I’m shopping for tires to put on an 18 x 7 inch rim. I’ve been playing with one of those tires calculators and trying to figure out what size tire(s) I can use.

The stock tire on my 05 Baja Turbo is 225/60/16. If I select a 235/45/18 tire, I get a diameter difference of 1.13% (or 1.139% too fast). That tire fits rims from 7.5 to 9 inches. If I compare the OE tire to a 245/45/18, there is a diameter difference of only .23% (or .219% too slow).

What I don’t understand:

1. Am I correct is thinking these tires will fit a 7-inch rim even though they are designated as fitting a 7.5-inch rim?

2. Why would a 245 tire offer a closer fit than a 235? I don’t see how the width of the tire accounts for the difference in diameter. The diameter difference between the OE tire and the 245 is a mere .04 millimeters.

3. Tire Rack recommends a 235/45/18 tires for an 18 x 7 rim. Why not the 245/45/18, which seems to offer a better fit? Based on their recommendation, I wonder if my logic is faulty.

I just don’t understand enough to make a sensible purchase decision, is what it comes down to.

There are other factors to consider as well: availability of tires in the 235 and 245 sizes, for one. And there’s cost of course. And the biggest decision of all, wouldn’t it make more sense to buy the wider tire?

 

On a 7 inch rim you don't want to go wider then 225 and 225/50/18 would be .9% bigger in circumference. What that means is that if you're driving 60.5 your speedometer will read 60.

 

You can go wider than 225 on a 7" wheel (IIRC most 2wd 1gen Expeditions came with 255/70 on a 16x7 wheel), but you must be reasonable - ie follow the tire maker's recommended range fo wheel width. You can go .5 in wider or narrower & it will most likely fit, but, again, it is not recommended.

You could go with the 225/50s, 235/45s, 235/50s 245/45s. I don't think 225/50/18 is a real size though - TireRack showed no tires in that size. The Inifiniti M45s come w/ 235/45/18s & IIRC the CTSV comes w/ 245/45/18s & I have sold a set of 235/50/18s so those sizes are viable.

http://www.tirerack.com/tires/tiretech/general/size.jsp
http://www.tirerack.com/tires/tirete...al/plusone.jsp

 

Your factory tires are 225/60/16 (225=width of the tire in mm, 60=Profile which is 60% of the width of the tire and 16 is the OD of the rim in inches). The profile is a percentage of the width and that is why the width will affect the OD of the tire. This is the way to figure out the od of a tire without the tire calculator. (I will use a 225/60/16as an example)

(1) multiply the width and the profile (225x.60=135)
(2) multiply x 2 (135x2=270)
(3) add the od of the rim (1"=25.4mm so 16x25.4=406.4) (270+406.4=676.4)

I am by no means a tire pro but I think Esoteric is correct about going bigger than 225 on a 7 inch rim. I have heard of people mounting 225's on a OE WRX rim which is only 6.5" wide.