Having been into Fieros since the late 90's I should know the answer but not sure about it. GM specified 215 x 60 x 15 rear and 205 x 60 x 15 front tire sizes. Here are my theories as to why:
1. Rear tires need to be bigger as more weight is on the rear wheels.
2. Without power steering, smaller tires on the front makes steering easier.
3. To balance a suspension that was designed for a smaller front profile.
4. GM decided that it looked better that way.
5. Handling is improved this way
6. For cost savings

Anyone know for sure what the real reason for specifying different rear and front size tires was? Its time for me to go to 225 x 45 x 17's and just wonding if I should go to 215's on the front.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I'm not sure anyone knows the true "GM" answer to your question, and as you've surmised, there are likely several reasons. IMHO, I'd suggest that the most important were to a. reduce inherent oversteer in a rear weight biased car by giving a larger tire contact patch at the rear end and b. keeping the steering effort to a minimum at the front end which was a major criticism in most of the magazines at the time.

quote Originally posted by Dennis LaGrua: Having been into Fieros since the late 90's I should know the answer but not sure about it. GM specified 215 x 60 x 15 rear and 205 x 60 x 15 front tire sizes. Here are my theories as to why: 1. Rear tires need to be bigger as more weight is on the rear wheels. 2. Without power steering, smaller tires on the front makes steering easier. 3. To balance a suspension that was designed for a smaller front profile. 4. GM decided that it looked better that way. 5. Handling is improved this way 6. For cost savings Anyone know for sure what the real reason for specifying different rear and front size tires was? Its time for me to go to 225 x 45 x 17's and just wonding if I should go to 215's on the front.

My understanding is that the size offset only affects the 88 model and it compliments the suspension redesign and wheel offset.

All the '86 to '88 GT's had 205 fronts and 215 rears. (edit to add: and Formulas)

[This message has been edited by Bloozberry (edited 09-28-2012).]

Yep. All GT models starting in 1986 equipped with the diamond lace wheels received a slightly staggered tire combination stock, as Blooz said 205s and 215s. I think as Fieros got older many owners simply replaced all four with the same width tires since it probably made more sense to have the ability to fully rotate the wheels out, rather than having to deal with wider tires up front if one swapped their wheels around like 'normal'. Otherwise the only asthetically way to rotate the tires would have been side-to-side.

I always thought the reason for the staggered tire setup starting in 1986 was simply asthetics. Depending on the make of the tire, a 215 could look fairly wide sitting up front, while a 205 may look too small sitting on the rear with the fastback bodywork. I was just always under the impression it was meant to keep asthetics in line.

im running non stock rims on my 86 gt with 215/60 r15 in the rear and in the front and i notice no difference in steering. i have also ran it with 1 inch lowering springs in the front and i got WAY better steering. im wondering if i used the stock rims could i get 215s in the front?

In that case it may have been a safety issue. The front end is a little on the light side along with the short wheel base and if my trip home in the rain recently is any indication the narrower tire up front makes it easier for the tire to slice through the puddles and maintain contact with the road. I have 225s up front and ran into some heavy rain with a good bit of puddling on the road and a few times had to slow down to 35 mph on a 70 mph hwy after literally having to allow the car to take me left or right across the lines until I could coast down as it felt like any moment the car would be swapping ends.

I never experienced it with the 215s and the tires on the car are not worn out although they're getting close. It felt like the front end was lifting and even at the lower speed the deeper the puddles got the harder the front end was pulled.

quote Originally posted by Niterrorz: im running non stock rims on my 86 gt with 215/60 r15 in the rear and in the front and i notice no difference in steering.

Bear in mind that the difference between a 205 and a 215 is 49% tire tread in front vs 51% in back, so it's not surprising you didn't notice any difference.

quote Originally posted by Niterrorz: I'm wondering if i used the stock rims could i get 215s in the front?

The stock 15" wheels for the '86 and '87's were the same width (7") front and rear so there would be no problem running 215's on the front. The '88 front wheels were an inch narrower at 6" wide so if you ran 215's on them, the sidewalls would bulge out more.

quote Originally posted by Bloozberry: The stock 15" wheels for the '86 and '87's were the same width (7") front and rear so there would be no problem running 215's on the front. The '88 front wheels were an inch narrower at 6" wide so if you ran 215's on them, the sidewalls would bulge out more.

hey sweet thanks! i just think the 2 differnt size tires look goofy as comapred to the same on all four corners. and its time for a tire refresh so looks like ill be sticking with 215s

quote Originally posted by Joseph Upson: In that case it may have been a safety issue. The front end is a little on the light side along with the short wheel base and if my trip home in the rain recently is any indication the narrower tire up front makes it easier for the tire to slice through the puddles and maintain contact with the road. I have 225s up front and ran into some heavy rain with a good bit of puddling on the road and a few times had to slow down to 35 mph on a 70 mph hwy after literally having to allow the car to take me left or right across the lines until I could coast down as it felt like any moment the car would be swapping ends. I never experienced it with the 215s and the tires on the car are not worn out although they're getting close. It felt like the front end was lifting and even at the lower speed the deeper the puddles got the harder the front end was pulled.

This is exactly why the sizes are offset. The skinnier tires up front make it easier to push water on the pavement to the sides, and the larger tires in the rear give a greater contact patch for maintaining traction to the drive wheels. The combination helps reduce lower the chances of breaking traction in certain driving conditions.

There may be some benefit to reducing understeer, too. If you raise the front end of a vehicle, it will not handle as well in a curve and will have serious understeer. Raise the rear too much and you get serious oversteer. We know the Fiero already suffers from understeer and it may increase that problem with matching size tires, though the increase may be minute or imperceptible to us.

A smaller contact patch in the front encourages more understeer, unless you're talking about rain. The tire size does affect the ride height front vs rear, but if the effect of that was significant it could more properly be adjusted with spring selection.
Manufacturers like understeer, it helps hold down insurance rates. Steering effort was also an issue. I think it's silly when people complain about non power steering, but it bothers some people who aren't used to it. Give them 10 minutes and they'll get over it, but on a test drive it may affect sales.

[This message has been edited by armos (edited 09-28-2012).]

mid rear engine cars like the fiero have an inherent handling flaw called snap throttle oversteer , which has been discussed many times on this forum .when you hit the limit in a corner and the rear of your car starts to come around the natural reaction is to get right off the throttle and steer in to the slide .shutting the throttle completely causes the slide to worsen very quickly , you wont be able to catch it with the steering wheel .having a larger tire in the back helps to minimize the problem .GM did not go that far with such a small difference but it is smart to always have a larger tire in the back .

quote Originally posted by wftb: mid rear engine cars like the fiero have an inherent handling flaw called snap throttle oversteer , which has been discussed many times on this forum .when you hit the limit in a corner and the rear of your car starts to come around the natural reaction is to get right off the throttle and steer in to the slide .shutting the throttle completely causes the slide to worsen very quickly , you wont be able to catch it with the steering wheel .having a larger tire in the back helps to minimize the problem .GM did not go that far with such a small difference but it is smart to always have a larger tire in the back .

That might be true, I've certainly heard of it anyway. Funny thing is, the only time I completely lost control of a Fiero was when I stayed in the throttle. Other times I reacted the "normal" way and recovered it. So I've sort of taken the popular wisdom on this with a grain of salt.

[This message has been edited by armos (edited 09-28-2012).]

you have to back off the throttle to about 25% when the rear steps out .if you stay in the throttle you are going to lose it .either way , if you have a larger tire patch in the back ,the problem goes away .most people put way too large of a front tire on their fieros anyway .the car is not that heavy and a 225 is never going to warm up enough to give the grip we are looking for .

How do I find out what are the correct tires on my car? I don't want to have speedometer reading issues. The car needs tires, and I'm not confident that the ones that were on it were correct.

It's an 87 5 speed 4cyl.

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I have a 1987 Fiero, 4 cylinder 5 speed. With an 88 engine.

quote Originally posted by Stainless1911: How do I find out what are the correct tires on my car?

The door sticker will tell you OEM tire size(s) as well as inflation pressure.

And if the door sticker is gone, then check out the VIN code tag on the wheel arch under the hood on the driver's side. You'll see one of the following codes:

QDX = P195/70R/14 black sidewalls
QDY = P195/70R/14 white letter sidewalls
QFE = P185/75R/14 black sidewalls
QFG = P185/75R/14 white letter sidewalls
QPU = P215/60R/14 black sidewalls

There are a few other codes but unless your car is an "export" or an end-year model, you should find one of the codes above. If not, then post the code you find that starts with a Q.

quote Originally posted by Niterrorz: I'm running non stock rims on my 86 gt with 215/60 r15 in the rear and in the front and i notice no difference in steering. i have also ran it with 1 inch lowering springs in the front and i got WAY better steering. im wondering if i used the stock rims could i get 215s in the front?

You may be onto something. There is about a 1/2" difference in diameter between a 215/60-15 rear tire and a 205/60-15 Fiero front tire. So with the smaller front tires the front sits slightly lower. Your experience with improved steering when using 1" lowering springs in front may be the very reason GM chose to use smaller front tires. I am now convinced that the reason is for easier steering and as a side benefit aesthetics.
My new tires sizes ( for aesthetic appeal) will be 225/45 -17 rear and 215/45-17 front. They mirror the O.D. of the OEM Fiero tires pretty closely and will fit without interference

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Dennis LaGrua: You may be onto something. There is about a 1/2" difference in diameter between a 215/60-15 rear tire and a 205/60-15 Fiero front tire. So with the smaller front tires the front sits slightly lower.

Bear in mind that a 1/2" difference in diameter only results in a 1/4" difference in ride height. That's likely negligible in terms of noticeable performance gains or losses.

I'm thinking it's all of those reasons. For one, it definitely "looks" cooler too....

quote Originally posted by Bloozberry: Bear in mind that a 1/2" difference in diameter only results in a 1/4" difference in ride height. That's likely negligible in terms of noticeable performance gains or losses.

That's possible but a 1/4" difference in alignment specs is considered a huge difference and will make significant changes to a cars handling.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Yes, but 1/4" difference in toe, camber, or caster is comletely different than 1/4" change in tire radius. Changing the tire radius by 1/4" doesn't change the suspension geometry at all (unless you consider the corresponding effect on the height of the CofG which is 1.3 meters behind the front wheel centerline, significant).

Here's what my 1985 GT Service Parts Indentification sticker has on it:

QPV 215/60R14 white-lettered, steel belted (84-85)

The tire spec changed in 86 with the fast back GT. The 85 was still a notch back with all tires the same size.

quote Originally posted by Dennis LaGrua: You may be onto something. There is about a 1/2" difference in diameter between a 215/60-15 rear tire and a 205/60-15 Fiero front tire.

But the stock front size is 205/65-15, not 60. So the diameter is 25.2 in both front and rear.

quote Originally posted by Bloozberry: ...IMHO, I'd suggest that the most important were to a. reduce inherent oversteer in a rear weight biased car by giving a larger tire contact patch at the rear end and b. keeping the steering effort to a minimum at the front end which was a major criticism in most of the magazines at the time.

I'd like to believe this is true. At least with the 88s.
The base 88 coupe had the same tires/wheels at all 4 corners, and no rear sway bar.
The GT and Formula got staggered tires and wheels, along with the addition of the rear bar.

When I upgraded to 17s, I attempted to keep a similar front/rear relationship with my stagger. From 205/215 to 215/235. It seemed to work quite well.
On those few occasions where I got to push the envelope (Waterford, at the 20th, comes to mind) it seemed very neutral. Neither end wanted to slide before the other. I did get into a couple of nice, but very controllable 4 wheel drifts. (I usually don't drive that hard on the street, and bla, bla, bla...)

quote Originally posted by dobey: But the stock front size is 205/65-15, not 60. So the diameter is 25.2 in both front and rear.

This isn't true. The stock front tire profiles for those cars with 205 width tires were 60 series, NOT 65. The only RPO code for 205 series tires is is QZT which is P205/60R/15.

All of the above seems to be correct.
From misc sources:

With the 1988 model year came a completely new front and rear suspension with vented disc brakes at all four corners - what Pontiac's engineers had planned for the car to have from the beginning. The front suspension geometry was changed to decrease the scrub radius thus decreasing steering effort without adding a bulky power steering system. The camber curve was also much improved, the dampers are moved inside the coil springs, and new sealed bearing/hub units were used in 1988.

This new suspension came with staggered wheel sizes on WS6 suspension equipped models, with 15-inch (380 mm) by 6-inch (150 mm) wide wheels up front and 15-inch (380 mm) by 7-inch (180 mm) wide wheels in the rear for improved handling balance and to offset the slightly increased front track that resulted from the improvements.

Take a look at cars like the Porsche's or Ferrari's. They typically have significantly larger tires in back than in front for many of the reasons mentioned in other posts. Putting the engine in the middle or the back changes what happens in turns. The big reason for putting the engine as close to the middle is to improve handling in turns. However, the lever arms are not exactly the same even though the weight distribution is 50-50. It is really weird to see a Porsche lift a front wheel in a hard turn.

We just went with 205's front and back. We will need new ones in 20 thousand or so, so I definitely want to verify if our car has two different size sets of wheels. I was not aware that the wheels were intended to be different sizes, just the tires.

It definitely makes it easier to rotate them. Unless you race or take curves at 30 MPH over the advisory speed, the Fiero sticks to the road like glue either way.

Speaking of even handling. I have tried before to lock up the brakes and can't do it!! Stomping as hard as I can just brings all 4 wheels to a very quick stop. Never had a car that did that before. Really cool.

quote Originally posted by Dennis LaGrua: Having been into Fieros since the late 90's I should know the answer but not sure about it. GM specified 215 x 60 x 15 rear and 205 x 60 x 15 front tire sizes. Here are my theories as to why: 1. Rear tires need to be bigger as more weight is on the rear wheels. 2. Without power steering, smaller tires on the front makes steering easier. 3. To balance a suspension that was designed for a smaller front profile. 4. GM decided that it looked better that way. 5. Handling is improved this way 6. For cost savings Anyone know for sure what the real reason for specifying different rear and front size tires was? Its time for me to go to 225 x 45 x 17's and just wonding if I should go to 215's on the front.

My guess is to make it understeer. That freaks people out less than oversteer. They did this on the wife's SRX because it's rear-happy at the limit. From my few "incidences" with my Fiero, it wants to oversteer at the limit.

The old joke is - Understeer is when the driver is scared and oversteer is when the passenger is scared.

[This message has been edited by TK (edited 10-02-2012).]

I heard a different one.

Understeer is when the front of the car hits the wall first, oversteer is when the back of the car hits the wall first.

quote Originally posted by Ray_and_kevin: ...I definitely want to verify if our car has two different size sets of wheels. I was not aware that the wheels were intended to be different sizes, just the tires. ...

The only cars to have different size wheels, front to rear, were the 88 GT and Formula. (15x6, front; 15x7, rear)
All 4 cylinder coupes and SEs (and the 85 GT notchback) had the same size wheels (14x6) and tires, front and rear.
This is with the exception of the base coupes with steel wheels, or the tiny turbo fin wheels. But they were still the same size front and rear.

86 and 87 GTs had the same size wheels (15x7) front and rear, but had the 205/215 staggered tire sizes.

[This message has been edited by Raydar (edited 10-02-2012).]

quote Originally posted by Stainless1911: I heard a different one. Understeer is when the front of the car hits the wall first, oversteer is when the back of the car hits the wall first.

I like that one too!