I need HELP with Wheel Fitment and Wheel Choice for my 66 GTO Please!

[old66tiger]

Hey Guys,

I am brand new to the forum. Glad to be here. Little background to start. My GTO started out as a stock 389 tri-power 4-speed convertible. Over the years, it has undergone a 455 5-speed transformation and will be getting a set of 4-wheel disc brakes from the right stuff to make this thing stop instead of keep you wondering. So, I am on the hunt for wheels and tires. I was set on 17's until I saw a 65 chevelle the other day that had 18's on the back. I am sold on the 18's for the back and possibly 18's for the front, but I need some help here.

The car is pretty much stock except for the tubular suspension out back. It has the stock 10-bolt posi. I want something that fills the wheel wells but not something so big where the tires are rubber bands. Also, the rear tires need to tuck into the wheel wells about an inch or so. No high jackers here. I am running air springs (bags) in the coils to keep things a bit firmer.

First off, I think 18X9's with a 5.5" backspace will fit on the back but I am not sure. Anyone know if they will? Maybe 255-45-17 tires to boot?

Second, what wheels would go well on the front? Should i go with 17X8, 18X7 or 18X8's I want something that looks proportunate. and won't look too small.

Third, I really like the look of the Boyd Junkyard dog rims. I saw they offer them up in cast which is not my first choice, but it is much more feasible that the billet variety. I am also considering the Billet Specialties Dagger in the cast series. They are similar but from what I have seen so far, I still like the Boyd's a bit better, but I am still in the planning phase.

Lastly, who has the best deals on Boyds and Billet specialties?

Thanks in advance guys. Looking forward to reading all of the threads on this forum

[b-man]

I have 18X9s all the way around on my '64 Tempest, 245/40s in front and 275/40s in back. My '64 rides on essentially the same chassis as your '66, the wheelwell sizes are very similar.

My rear wheels have 6.57" of backspacing, the fronts are the same as the rear but I'm using a spacer that effectively gives the front wheels 5.5" of backspace. The 18X9 is a very tight fit, an 18X8 would be easier to use and should fit well with a 5" backspace.

In mid-year '66 Pontiac changed to the 1" wider rear ends (1/2" wider per side), so be sure you know which one you have. My car has a later wide rear end, for the early narrower rear just use 1/2" less backspacing.

When calculating the rear wheel backspacing and going from drums to discs you absolutely must take into account the added thickness of the disc brake rotor at the hub flange. A drum is .095" thick at the mounting flange (at least mine were), the disc brake rotors I'm using are .280" thick in the same area. Might not seem like a lot to some, but when you're trying to stuff in some wide rubber that extra .185" (about 3/16") made a difference (in my favor) by spacing the wheels out a bit.

Link to my Tempest build thread, amongst all the other nonsense is the wheel and tire fitment info:
https://www.pro-touring.com/forum/showthread.php?t=48361

Welcome to the Pro-Touring forums and good luck!

Bart :smoke:

[old66tiger]

Bart, You are everywhere...Besides moderating the PY board, you are over here posting your work on the resto mod. nice job BTW. Quite the brake system...

Anyway, I measured up the back of my car with the drum off and I have about 6" of spacing on the back side and 4.75" on the front. So, there must be something definitely different between our cars. I also measured the rear spacing at the most absolute closest portion of the tire which is actually at 2:00 when looking at the side of the car. Funny that the wheel wells are shaped such that that portion of the tub sticks out further than the 12:00 location. Anyway, I wonder if a trip to discount tire would be good at this point to see if they have a fixture

[b-man]

When I first took my measurements for the rear wheels I had the original '64 narrow rear in the car, with the brake drums in place.

Came up with 6.750" on the inside and 5.000" on the front side at about the 12-o'clock position. Since I was planning on using the later wide rear end I just added 1/2" to the back side (now 7.250") and subtracted that same 1/2" from the front side (now 4.500").

These numbers added together (11.750") allow enough room for a 9" wide wheel (actually 10" wide overall from lip to lip) leaving .875" (7/8") of room on each side between the body and the rim lip. Tight fit once you put a tire on it that will stick past the rim edge by about .250" (1/4"). This leaves only .625" (5/8") between the body and the tire sidewall.

I subtracted .875" from my inside clearance number of 7.250" and arrived at 6.375". After adding in .185" for the added thickness of the rear brake rotor (.280") compared to the old rear drum .095") it looked like 6.560" of backspacing would do it. Luckily I found wheels with 6.570" of backspacing, only off by .010". :)

I don't hammer or roll the outer wheelwell lips on any of my cars, a little hammer work on the inside is as much as I'll do.

From the measurements you've taken it looks like you're being a little more conservative with the straightedge and tape measure. Your findings of 10.750" of room inside being 1" less than what I came up with seems a bit off, your wheelwells should be at least as wide as mine inside. Measure a few more times, I did just to be sure. My guess is you have the earlier narrow rear, it should measure 61" from drum to drum.

You will find the biggest chance for interference will be at the backside rear of the inner wheelwell, but it's not a big problem as long as you're not running tires that are too much over 26.5" tall. I had to do some hammering on my '64 GTO in that area, but the tires are 28" tall 275/60-15s and tucked in very tight as well.

I would say that a 6.00" backspace on an 18X9 wheel should be very close if you're running rear disc brakes and the early narrow rear end. An 18X8 would fit best with 5.5" of backspace if you decide to go a little more conservative in the rear.