Monday, January 22, 2018
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Attention 65-66 Mustang Owners

Q. - Will your kit fit the 1965-1966 Mustang chassis?

A. - YES and NO.

1965 Mustang Fastback with a
2000 Cobra R Independent rear suspension.

This sweet ride is owned by fellow Hoosier Dale Irwin




Back in January we promised an answer to the question of whether or not our kit to install the IRS from a late model Cobra would fit into the 65-66 chassis, and we thought we would have a quick answer. The 1965 fastback above was going to be used as our guide for any changes that would need to be done to make our system fit the early cars.

What we didn't know at the time is that this car came with a set of beautiful rear fender flares, which look great but didn't help in one key aspect of determining fit on the early chassis.

The flares on this car were great, but it prevented us from confirming the width
of the IRS unit to the original wheel flares.

We knew the IRS setup was an inch wider on each side in the 67-8 chassis, and we knew that the 65-66 standard axle is an inch shorter than the 67-70, we just didn't know if the wheels would fit under the standard lip of a 65 Mustang!

First things first. Since we knew we would have tons of room to put massive amounts of tire under this car with the new flares, our first goal was to see if our kit will physically bolt into the 65-6 chassis. Answer - YES! The frame rails and position in the rear of the chassis allow for the physical installation of the IRS unit.

Trick slotted and plated rotors courtesy of SSBC!

Only three real modifications were needed for the installation of the kit into the 65 chassis. We had to notch the upper left hand corner of the front bracket to clear the indentation of the rail assembly. The rail itself is thinner and will require a spacer on the outside to make the kit bolt together properly. Inside, this drip rail on the early car (middle photo) is right in our way, so we'll have to make a longer insert to clear the strengthening rib. The final photo is the floorpan of a 70 Mustang. Note no rib!

Looking to the inside of the wheel well, there is plenty of space with the early cars. There is about an inch to go before you start having clearance problems with the rear bracket and the coil over shock - just as you do with the later 67-70 installations (we note this for a reason).

No clearance problems on the inside of the wheel well. Our alignment guy says a 9 inch rim is max before you start hitting things.

So what's the problem? Here's our problem - the unit is wider than the factory Mustang wheel lips. The stock 65-66 wheel lips are approximately 66 5/16 inches wide from inside lip to inside lip.

Using a real high tech method of lug nuts and string, we measured a 65 Mustang that still has it's original quarter panels (repo panels may be different!). Inside lip to inside lip is approximately 66 5/16 inches wide.


Here's a couple of shots of the irs in a 66 provided by KR Performance and Restorations showing the tight fit of the irs in the early chassis. You have maybe an 1/8 inch to work with. You'll definitely need to roll the lips under, and probably need a slight radius on the lips as well.


Now, using the stock late model Mustang rims (17x8 with a 5.72 inch offset) and 245/45 series late model tires, we measure 69 1/2 inches from outside tire to outside tire.

Missed it by that much!


So, what is the solution?

We need around 3 inches to make this work. The solutions to make it fit are as follows:

  1. Large, way cool wheel flares as shown above. A bit radical, but it does work.
  2. Roll the rear fender lips under. This will provide about and inch total additional clearance.
  3. Wheels with a larger offset. We mentioned there is room on the inside to offset the rim some more, but the wheel is going to be very flat. We're looking into wheel manufacturers and hope to have a list of recommended wheels soon.
  4. A mild roll of the wheel lip to gain additional clearance. A proper body shop can expand the wheel lips and make them look nice.

Probably a combination of the above items are needed to make the wide unit fit the 65 chassis. Sooooo, if you were thinking about big flares or some other means of widening the rear of your car, the IRS WILL fit the 65-6 Mustang - with some modifications! Thanks to everyone who has been patient with us!