This has probably already been seen, but there ya go anyway...great site

 

 

this one?

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

 

This one

http://www.dakota-truck.net/TIRECALC/tirecalc.html

 

oops
I guess that I forgot to put the link in:
http://www.tolan-hoechst.com/cars/tirecalc.htm

 

225 vs 235 - 17s

225 235

Width +.39 +.79
Sidewall height -.25 -.07
Overall height -.49 -.14
Circumference -1.55 -.43

Given this information, the 235 seems a MUCH Better Option than the 225, at least closer to Spedo.

 

wow, talk about archive material!

 

Yes, that is the one....

 

That would be going from 50 to 45 aspect ratio? Yup, 10% less,
need 10% more someplace else..

 

Table moved to here:

<pre>
<font size=+1>

225 235
------------------------------------------------
Width +.39 +.79
Sidewall height -.25 -.07
Overall height -.49 -.14
Circumference -1.55 -.43
------------------------------------------------

</font>
</pre>




For accuracy to speedo -- yes!

However, some tires in a 235 (like the Kumho @ 27.5lbs) weigh a bit more than the stock tires. Since the bulk of this weight is concentrated at the steel belts, it makes for a loss in acceleration. (The Kumhos in a 225/45-17 size have a weight close to the stock/OEM MXM4 tires.) Finally, most of the 235/45-17 tires have a minimum rim width of 7.5” (BMW and others car makers sometimes put tires on ½” undersized rims.)

So, the 235 < -- > 225 issue depends on the brand and if it is going on the stock rim.

Then you have 235/45-17 Toyo T1S on stock rims -- they fit and work great, but the alignment is a bitch (due to the thick sidewall protector and funny outside flange on the stock rim).

Finally, while the calculator is great for wheel and tire sizing issues (combined with offset calcs), it will NOT tell you the actual speedo error. It is better to look at TireRack (or Michelin) for the stock Revs/Mile spec to compare against the Revs/Mile spec of the replacement tire. Different tires have different loaded radii depending on the construction of the tire.

 

Or get a GPS and do your own calibration tests (I found the stock tires to be off app. 1.2% ..

 

IMO -- the speedo is all over the place and is rather optimistic.

There was that guy who had a AAA test and 150+ on the speedo was no more than 147 MPH

(Perhaps some 205/40-17 replacement tires would help some of the speeders avoid more tickets)

 

Hmmm... I don't think the GPS has a crystal ball satellite feature for getting different replacement tires...

Oh, and then you can recalibrate (via the GPS) as your tires wear out -- since this changes the diameter of the tire by:


25.4" when the tires are new to 25.4" - 20/32" when worn out