Im sure there's some sort of formula but I'm too lazy to look. Im sure one of you knows, so.......

I have 225/45-18 tires on my ride now.

What is the overall difference in height between 225/45-18 and 215/60-16 tires? Will the 16's whack out my speedo?

I am debating adding a new set of wheels. Thank you so much.

 

The overall hieght on the 18" tire is 659 mm or 25.9 inches
on the 16" it is actually taller it is 664mm or 26.1 inches as far as the difference on your speedo it wouldn't be realy noticed goodluck.

 

Same size tires can vary a little bit too.

Check out the Tireracks web site and look a the revolutions per mile of each tire. My 18 inch and 16 inch only vary 2 revs per mile but when I was looking some were a little more revs.

This will give you a good idea how close they are.

 

On my GMC Safari with the incorrect tire - not sure what it is at this time - I am off a full 5 mph.

Also, even on my 2007 HHR with 17s, according to my navigator, I'm off 2 mph. I wonder if this is correct. Don't they recalibrate when they put 17s on the Special Editions?

 

Its typical for factory speedos to be off a little..... there is even a slight differance between a new set of tires and a worn set of tires..... It is why police car speedos all used to say right on them that they were calabrated....and even they were usually allowed +/- 1%

Heck all late model Honda Goldwings read about 6% fast and they all come with the same tires........Cars aren't held to any tighter standards that motorcycles.

 

I know when I was in the process of buying mine, my car is a 2lt and it had the painted 17" wheels on it but when I looked at it at the dealer, 3 of the wheels had real bad curb rash on them and the dealership would not replace them. I ended up having them take the 16" aluminum wheels off of my 07 1lt and put them on the newer one that I was buying. I asked about the speedo being off and they did some inveestigating and figured out that with the 16" wheels my speedo would be off .5 mph's at 60 mph( so I would be doin 59.5 mph instead of 60) . hopefully that will help you out.

 

I'll take your SS wheels, Don.

@

 

HipHotRod,
Here's the forumula yer looking for:
http://www.miata.net/garage/tirecalc.html

Mo

 

The Formula T20 posted is good but incase your out looking and dont have the option of the internet here is the actual formula.

*Example Tire is 235-35-19*

Step 1) 235mm divided by 25.4" = 9.25" (section width)
Step 2) Multiply 9.25 (section width) by .35 (aspect ratio)
9.25 x .35 = 3.24 (section height)

*To find the outside diameter*
Multiply 3.24 (section height) x 2 then add rim diameter.

Step 3) 3.24" x 2 = 6.48" + 19" = 25.48" Outside Diameter.


Hope this Helps