switching 14" axels to 15"

[gotdust57@yahoo]

i have a 73 duster with 489 8.75 3.55 suregrip and would like to get bigger tire on for better traction, i have 14" tires now and would like to maybe put 15" tires on back, can i just switch axels to fifteen inch or is there another solution to my problem, if i stay with 14" what is widest and tallest tire i can put on without hitting springs, any ideas are welcomed? thanks

 

[dgc333]

The axles have nothing to do with the size rims/tires you can put on the car. A-body cars through 72 (and non disk brake \6 cars through 76) had a small 4" bolt circle for the wheels. You can get 15" wheels from the aftermarket that fit the 4" bolt circle but have limited choices. The more common thing to do is up grade to the more common 4.5" bolt circle then the wheel choices are limitless.

Stock 14" wheels whether they were 4" bolt circle or 4.5" bolt circle would be 5.5" wide and about the biggest tire you could fit on these rims is going to be a 225/60-14 tire. If you can find wider 14" rims the biggest you could fit under the front without interfering with the fenders is going to be a 235/60-14 and the largest 14" tire that is still readily available is a 245/60-14 which will fit under the rear with no issues.

If you go to a 15" rim with the proper width a 225/60-15 is about as bif as you can go on the front and a 255/60-15 or 275/50-15 will fit under the rear with no issues.

If you carefully select diameters, width and back spacing 16, 17 and even 18" rims with 50 series or lower aspect ratios will fit.

Your 73 Duster would not have come from the factory with the 8.75" rear end. If it was a 340 car or had the optional heavy duty rear end it would have been an 8.25 rear end. In either case the car would have the larger 4.5" bolt circle.

If the 8.75 in your car was swapped from an earlier A-body car it would have the smaller 4" bolt circle. Your choices are to; have the axles and brake drums redrilled, or, have new 4.5" bolt circle axles made with the B-body offset and you could use readily available B-body brake drums.

 

[gotdust57@yahoo]

thanks, and yes in did install 8.75 it had 7.25

 

[2 Darts]

I agree. Watch backspacing to keep a wide tire out of the fender and off the rear springs (in stock location). I'm using 15x7 cop car wheels with 235/60-15's with no problems. The same wheels with 255s work ok on the rear, but not on the front. My problem with putting a larger tire on the rear for a street car is that it exacerbates understeering.

 

[dgc333]

It's true a wider tire on the back will move the balance towards understeer but our old cars are already have extreme understeer where you will completely lose traction with the front wheels without losing the backs so wider tires aren't going to change that but our cars, especially with v8's, have traction issues that a wider tire can help with. You can make a much more balanced car with a wider tire on the back.

FWIW, I have 225/60-15 (7" wide rims) on the front and 255/60-15 (8" wide rims) on the back of my 68 A-body Barracuda. I used to have 235/60-14's on all 4 corners. I can drive much harder out of corners now with the bigger rears without fear of power induced oversteer. And, I don't have anymore or less understeer going into or through corners. Plus there is a big improvement in straight line traction with the wider rear tire.