph with 215s it should be a little more than that but if you know how much your off well drive slower

if you have 215s and the speed limit is 65..............drive at 60 and you should be fine

the added benefits of wider tires will make up for any math problems you have to think of

 

on tibs our stock tire size is 205 / 50 R15.

205 is width of the tire, in mm
50 - is profile height % (percentage of width)
R15 - outer diameter of the rim

calculating profile height
width / 100 * profile height %
205 / 100 * 50 = 102.5 mm (or 102.5 / 25.4 = 4.04 inch)
so profile height is 4.04 inches

calculating outer diameter of the wheel
outer diameter = profile height + rim outer diameter + profile height
outer diameter = 4.04 + 15 + 4.04 = 23.08

So, outer diameter of the stock wheel is 23.08 inches.

So basically if you keep the outer diameter of your wheel around 23 you will be ok (minimal change in riding height and speedo)

So lets see, here's the outer diameters of some tire sizes:

205/45 R16 = 23.3" *closest, but is a 16"*
205/50 R16 = 24"
205/45 R17 = 24.2"
205/40 R17 = 23.5" *here's your best bet*


Anyway, the speedo needle can be adjusted yes, but only linearly. The tires affect the speedo exponentially, as speed increases so does the %age of MPH your speedo is off by. I suppose if you really wanted to get into math you could find a best fit line to decide just how much you want to take your dash apart off and adjust your speedo by .. but we're only talking about a few MPH if you just buy the reccommended tire size, so why worry about it!


*just check out this website you can see for yourself .. it's a little java-based calculator .. http://www.miata.net/garage/tirecalc.html

 

Here's a good calculator where you can compare a bunch of different tires:

http://www.1010tires.com/TireSizeCalculato...p?action=submit

A 215/40-17 is 23.77 inches and will show a 3.04% SLOWER speed than what you're actually going. (I dunno about you, but if the speedo's going to be off, I'd rather it be slower than faster)

So:
55mph = 52mph
60mph = 58mph
80mph = 77mph

 

Umm...my tib came with 195/55-R15's stock...

 

oh shit, did i f*** up? lol..

 

That's even better for the 215/40-17's they're 23.20" high.

It's only a 1.405% difference on the speedometer, which is close enough to not have to worry about the math.

55mph = 54.22mph
60mph = 59.15mph
80mph = 78.87mph
100mph = 98.60mph

I'm running 205/40-17's on my Elantra and I feel like I could use a little more width on the tire for it to look "just right".