Ok I have a 1978 Chevy Camaro Type LT and it has a 305 with stock heads except for a cam I put in it and a quadrajet four barrell on it and it has a 241 axle ratio which is a posi. Now the current tire size is 205 70r 14 I have an extra set of z28 rims of my 1979 z and i want to put those on my 78 They are 225 70r 15, which i would like to put 235 70r 15 or even a 245 on it will this effect the axle ratio and will this work. And should I go with a 60r instead of a 70r? What do you guys and gals think.

 

Calculated tire diameters, and resulting speedo errors, assuming the speedo is correctly callibrated for the 205/70-14 tires:

205/70-14 = 25.3" = 0% error = 2.41:1 axle

225/70-15 = 27.4" = speedo reads 7.7% lower than actual speed = feels like 2.23:1

235/70-15 = 28.0" = 9.5% lower than actual = feels like 2.18:1

245/70-15 = 28.5" = 11.2% lower than actual = feels like 2.14:1

As you can see, when you increase the wheel diameter from 14" to 15", you have to use a lower "aspect ratio" tire.... the 70 aspect ratio is too large. You have to drop to a 60 or 55 series tire. Best choices would be:

225/60-15 = 25.6"

235/55-15 = 25.2"

245/55-15 = 25.6"

Now you come to the next problem.... the width of the tire (205, 225, 245) is limited by the width of the wheel. There is a specific range of wheel widths required for each tire size. You have to know not only that you have a 15" wheel, but you have to know how wide they are (6", 7", etc) in order to determine what width tire will fit on them correctly.