Changing From 14" Rims to 15" Is There A Formula?

I am changing from 14" rims and tires (215 R70) to 15" rims and new tires. Is there a "magic" formula that will convert the 215 R70 14 to a different tire size on a 15" rim?

 

The Miata.net Tire Size Calculator should help you out.

If you want to keep the 215mm width, then a 215/65/15 is what you need. Otherwise, you can play with the sidewall and tire width variables to your liking.

 

14+1=x
x=15

 

Getting the right aspect ratio is more important to the visuals than staying with the same size front tire. Inch for inch with the same aspect ratio tire in a 15" diameter will raise your car 0.5" and fill the wheel opening with a little more tire. Post a photo of your ride. Go to www.coker.com or www.dbtires.com to see what height your new size will be. Your speedo will be marginally out but you can calculate that by going to www.currieenterprises.com and click on calculators on the right sidebar - but wait - here is a direct link....

www.currieenterprises.com/cestore/calculators.aspx

 

play with this a bit. http://www.onlineconversion.com/tire_size_conversion.htm

The tire you have now is 25.3 or should be tall.

A 195 70 15 would be 25.7 tall but may not have the right visual look as Weasel mentioned above. A 205-70-15 is 26.3 tall according to the chart but may look better Than the 195

 

Several considerations here:

Calculating a tire size is fine but you really, really want to double-check the manufacturer's spec for the tire. The Tire Rack website has extensive manufacturers' dimensional details for the tires/tire lines they carry.

A tire with a lower aspect ratio and the same section width is a smaller tire, and will have a lower load rating. So, for instance, a 205/60-14 is a smaller tire than 205/65-14, which is a smaller tire than 205/70-14. As a general rule, you need to compare loaded rolling radius and load rating, then fudge the section width if you have to.

 

Roughly right formula is - for an inch rim upsize you need to up width by 10mm and lower aspect ration by 10 , ie: in your case 215R70X14 would become 225R60x15. Tire load capacity is proportional to the VOLUME of air at the tire patch, so as you go down in aspect ratio you need to go up in width to keep the same load carrying capacity.

But best way is to get mfgr's specs , as stated above , and get the diameter and width you want, and make sure you have enough load capacity.