changing tire sizes

[reedbecky]

Hi, I'm new to this post and I have a question concerning the tires on my '66 Monza: Since 13 inch tires are scarce, how much will the speedometer be "off" if I switch to a 14 inch. The tires on my car I believe to be vintage to the car since the size on the are B78-13. I don't trust them and can't find tires in my area to fit the car. Any help in this area would be greatly appreciated, thanks.

[Corvair.crazy]

Mine is ten miles faster, I have 14"

[reedbecky]

Thanks for the reply, 10 miles isn't that hard to remember. Hopefully I'll have it on the road by summer. The car is basically a "barn find" with less than 46,000 miles in it. It is all original, but need a new top and paint for the most part.

[mart]

try using the Kouki tech tyre size calculator and it will tell you which tyre size has the same rolling radius as your old tyres for any diameter

[caraholic4life]

Here is a comparison tool I have used. http://www.miata.net/garage/tirecalc.html

The important thing is the revolutions per mile for the tire. The tire size is only secondly important to how it will fit in the wheel opening along with the proper offset and width of a given wheel.

Here are a couple of examples of tire specifications:

A 195/75/13 tire would have a circumference of 77.0 inches and turns 823 revolutions per mile.

A 205/60/15 tire would have a circumference of 77.6 inches and turns 817 revolutions per mile.

The 15 inch tire is slightly larger in circumference than the 13 inch tire and will result in a 7% slower speedometer/odometer reading.

With the 15 inch tire your speedometer in theory would read 60 MPH when you are actually traveling at 60.4 MPH.

A 235/40/17 inch tire would have a circumference of 76.7 inches and turns 827 revolutions per mile.

With the 17 inch tire your speedometer in theory would read 60 MPH when you are actually traveling at 59.7 MPH.

This of course is also going to depend on the accuracy/calibration of the speedometer in the car.

The comparison chart also provides Sidewall, Radius, and Diameter information as well.

If you do a search on this site you will probably find a lot of different posts related to this subject. There will also be several posts with wheel diameter, width, offset, and tire size used on owners cars as well as their individual experiences and opinions. Good luck finding the wheel and tire combination that is right for you.

I hope this is of some help.

[reedbecky]

Thank you for the information you posted, it is an immense help. I was thinking of using some chevy rally wheels on it and with your data it makes it a reality. Thanks again and hopefully I'll be on the road in the spring............

[4carbcorvair]

You may want to consider a 15 or 16 inch wheel. Our tire guy at work mentioned recently that 14 inch tires are starting to go away like 13's and even some 15 are not available in certain sizes anymore.

I'm running a 16.

[reidmier]

On a late model, if you use 205/60-15 tires on 15" wheels, you will end up with a wheel/tire diameter and circumference that is almost exactly the same as using 185/80-13s and your speedometer should still read correctly. More important will be making sure you get the proper offset and backspacing on whatever wheels you choose. If you use 6" wide wheels, you will want a backspacing of around 4". If you use 7" wheels like I did, you'lll want the backspacing closer to 4-1/2" to 4-3/4". Chevy Rally wheels look great on a Corvair, but try stay with 7" wide or less to minimize clearance issues. I found this site to be immensly helpful when I was searching for new wheels: http://autoxer.skiblack.com/tires.html