For what it's worth, I dug out a set of '88 GT wheels and a grey lace wheel from an '87 GT, and measured them. Here's what I got:

'87 wheel, 15x7" code XBZ, offset 29mm, using these measurements: total lip to lip rim width was 7 15/16", backspacing to rim lip was 5 1/8".

'88 rear wheel, 15x7": Same as above, including XBZ and part number 10044192.

'88 front wheel, 15x6": offset 37mm based on a lip to lip width of 6 15/16" and a backspacing to the lip of 4 29/32". The 6" wheel has the offset cast into it, and the legend FRONT ONLY in several locations as well.

The OSG seems to be in error regarding wheel info, at least on the lace wheels. This weekend I'll pull the wheels off my Formula and measure them as well.

JazzMan

Perhaps you are mistaken on your method of measurement. I have always seen, in every source on fieros, that the offset is 35mm on 84-87 lace wheels...and 42 mm?? on 88s. I know that I put on beretta GTZ wheels and found that the wheel rim was ~7mm further in from the body, and they had 42mm cast into the inside.

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87 GT Auto, Beretta GTZ Wheels, 225/50-16 Dunlop SP Sport A2s, Poly cradle/swaybar bushings, KYB shocks/struts, RCC Bumpsteer kit, 1" front & 7/8" rear swaybars, NYM rear springs (stiffest), H4 headlights, Fierowarehouse Scorpion scoop, 88GTNeverfinished tails, Repainted all silver, Fiero Store hitch, Accel SSR wires, Rapidfire plugs, K&N filters.
Future mods: Rear control arm poly bushings, Front mounted battery, Hood pressure vent.
3800 S/C after all that!

I can back up JazzMan's info. I had a set of 86-87 GT wheels. They were identical in every way except paint color to the 15x7 wheels on my Formula.
The OSG is incorrect. Unfortunately in this case, since it's one of the oldest sources, most people seem to have gotten their information there and quoted it on other pages.
Before anyone beats me to it, the 22P also lists a different part number for the 88 wheels. It's wrong, too.
They're an identical casting. Been there.

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Raydar

88 3.4 coupe. 17s, cut springs 'n all.

quote Originally posted by Bigfieroman: Perhaps you are mistaken on your method of measurement. I have always seen, in every source on fieros, that the offset is 35mm on 84-87 lace wheels...and 42 mm?? on 88s. I know that I put on beretta GTZ wheels and found that the wheel rim was ~7mm further in from the body, and they had 42mm cast into the inside.

Nope, measurement methodology was quite correct, though the instruments I used only had a resolution of 1/32" (0.8mm). It's actually quite easy to measure the offset, needing only a good ruler and a stiff straight edge. Since the resolution of 0.8mm is quite a bit better than the differences in the OSG and my wheels, I'd say that the OSG is incorrect, as much as I hate that to be.

JazzMan

Ok...

So if I'm reading this correctly, what your saying is...

My 87 GT Diamond Spoke 15x7 rims are not 35mm offest???

Where is the error?

I was going to buy aftermarket rims with 40mm offset. (wheel would sit in 5mm more)

If this info is correct (29mm offset) that means my wheel (and a whole bunch of Fiero owners on here with 40mm offset) would sit in 11mm more..... not 5mm.

That means the total track would be narrowed by 22mm instead of 10mm.

Hmmmm..... kinda ill.

Was going to order my wheels tomorrow. Going to hold off untill I look into this further. There goes 2 weeks of wheel shopping down the drain.

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[This message has been edited by Skybax (edited 09-04-2003).]

quote Originally posted by JazzMan: Nope, measurement methodology was quite correct, though the instruments I used only had a resolution of 1/32" (0.8mm). JazzMan

Hey Jazzman, You're almost right on. That 0.8mm will get you every time.

I performed the same measurements a while back using very accurate means (dial calipers, gage blocks, squares, etc...).
https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20020825-2-022316.html

The '88 front wheels are 15 x 6 x 37mm
The '88 rear wheels are 15 x 7 x 30mm

Any source that tells you otherwise is wrong. Flat out wrong.

As for non-88 wheels, I've been told that all of the 15 x 7 lace wheels have the same offset, but since I've only measured an '88, I can't positively confirm or deny.

-Bruce at FTF Engineering

quote Originally posted by Skybax: That might be where your error is. Rim width is measured from the inside of the lip, not the outer, which would give you 7" and not 7 ~ 15/16" That could be throwing your offset off from 35mm to 29mm. No?

No.

The offset is the relationship between the plane of contact between the bolting surface on the wheel and the midline of the wheel proper. So, if you measure the overall width of the rim, divide by two, that gets you the wheel midline, then measure the backspacing from the same point you measured the overall width from, then subtract one from the other, you get the offset, the difference, between the midline and the bolting surface. If you measure the overall width from the tire bead surface on both sides, assuming the rim lips are the same thickness (I checked, they are on the wheels I measured), then measure the backspacing from the same point, you get exactly the same number.

I'm afraid the OSG is in error, not me. BTW, I worked at SuperShops for a year and a half, and attended many training sessions to learn all of this interesting info.

JazzMan

quote Originally posted by JazzMan: The actual calculated offset in mm was something like 29.4 or so, so I just rounded down. I compared an '87 and '88 7" wheel side by side, and they are identical in all respects

So you confirmed that the pre-88 lace wheels are identical to the 88 rears, right? That would mean that all four wheels on pre-88 cars are 15 x 7 x 30mm.

One thing to be careful about though... Just because all the 7" lace wheels came from the same casting doesn't mean that they machined the same amount of material off for all years. Did you actually measure a pre-88 wheel too or just compare castings?

One more question. You measured it. I measured it. A few others in this thread measured it... Why is it so hard for people to believe the numbers?

Is it simply a matter of correcting the disinformation floating around out there by fixing things like the OSG for instance?

Hey Ogre, if you're watching this, how about putting a note in the cave about the wheel offsets?

-Bruce at FTF Engineering

quote Originally posted by FTF Engineering: So you confirmed that the pre-88 lace wheels are identical to the 88 rears, right? That would mean that all four wheels on pre-88 cars are 15 x 7 x 30mm. One thing to be careful about though... Just because all the 7" lace wheels came from the same casting doesn't mean that they machined the same amount of material off for all years. Did you actually measure a pre-88 wheel too or just compare castings? One more question. You measured it. I measured it. A few others in this thread measured it... Why is it so hard for people to believe the numbers? Is it simply a matter of correcting the disinformation floating around out there by fixing things like the OSG for instance? Hey Ogre, if you're watching this, how about putting a note in the cave about the wheel offsets? -Bruce at FTF Engineering

Hey, I believe it !!!
Even saved the numbers for future reference.
Thanks, now we need to just get it changed everywhere its been posted on the internet !!!!

quote Originally posted by FTF Engineering: Did you actually measure a pre-88 wheel too or just compare castings? -Bruce at FTF Engineering

I've measured the backspacing of '88 vs. pre '88 wheels and they're identical according to my tape and a straight edge.

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Doug Chase
Chase Race
Custom roll cage fabrication available
Custom exhaust fabrication available
'87 Fiero SE V6 5-spd for sale $2200

Something just doesn't make sense here. I agree that the OSG numbers are incorrect. But I also have to question Jazzman's numbers too.

Here is my basis for questioning his numbers: I replaced the rear wheels on my 88 GT with 16 x 6.5 x 39mm. before replacing the wheels I measured the distance from the rotor face to the outer face (lip) of the wheel. I measured this same exact measurement after replacing the wheels and got a difference of approx 10.3mm.

Now, IF the wheels had the SAME offset the wheel lip should have moved inward .25" (6.3mm). (7" - 6.5" = .5"/2 or .25") Correct? But the wheel lip moved inward an additional 4mm to a total measurement of 10.3mm

For this to occure the original 15x7 offset would HAVE to be 35mm. If, as Jazzman is suggesting, the original offset is 30mm then my new wheel should have moved inward a total of 15.3mm which it did not.

So, what is going on here? Hmmmmmm....I'm confused.

[This message has been edited by turbotoad (edited 09-05-2003).]

- You have 16x6.5 and 39mm offset. (you must have a Toyota rim)

- That means your new rim sits inward approx 6.25mm more (on the outside from stock apperance) not including the offset.

- Then when you add the offset difference (29mm to 39mm) that's another 10mm.

- All said and done... this would mean your rear wheels sit inward (from the outside apperance) 16.25mm more than stock.

That's quite a bit. And that's why I'm slightly dissappointed because the rims I was going to buy are 40mm offset. I know many here run 40mm offset. But if the stock rim is truely 29mm and not 35mm, I have to seriously re-think my wheel choice selection and keep in the 25mm to 35mm range.

PS: On the inside, it only sits in 3.75mm more.(because you went from a 7" rim to a 6.5" rim)

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[This message has been edited by Skybax (edited 09-05-2003).]

quote Originally posted by turbotoad: Here is my basis for questioning his numbers: I replaced the rear wheels on my 88 GT with 16 x 6.5 x 39mm. before replacing the wheels I measured the distance from the rotor face to the outer face (lip) of the wheel. I measured this same exact measurement after replacing the wheels and got a difference of approx 10.3mm. Now, IF the wheels had the SAME offset the wheel lip should have moved inward .25" (6.3mm). (7" - 6.5" = .5"/2 or .25") Correct? But the wheel lip moved inward an additional 4mm to a total measurement of 10.3mm For this to occure the original 15x7 offset would HAVE to be 35mm. If, as Jazzman is suggesting, the original offset is 30mm then my new wheel should have moved inward a total of 15.3mm which it did not.

So you're saying that you replaced the original 15 x 7 x 30mm rears with 16 x 6.5 x 39mm?

If that's truly the case then you are absolutely right, the tire bead mounting surface of the new wheel should move inboard by 15.3mm when compared with the tire bead mounting surface of the old wheel.

Measuring anything to the face of the wheel would require that the thickness of the wheel lip was identical in order to be accurate. Same goes for measuring anything to anywhere on the tires.

Remember, when they say a wheel is 7" wide, that means it's seven inches between tire bead seating surfaces. The total width of the wheel will always be greater than that.

So... One possibility is that the new wheels have a different thickness flange? You're only looking for 5mm, right.

Out of curiosity, you said you measured before and after and got a delta of approximately 10.3mm. This is .4055 inches... Seems like an odd number to "approximate". I would expect your measurement to be "approximately .4 inches" which is actually 10.16.. What measuring devices are you using for your readings?

Also, I wanted to double check... You went DOWN in wheel width from the original 7" to a 6.5" wide wheel? Seems like an unusual move... Most people want wider.

-Bruce at FTF Engineering

quote You went DOWN in wheel width from the original 7" to a 6.5" wide wheel? Seems like an unusual move... Most people want wider.

Toyota

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quote Originally posted by FTF Engineering: So you're saying that you replaced the original 15 x 7 x 30mm rears with 16 x 6.5 x 39mm? If that's truly the case then you are absolutely right, the tire bead mounting surface of the new wheel should move inboard by 15.3mm when compared with the tire bead mounting surface of the old wheel. Measuring anything to the face of the wheel would require that the thickness of the wheel lip was identical in order to be accurate. Same goes for measuring anything to anywhere on the tires. Remember, when they say a wheel is 7" wide, that means it's seven inches between tire bead seating surfaces. The total width of the wheel will always be greater than that. So... One possibility is that the new wheels have a different thickness flange? You're only looking for 5mm, right. Out of curiosity, you said you measured before and after and got a delta of approximately 10.3mm. This is .4055 inches... Seems like an odd number to "approximate". I would expect your measurement to be "approximately .4 inches" which is actually 10.16.. What measuring devices are you using for your readings? Also, I wanted to double check... You went DOWN in wheel width from the original 7" to a 6.5" wide wheel? Seems like an unusual move... Most people want wider. -Bruce at FTF Engineering

I wasn't really planning on replacing the rims at the time. I had a Pontiac Vibe at the time and came across a great deal for an entire set of Vibe (Toyota) wheels and tires at a local Belle Tire. So I bought them originally thinking that I would use them as a second (Winter) set of wheels for the Vibe. But When I got home I noticed that the lug pattern was the same as the Fiero and I put the wheels up next to the Fiero wheels. They looked awsome and I decided to put them on the Fiero. However, I had to have the center hole bored out to fit the GM hub and I also bought some 225/50/16 tires for the rear.

I realize we're only talking 5mm here. But I wouldn't think that the difference in material thickness of the wheel lip wouldn't be that much. I could see maybee 1-2mm.

I used a carpenders square held against the outer lip and then used a measuring tape to measure to the face of the rotor. Obviously not the most acurate way to do it, but it was close enough for me at the time. I suppose it's possible with the difference in material thickness plus the inacuracies of the measurement method to be off by 5mm. All I know is that my "new" rim definetly measured less than a 15-16mm delta. At least to the outer face of the lip anyway.

Anyone out the have access to a CMM machine that could accurately measure this?

[This message has been edited by turbotoad (edited 09-06-2003).]

[This message has been edited by turbotoad (edited 09-06-2003).]

quote Originally posted by FTF Engineering: So you confirmed that the pre-88 lace wheels are identical to the 88 rears, right? That would mean that all four wheels on pre-88 cars are 15 x 7 x 30mm. One thing to be careful about though... Just because all the 7" lace wheels came from the same casting doesn't mean that they machined the same amount of material off for all years. Did you actually measure a pre-88 wheel too or just compare castings?

I have a set of 4 '86GT lace wheels, and 3 sets of '88 lace wheels. I measured two of the four '86 wheels, and two each of the front and rear '88 wheels. The rear '88 wheels are the same as the '86 wheels.

quote Originally posted by FTF Engineering: One more question. You measured it. I measured it. A few others in this thread measured it... Why is it so hard for people to believe the numbers?

Dunno. I can only report what I see and know, and having taken metrology classes as part of my degree I have to believe that I know how to make good measurements.

JazzMan

quote Originally posted by turbotoad: I had a Pontiac Vibe at the time and came across a great deal for an entire set of Vibe (Toyota) wheels I realize we're only talking 5mm here. But I wouldn't think that the difference in material thickness of the wheel lip wouldn't be that much. I could see maybee 1-2mm.

Cool. Sounds like you got yourself a nice set of wheels.

Other than the potential problems with the measurements, I don't know why
you got the numbers you did.

One other idea is maybe the replacements weren't 39mm? I don't know anything about the Vibe, but maybe it uses and offset greater than 39? A 45mm offset would explain what you found?

Thanks also to Doug and JazzMan for confirming that the 88 rear wheels are
idententical to prior years.

-Bruce at FTF Engineering