I will be ordering four BFG Comp T/A VR4's in the near future for my '87 GT. They come in both 205/60/15 and 215/60/15. I know stock size had the 205's up front and the 215's on the rear on my car. I have heard wider tires in the rear helps cure understeer problems in mid and rear engined cars, but why? Wouldn't this just provide more traction in the rear, making MORE understeer? Somebody enlighten me.

If I do end up going with one size all around, should I go with 215 or 205?

hey Shiner... I'm not very familiar with the correct tire size to use soooo... the may not help you but I have a 87 GT also and I'm running 205/60/15's in the front, and 235/45/15's in the rear...
I know that i WILL GET IT FROM SOME MEMBER'S for not using the correct tire size BUT.... it look's good and my a$$-a- meter feels that it really corner's great...

1FieroGTRacingInc.

I would say you are right about the difference just making the car understeer more at low speeds,however,the wider size in the rear does make the car nicer/safer at higher speeds.I used just a slightly larger size in the rear that was a real sticky tire compound as well compared to the front compound.The more power you have,the more that increase in traction in the rear will be appreciated.

Depends what rims you have. Sounds like you've got the 2 sized set. In that case use the recomended sizes for each rim size.

if you put the big rubber all the way round you pinch the beads on the front. (you pull the sidewalls in beyond specification.) If you put the small rubber on the big rim you V the walls out.

BEst thing to do is get both rim sizes and then study the tire makers specs. Find a tire that is listed to fit both rim widths. Not all will like it. Mount a tire on a rim it doesn't like and you will have problems.

Tires are measured on specific test rims. The "rule" is used to be that you can +- 1 inch wider/narrower than test with your rim. You have to check the specific tires tho because allot of the new tires are so stiff that they don't allow any variation from the test rim. Other tires can go -1 to +2 inches. Most of this info is on the tire makers web sites.

It's a pain in the ____ but you really want tires sized for you rims. This will put the tire in it's designed profile where it will perform it's best. Even in a 2 sized set like some of the GT's have.

also.... check your rims andmake sure the right tires go on the right rims and that they are all put on the right axles and, if you got unidirectional tires, that the rotation arrows face the right way. Never trust the shop with 2 sized sets. They have a nasty habbit of rushing things and screwing up these sets.

Ogre,
I have the 15x7" diamond spoke rims all around. Part of the reason I would like to get the same size all around is so that I could actually rotate the tires.

I still don't see any reason to use larger rubber on the rear though.

Another reason to use the narrower tires on the front is to reduce the steering effort when parking. I know some folks that have put 225's on all four wheels (pre 88's), and have had no problems whatsoever. Looks good too.

Quite a few tail heavy cars have gotten away with same size tires front and rear, Porsche 911 and Chev. Corvair as an example, so different size tires aren't usually a neccessity for street use.
On a rear heavy car, probably the limit for same size tires, in cornering mode, would be when the rear springs and sway-bar are as soft as possible, and the front sway bar is as stiff as possible (when the inside front wheel leaves the ground whilst cornering). The rear being soft gives maximum traction, the front being stiff(particularly the bar)giving the least traction due to tire load sensitivity.
An explanation: The outside front tire is being overloaded somewhat with double the weight as normal, and with double the weight, will not give double the traction(Tire Load Sensitivity). The inside front tire is contributing nothing, if it is in the air. Since the fronts are now doing most of the work of keeping the car level, the rears are more evenly loaded. And they can make the most traction.
The other limit for rear tire size would be if traction while accelerating (or accelerating while turning)was acceptable or not.
One final factor could be the tire needs to be larger because of it's load capacity rating (pure front to rear weight considerations). A Fieros' weight distribution is about 43/57.
In a nutshell, it's usually easier to make a rear heavy car stable and good handling with larger, wider tires on the rear with reasonable size sway bars and springs. But it puts a certain burden on the owner for the reasons you already stated.

In your case shiner since all rims are the same, no there's no reason at all you can't match all 4 wheels. The majority of cars on the road, even the hiperfs, mostly use matched tires. Only RWD applications that can't stick the engine hp realy ever need bigger rears. Suspension tuning is much easier and more predictable when all tires match.

I took a quick look at just one good year product 215 all around should be fine. you can likely run a 65 profile without problem. that's a reasonably short sidewall. If you think they might be a bit stiff to turn a 205/60 and 205/55 are available too. These are all measured on a 6.5 wide rim and are aproved for your 7s.(eagle GT+4 is the tire I used for quick reference. It's just a brand I've tire I've handled before.)

Like I said. Allot of it depends on the exact tire you choose. BTW. 215/60 will be measurably taller than 205/60.

this is bfg's tire web site http://www.bfgoodrichtires.com/ you need acrobat to look at the exact specs and I presently don't have it installed. My guess would be the 205 all around.

[This message has been edited by theogre (edited 05-02-2000).]

Okay, the last two posts really helped I DO plan on doing some tuning to dial out a good amount of that understeer using sway bars and whatnot, so it sounds like going with 215's all around might be my best bet, depending on the tire. I would like to keep the sidewalls as short as possible, within reason, so I don't think I will go above 60 series. Anything lower than that starts to get expensive pretty quickly. Anyway, I am planning on Koni's, new sway bars, a complete poly bushing upgrade, and POSSIBLY lowering springs, even though I don't like how it changes the geometry. With all this, especially the fully adjustable Koni's, I should be able to compensate for the smaller rear tires.

The above are correct, but haven't got to the essence of the situation.

In 1984, the SE's had P215/60R 14's all around. Other combinations were used too, on other models.

The earlier cars were designed purposely to understeer. Remember the Corvair? Because of the understeer and the steering damper, people complained about heavy steering. Can't have it both ways.

So what to do? Put smaller tires (205's) in front to reduce steering effort. Hence, you don't have wider tires in back, you have narrower tires in front. The trade off is slightly less traction, but easier low speed steering, but even more understeer. Narrower tires in front = more understeer. Wider tires = more oversteer. Except the Fiero's suspension geometry is greatly biased towards understeer. A rear sway bar offsets some of this and makes the cars handle a lot better.

If you go too wide in front, you may not like the steering effort at normal driving speeds. Or the front will "hunt" on straight stretches (similar to oversteer).

For my '64 Corvair, I put more air in the backs and less in the front (as recommended by Chevrolet) and used softer tires in the front compared to the back. It had very good handling with that set-up. (See slip angles)

Pick your poison: performance/comfort.

[This message has been edited by Ed (edited 05-02-2000).]

Check this site out too...Some good stuff on Fiero Handling...I think it was Batboy who posted this URL in a thread. I'm sure some of you have seen it before but if not check out the 1st 3 paragraphs on the Herb Adams link.

http://www.homestead.com/fierorock/Suspension.html

Ozzy

Ed,
I have 205/60/15's up front right now, and even with the quicker steering ratio that came on my car, it doesn't bother me one bit. I am a very big guy, so it's pretty easy for me to muscle it around in parking maneuvers.

Ozzy,
I read everything on that page, that's what got me started on this topic Herb said he put larger tires on the rear, but it gave no explanation as to why he did that.

The '87 suspension seems to handle MUCH more neutraly than what's on my '84, but it's still a little bit too much understeer for my taste.

Shiner,
I have the same wheels, same suspension as you on my 86 GT, and I went up one size to 225/60/15 out back and 215/60/15 up front. 33# air pressure all around. They fill the wheel well up nicer than the stock sizes, but if you lower it 1.5" like I did, the fronts will rub the inner fender liner at full lock, until they get some miles on them. Roughly .5" bigger diameter, so speedo inaccuracy is negligible. I have the BFG Comp T/A HR4, with 45,000 miles so far. I have yet to spin it around unintentionally, but I installed the springs, polyurethane and rear anti-sway bar at the same time as the larger tires. Big improvement over the original, by the seat of the pants meter.

You're forgetting one thing here. Fieros looks AWESOME with bigger meats in the back!

I guess if I had a zapper 350 like Batboy in the engine bay, I'd get more use out of something wider out back but I like the way the 225's fit inside the fender lip.

You guys are giving the extra 1" of rubber in the back too much credit. Really the way you drive should dictate what kind/size tires you use. I personally use 205s in front and 225s in back with a very sticky compound on both. I find with smooth hands, the backend will break loose with the front resulting in a perfect four wheel drift.

Shiner, read the Herb Adam's page again. He says one of the problems with the Fiero is the weight of the engine is behind the center of gravity of the vehicle. Placing bigger wheels on the rear helps to reduce the effects of this problem.

Ozzy

If you don't like the way a rear weight biased car understeers, maybe try getting on the throttle BEFORE the corner.
If you still have problems,maybe some horsepower,or a small swaybar will correct things.

LowCG,
There really was no need to try to make an ass out of me. I know about throttle on oversteer. The problem is that the car is not setup for fast cornering, it's setup for safety of Joe Driver, who wouldn't know what to do if the rear came around on him.

I just went from 205/60r15's on the rear of my GT to 225/60r15's... they look sooooo much better! Need to get some pics..

i've had 225/50 -16's all around for about 2 years, but i am switching to 255/50-16's in the rear after a coilover change. why? because i can. remember the hot rod mag. article with the super duty powered fiero? it had 315/35-17's on it. i'd do that if i could. price and the availability of the pieces they had stops me, but i love the look.