How big rims for my 2nd gen T/A?

[zamora7]

What is the biggest rim I can run on my 1980 Trans Am front and rear? I want to buy a set of black Z06 deep dish wheels for it but not sure if I can fit them without any amjor modifications. The rears are 18x10.5 and the front are 17x9.5, but really wanting 18x8.5 or 9.5 fronts. I will most likely be using c5 brakes upfront and rear just in case I go with 17 in wheel.

Offset of the wheels is 54mm for the front 17x9.5 and 56mm for the rear, 18x10.5.

[srh3trinity]

I am interested in the answer you get on this one too. I bought a '74 'Bird yesterday and I was thinking about the same wheels for now and then going with something a little more custom as the money rolls in. I believe the 70-74's have a little more room in the wheel wells than the later second gen firebirds and TA's.

[zamora7]

Well it seems like nobody in here is using these rims. I have seen some 2nd gens with big wheels, specially outback but not anything like OEM c6 wheels. Hopefully someone will chime in.

[critter]

You posted at 2:00 p.m. Give it some time.

I have no personal knowledge of those wheels but I suspect the offset for those rims would make them unusable. Most late model wheels are a positive offset and our old cars need a zero offset or something in that range. I can tell you that I'm running Boyd Smoothie II wheels, 18X10 rear with a six inch offset and 18X8 up front with a zero offset. Those will work if you're willing to pound down the seatbelt bulge on the rear wheel well. Tires are a 295/35 out back and a 255/40 up front.

[srh3trinity]

Critter,
What year is your car? Just curious because I have heard that the wheel wells are different based on the year with the earlier cars having more room.

To the OP, I haven't calculated the offset, but I think that you will need a spacer or adapter with the C6 wheels but I am not sure what size.

[zamora7]

To the OP, I haven't calculated the offset, but I think that you will need a spacer or adapter with the C6 wheels but I am not sure what size.

Well to be honest, I don't know how to calculate the offset on wheels, or what fits where and such, hence the thread. Now, something that needs to be noted is that I will be using either c5 or c6 brakes. If use c6 I have to run 18" rims, c5 needs 17" rims, but its the offset and stuff like that, that I don't understand how it works.

[formula]

time for a little wheel fitment 101, then.

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

Positive

The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative

The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

*Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).
https://static1.pt-content.com/images/noimg.gif
Now that that's out of the way....


There are two ways to play the wheel game.

Option A: Hay guys, what will fit?
In this scenario, you come on a forum and ask what will fit your car. This is very similar to asking a bunch of guys who the hottest girl in the world is--you'll be able to get a pretty general feel for what is going to fit, but at the end of the day, every answer's going to be a little different. This is because every car is actually different--brakes, suspension, manufacturing tolerance, and just how close you really want to cut it and how much work you're willing to put into fitment determines how much wheel and tire you can actually stuff in there. I was told by Year One that my car was actually impossible, at one point--which came as quite a shock to both me and my apparently-imaginary automobile. I'm not saying that they don't know what they're talking about--at least not right now--but what I am saying is that everyone has their own opinions.

Option B: If you want something done right...
Realistically the only way you can be absolutely sure something will fit is to get out there and find out. To really truly do it right, you need to save your wheels for last--set up everything else the way you want it, so that you have the amount of space you'll have finalized--then actually measure the space and see how much wheel you can get in there. Then you either find or order wheels to fill the space.

That being said, since you've already ventured down the first path, i'll save briefing everyone on exactly how one should measure and offer my own opinion--18s will be fine on any corner, you can pretty much compensate for any (reasonable) wheel diameter with an appropriate tire to maintain the same overall diameter. 9.5's gonna put you at around a 275 tire on the front--that'll be fine if you're not going very low, but if you plan on getting down in the weeds, you're going to want to go with an 8.5 and a 255 or so. The rears are going to be nigh-upon impossible at that width, though, at least in a stock wheel well (unless you way-undersize the tire and make it look stupid).

At the end of all of that, you're going to end up with serious wheel adapters to run 'vette gear--on the order of an inch, give or take, if memory serves. better off getting wheels that actually fit, from a safety and cost-benefit standpoint. At least, in my opinion.

[wheelman]

First off - Props to Formula for a well written tech article. In regards as to what will fit, there isn't a magic app guide to go by. There are too many variables with clearance issues and a variety of combinations of suspensions and brakes and ride heights. I would ask that you look at the Tire MountMate test fit tool. The website is www.wheelworksinc.com . Click on Products and watch the short video. Newstalgia Wheel and Detroit Speed have the tools in stock as well as Yogi's. If you have specific technical questions, please send me an email and I'll try to answer your questions.

[zamora7]

Wow, great info guys.

Wes, thanks for the schooling. Everything is much simpler now, I think. Let see, if I was to order wheels with a bunch of negative offset it would bring my wheels outside the fenders, correct? And if I was to order deep dish vs. non-deep dish with the same offset, deep dish will srick out past the fender more, correct?

Wheelman, thanks for the info, I'm sure I will need that tool eventually.

Thanks again guys.

[JustinB]

Wow, great info guys.

Wes, thanks for the schooling. Everything is much simpler now, I think. Let see, if I was to order wheels with a bunch of negative offset it would bring my wheels outside the fenders, correct? And if I was to order deep dish vs. non-deep dish with the same offset, deep dish will srick out past the fender more, correct?

Wheelman, thanks for the info, I'm sure I will need that tool eventually.

Thanks again guys.

Yes, the more - offset, the further the wheel is pushed away from the inside and towards the outer fender. The more + the offset, the more you are pushing the wheel into the inner fender. "Deep dish" is a term I think they are using to describe the depth of the wheel lip, which may just be achieved by curving the spokes back towards the back of the wheel, giving you the illusion of a deeper wheel, but if the offset is the same, then the mounting surface of the wheel sits in the same place.

Another term you should get use to is "backspace". Which is measurement from the very back edge of the wheel, to the mounting surface of the wheel (which is where the wheel meets the brake surface). Most classic car wheel brands measure in this fashion.

Call us if you have any questions at all, more than happy to help.

[critter]

And just to answer your question, mine is a 1974 model.

[WS6]

Check out the very last page of my build thread and you can see what 18" rims look like on all four corners of my 79TA. Fronts are 8.5 and rears are 9.5 wide. Offset was not far off from what the car needs. The Corvette rims you desire will have a lot more offset and will require a much thicker adapter. That's not really a problem it just adds cost into the equation.

Having said that, if money allows I would buy a rim that is designed to fit the car without adapters. I have 40 lug nuts on my car. 40! That's a pain in the ass even if it's not something to deal with on a regular basis. I was on a budget though and these rims worked.

[zamora7]

Check out the very last page of my build thread and you can see what 18" rims look like on all four corners of my 79TA. Fronts are 8.5 and rears are 9.5 wide. Offset was not far off from what the car needs. The Corvette rims you desire will have a lot more offset and will require a much thicker adapter. That's not really a problem it just adds cost into the equation.

Having said that, if money allows I would buy a rim that is designed to fit the car without adapters. I have 40 lug nuts on my car. 40! That's a pain in the ass even if it's not something to deal with on a regular basis. I was on a budget though and these rims worked.


Wow, you did an excellent job, I actually have some 18" tires that I plan on using and they measure around 24" or 24.5", the same as the current tire and wheel combo currently on the car. I'm thinking that it would be okay with 18's all around as long its ordered with the correct offset for clearance with the new braking components.