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Proper Inflation

Read this article at Tire Safety. It covers most of it. (See Also, the FMVSS data about inflation pressure below.)

Best Handling

Make sure all your rims match!!!!! This is critical, especially on an older car. The odds are very good someone has replaced a rim and even if they look the same cosmetically they might have different offset, width or even diameter. This is a common problem when Aluminum wheels that look alike are pulled from another model car. It is even more common with steel wheels. I have seen cars that had 4 different rims on them.

I always prefer 4 identical tires on a car. For any normal driving this is your best handling unless the carmaker specified an uneven front to rear size setup. Some people want the uneven setup because of a bigger engine. Either way you want to match brand and type all around even if your run two sizes.

Make sure you have proper inflation. Always start at the original tire pressure! (I think it's 30-PSI for all Fiero tires.) For regular driving the OE pressure should work for any tire that will fit the car.

Tires have changed quite allot and If you are driving hard or auto crossing, you may need to tweak the inflation up or down a couple PSI to find the best tuning. The way radial tires are built just one or two PSI change can have a significant difference. Even a fraction or a PSI can make a difference so make sure you have a really good tire gauge.

I want more grip!

When you check tire specs, check the actual tread width. The Section Width, three-digit part of the tire size, is not the Tread Width and there is quite a bit of variation from one tire to the next, even when published tire size is the same! I've seen an inch or more range of tread width for some sizes.

If you have a narrow treaded 195 you can get more tread just by picking a different 195. Don't just assume that a 205 or even 215 gives you a big tread increase over a 195. It is actually possible to see 195 with the same tread width as a larger tire. (Allot of sizes overlap like this.)

Depending just what tires are involved, you might not gain any thing or you may only have a fractionally larger tread patch on the road. Tread patch is mostly determined by rolling diameter, tread width, and inflation pressure. (Ignoring wrinkle wall tires for now.)

Mounting Wider or Narrower Tires

Note: Just because a wider tire fits, don't assume you will get better handling. You can actually make things worse. It all depends on the tires and how the car is setup. This just explains some of what to watch when fitting a tire to a rim.

Before you plan on installing a non-stock tire, check the tire maker's web sites or catalogs. They list the range of rim width they recommend for a given tire. This is fairly important, as some tire designs and vendors are less tolerant of rims that are too wide or narrow.

Here are some examples from Cooper. (As published February 04)

Type Size Approved Rim Measured on Rim
Cobra Radial G/T P195/70R14 5.00 - 6.00 6.00
Weather-Master S/T 2 195/70R14 5.00 - 6.50 6.00
Weather-Master XGR 195/70R14 5.50 - 6.50 5.50
Cobra Radial G/T P215/60R14 6.00 - 7.00 6.50
Cobra Radial G/T P225/60R14 6.00 - 7.50 6.50
Cobra Radial G/T P235/60R14 6.50 - 8.00 7.00

The "measured on rim," or "Meas. Rim Width" as Cooper lists it, is the rim used to determine the tires published size data. Notice the different specs for the three 195/70s even though they are the same size.

All the tires above are approved on a 6-inch rim except the 235. Some brands of 215 or 225 may also be out of spec on a 6-inch rim.

The "real" effect of alternate tire sizes on Speedometers

Many people will say that speedometer error caused by changing tire sizes doesn't matter. They say, "Oh, it's only X% off. So what???" I used to think that, and then after I did the gear ratio article I wondered what else was screwed up.

Note: This article assumes you are starting from 195/75R14 for all calculations. If your car originally came with 13" or 15" tires, you'll need to adjust the numbers. This article assumes you car has the original speedometer gears for the tires it came with.

Smaller Tires

It's fairly common to drop the profile one number to get a stiffer tire. I've done this many times. Last time I bought tires, I changed from 195/75R14 to 195/70R14. Like allot of people, I didn't figure it would make much difference. Let's see...

Most people only look at the speedometer, forgetting completely about other things. The Miata.net tire calculator says my speedometer is now reading 3% too fast or at 60 you're doing 58.2 MPH. No big deal there. Reading fast is better than slow. At least I won't get a ticket with it.

The effect on Gear Ratio we've covered already, 3% doesn't make much dif here either. So what's left?

Well, the one everyone overlooks is the Odometer. It's now off as well. What this means is that for every mile my Odometer says I've gone, I've actually traveled 158.4 feet less than that. This means for every 33.3 (rounded) miles the Odometer says I drive, the car has actually moved 1 mile less.

The part that stinks is the car is doing nearly the same work as it would but getting less distance. Example: If, using the Odometer reading, the car get's 27 highway MPG with OE tires, and the car still says it gets the same with the slightly smaller tire, the fact is that I've actually lost almost 1 MPG. Over the life of a set of good tires, that's a significant amount of extra fuel burned.

Or, if we simply take 27 x .03 we get .81 miles lost.

Figure many tires last 40,000 miles or more...

That "insignificant" 3% change has/will cost me about 44 gallons of fuel spread over just 40,000 miles. (1200 missing miles / 27 MPG = 44.44) That's more than 4 tanks of fuel. I can drive back and forth to work for a month on that much fuel. It rapidly gets worse if you get lower MPG, like constant city driving. Just dropping to 25MPG will hit you for 48 gallons. Dropping to 20MPG, where many V6's hang out, costs 60 gallons more fuel.

Larger tires?

For one or more reasons, many people go to larger tires... So lets say you were 3% the other way, reading slower than you are actually moving. For example: 195/75R14 to 205/70/15. Miata.net says the error is 3.1% slow and speedometer reads 60 but your going 61.8. I rounded to 3%. No big deal here, it's not enough to trip most speed traps.

You may gain MPG, but loose torque to the ground just like the Gear Ratio article says. You also cover more ground than the Odometer reads. So for 40,000 miles of Odometer reading, you'd actually have driven 1200 miles farther. (Depending on the exact tire involved, you could actually loose MPG. City MPG would likely be hit more than Highway. In this case I mean real MPG not what you'd get using the Odometer.)

2-2.5 is a pretty reasonable/common range of change. If you change the tire diameter by 2.4 inches, the speedometer is now reading 9.1% too slow. You're reading 60 but the car is moving 65.5. (195/75R14 to 215/70R16)

A 3.4" increase in diameter, easily done with 17" and larger rim/tire combinations, results in a whopping 13.1% error. Now you're reading 60, but doing nearly 70! (67.8)

Unless they are feeling picky for some reason, the police often won't waste time on someone doing 2-3 MPH over the speed limit. They are after the people who are clearly blowing off the limit because it means more points and bigger fines.

So using our 13.1% example at a common highway speed limit... If the speed limit is 65, and that's what your speedometer says, you're actually doing 73.5MPH, more than 5 over the limit. If you're like many people who commonly run a few miles over on the speedometer, at 70 you're doing 79.2, clearly more than 10 over the limit.

Even at our more conservative 9.1% error, at 65 you'd be doing 70.9. That's still 5 over the limit and enough to be a ticket magnet in allot of places.

Now what?

For these reasons your best choice if you want lower tires, with minimal change of tire circumference, change from the 14" to a 15" or 16" tire with lower sidewalls. According to the Miata.net calculator, the following combinations are really close to the OE 14" tire. (I don't know if they really exist. They'll at least give you an idea. Use the Miata.net calculator to compare your tires before you buy them.)

205/65R15 Error = 0.1% too fast (Read 60, do 59.9)

205/60R16 Error = 0.7% too slow (Read 60, do 60.4)

215/55R16 Error = 0.8% too fast (Read 60, do 59.5)

What this also means is that if you are going to run tires with a significantly larger circumference, you really should consider recalibrating the speedometer. This adds to the expense of replacing the tires, but it could save you a fortune in fines and increased insurance rates.

Final Note

If you've done something like installed an AOD transmission, which I understand some people have gotten from minivans, you want to note what tires were used on the donor. If the van used a big 15" tire, you could likely change your Fiero tires to something with the same rolling diameter. This would actually correct your speedometer if your Fiero has 14" wheels. Same thing works for putting a GT transmission into a coupe. (Many GT's have 15" wheels and thus different speedometer gears in the transmission.)

Hub Centric Vs. Lug Centric

Hub centric mean the wheel will center using Hub/axle as a reference. Most OE Wheels are Hub Centric. Most Aftermarket Wheels need adapter rings to use the hub as center reference.

Lug centric mean the wheel will center using Lugs as a reference. This method works well but has rules...

In Lug Centric method:

Cone's are self centering only when wheel don't see any load... Loading the wheel, like dropping the jack to stop wheel spinning when torquing lugs, cause lugs bolts to slightly bend and other interference.

Torqueing need three steps at minimum: All lugs hand tight in star pattern, torque 50ft/lb in star pattern, then torque to spec in star pattern. (Lugs too tight to fit fingers? Starting torque sequence with a wrench is ok but be carefully.)

Wheel Locks

Fiero Factory Locks are made by:

AMCOR Wheel Locks

12955 Inkster Rd.

Livonia, MI 48150

(800) 831-9604 or (734) 458-8920, 8:00 a.m. - 5:00 p.m. Eastern time.

  1. If you have original lock and doc's for car... One document has key code. Call amcor and order new key.
  2. If you go to Dealer and use Master Key then write down Key Code. Order key from Amcor. (Note: Not all dealers have the set of Master Keys. Call around.)
  3. You can buy new lock and key(s).

fierosound says "They are Amcor PN 057 and are available directly from Amcor." AMCOR Wheel Locks parts

Aftermarket locks:

McGard have styles of locks that fit Fiero studs that might better match aftermarket wheels. Notice as part number 24012 is NOT compatible with OE Fiero Wheels and lug caps.

This (24012) lock will not fit OEM plastic lug caps. Matching lock can only be purchased from GM Dealer.
Source: McGard

There are several other brands of locks running around. Shop them carefully. Most are poorly made and rust easily. They can often strip and leave you completely stranded.

FMVSS Data

I've added this because of a huge debate over certain terms, like the maximum inflation pressure you see on the sidewall. Here are the official definitions of those terms.

§ 571.109 Standard No. 109; New pneumatic tires.

S3. Definitions.

Maximum permissible inflation pressure means the maximum cold inflation pressure to which a tire may be inflated.

Load rating means the maximum load a tire is rated to carry for a given inflation pressure.

Maximum load rating means the load rating at the maximum permissible inflation pressure for that tire.

Section width means the linear distance between the exteriors of the sidewalls of an inflated tire, excluding elevations due to labeling, decoration, or protective bands.

This part is just for reference. It explains the FMVSS requirements for nearly all the labels you see on the side of the tire.

S4.3 Labeling Requirements. Except as provided in S4.3.1 and S4.3.2, each tire shall have permanently molded into or onto both sidewalls, in letters and numerals not less than 0.078 inches high, the information shown in paragraphs S4.3 (a) through (g). On at least one sidewall, the information shall be positioned in an area between the maximum section width and bead of the tire, unless the maximum section width of the tire falls between the bead and one-fourth of the distance from the bead to the shoulder of the tire. For tires where the maximum section width falls in that area, locate all required labeling between the bead and a point one-half the distance from the bead to the shoulder of the tire. However, in no case shall the information be positioned on the tire so that it is obstructed by the flange or any rim designated for use with that tire in Standards Nos. 109 and 110 (§ 571.109 and § 571.110 of this part).

(a) One size designation, except that equivalent inch and metric size designations may be used;

(b) Maximum permissible inflation pressure;

(c) Maximum load rating;

(d) The generic name of each cord material used in the plies (both sidewall and tread area) of the tire;

(e) Actual number of plies in the sidewall, and the actual number of plies in the tread area if different;

(f) The words "tubeless" or "tube type" as applicable; and

(g) The word "radial" if the tire is a radial ply tire.

S4.3.1 Each tire shall be labeled with the symbol DOT in the manner specified in part 574 of this chapter, which shall constitute a certification that the tire conforms to applicable Federal motor vehicle safety standards.

S4.3.2 Each tire shall be labeled with the name of the manufacturer, or brand name and number assigned to the manufacturer in the manner specified in part 574.

S4.3.3 [Reserved]

S4.3.4 If the maximum inflation pressure of a tire is 240, 280, 290, 300, 330, 340, 350, or 390 kPa, then:

(a) Each marking of that inflation pressure pursuant to S4.3(b) shall be followed in parenthesis by the equivalent inflation pressure in psi, rounded to the next higher whole number; and

(b) Each marking of the tire's maximum load rating pursuant to S4.3(c) in kilograms shall be followed in parenthesis by the equivalent load rating in pounds, rounded to the nearest whole number.

S4.3.5 If the maximum inflation pressure of a tire is 420 kPa (60 psi), the tire shall have permanently molded into or onto both sidewalls, in letters and numerals not less than 1/2 inch high, the words "Inflate to 60 psi" or "Inflate to 420 kPa (60 psi)." On both sidewalls, the words shall be positioned in an area between the tire shoulder and the bead of the tire. However, in no case shall the words be positioned on the tire so that they are obstructed by the flange of any rim designated for use with that tire in this standard or in Standard No. 110 (§ 571.110 of this part).

Copyright 1999, 2013 The Ogre's Fiero Cave, D.W.Lane, All rights reserved.
Not for reproduction in any media without written permission.