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Aftermarket Wheels for a GT

3847 Views 5 Replies 4 Participants Last post by  Mountaineer
again, letting my ignorance show, but I am ignorant when it comes to some of the terms and dimensions listed in some of the threads concerning backspace and offset in regards to wheels for Opel GTs. I am watching a set of wheels on e-bay that say they have a 35mm offset, 5 3/8" backspacing, and bolt pattern is 4 x 100mm. The only part I am sure will work for the GT is the bolt patter, but I have no clue to the rest. Can some please help me and maybe even educate me along the way?
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I was going to send you off to do a search for an excellent article by the late John Seaman (former Editor-For-Life of the OMC Blitz) but the link to his article (http://www.opelclub.com/public/blitz/articles/wheels.pdf), which USED to be on the OMC site, seem to be broken. And the article doesn't show up on the OMC site at the moment. And I hadn't saved it to my personal collection. Hmm...

OK, a quick primer on Opel GT wheel fitment:

First, you need the correct bolt pattern, and 4 X 100 mm is right. Same as a very common car, the older Honda Civic and Prelude (pre-'92 in the Prelude, somewhat later in the Civic I believe). Opel wheels are "stud centric", versus "hub-centric". In other words, the wheels are centred by the studs, not the hole in the middle.

Second, you need to make sure you have the correct back-space so the wheel doesn't rub on the rods and suspension pieces, and this is approximately determined by the wheel width and the offset. In simple terms, a "zero" offset (ET 0) would have the mounting hub face right at the centre of the wheel. A 25 mm positive offset (ET 25) would have the hub OUTBOARD of the centreline by an inch (25.4 mm to an inch) so the wheel centre is INSIDE the hub face by an inch. A 35 mm offset (ET 35) would have the hub face 35 mm, or about 1 3/8 " towards the outside. The bigger the positive offset (and almost ALL wheels are a POSITIVE offset) the more the wheel sits inwards, so it has a narrower track for a given width. And the bigger the ET (a standard term, that you will often see stamped on a wheel), the bigger the backspace.

OK, backspace. A 7 inch wheel is actually about 8 inches wide from the outer edges of the rim. The 7 inch refers to the TIRE mounting width. So a wheel that is advertised as being 7 inches wide is actually 8 inches wide at the rim edge, and an ET 35 will give a backspace of 5 3/8' (8 divided by two is 4, plus 1 3/8" equals 5 3/8"). So the wheel that you are looking at is probably 7 inches wide. And 5 3/8 " is TOO big for a GT. The stock wheel is 13 x 5, with a ET25, so it has a backspace of (add an inch to five gives you six, divided by two is three, plus 1 is) 4 inches. Depending on wheel diameter and tire sidewall bulge, a GT with a 15 inch wheel (a common "plus two" fitment) can tolerate a 4 1/2 inch backspace. So the math is seven plus one is eight, divided by two is four, plus 1/2 inch to get 4 1/2". A 1/2 inch is about 13 mm (ET 13, pretty rare), but experience has shown that an ET20 in a 15X7 wheel will fit, so long as the tire isn't too wide.

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Thank you Kieth. As always, a tremendous help, as well as educational. Am glad I checked before buying! Very well written as well... will save for future reference in case my aged memory fails to store it properly!
Yeah............ I'm gonna have to go and say, yeah, what keith said. I, in no way, could of done better.

Only thing to add is remember... You have a rear wheel drive car and most of these wheels are made for front wheel drive. Pretty much anything in the 25 - 45 mm offset is going to be for a FWD car.

Also, if you dont mind doing it, you can always convert your hubs to 5 x 114 mm. That opens up a the door for a whole lot more to choose from. Thats what Im doing.
That's right Thom. I meant to say that the Honda, being FWD, usually has wheels of about ET38 (or at least my old '88 Prelude with 14X6 wheels were that offset) and as much as ET45. Interestingly, those old Prelude wheels did fit my GT (six plus 1 is seven, half is 3 1/2, plus 38 mm or 1 1/2) with a 5 inch backspace. Or at least I didn't detect any interference. But since the wheel centre was inboard of the stock wheel centre-line, the handling left a bit to be desired. Not too bad, just not good enough to justify the swap, as they just weren't very attractive wheels.

A common wheel (well, not THAT common) that fits is for the earlier BMW 3 series (the E21, 320i from '77 to '83) or the older BMW 2002. They are usually ET20 or 24, but seldom come bigger than 14 inch (unless they are aftermarket wheels). Later 3 series (the E30, the older 323 and 325, with the 4X100 bolt pattern) might also work (I bought a set last month) but check the offset.

And of course you can use spacers, although that comes with it's own issues (have a look around and search this Forum for "spacers").

And as Thom mentioned, you can get your hubs re-drilled to a different bolt pattern, although I don't know what is involved in that.

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Hope the bump isn't an issue, but just adding for a knowledge base:

15x8 +25mm ET, with a 5mm spacer because the offset needed wasn't available. These were found by searching for a 1980 BMW 3-series if I recall correctly.

225/45/R15 Falken Azenis. 1" Lowering springs all around.

Note on wider tires: you're going to want power steering. Hopefully you don't need to parallel park, or you should be working on your curls.
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