1975 wheels 13" vs. 14"

[eaglecap]

My 1975 Ghia came with CR70-13 tires. Rims are 13 x 5 with 4 bolt pattern. The Mustang specs do not list a 14" option for 1975. I have found a set of 14" x 6" chrome rims from a 1979 Mustang. Will they fit and what size tire do I use to make sure I don't screw up the speedometer readings.??

 

[kram2]

I have 195/60/14 on the front and 225/60/14 on the rear. No rubbing and speedometer is still good.

 

[Johan]

To keep the speedometer reading the same, you need a tire with the same overall diameter or same revolutions per mile. I couldn't find size data on the original, but MVMA specs lists all tires for the 75 II at 888 revs/mi. I don't think that's quite right (195/70-13 should be about 849 revs/mi), but the speedo is probably set for that.

http://www.mustangii.net/1975/75_mvma_21.asp

Use this tire calculator to find a tire with a similar number of revs/mi:

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

The closest I could find to 888 revs/mi in a metric 13" is the 175/70-13 at 891. The closest in 14 would be:
185/60-14 (887revs/mi)
205/55-14 (882revs/mi)
225/50-14 (882revs/mi).

Keep in mind that with taller tires, you're going to be faster than the speedo indicates.

 

[Blue Thunder]

You can also slightly adjust your speedo with a different color cable gear.