Offset and Backspacing Made Simple:
This is an area that can easily cause much confusion. While these terms are often used interchangeably, they
are not the same. Further confusing all of this is the fact that rim width is the distance between the rim flanges
(where the tire bead contacts the rim), so that the actual wheel width from edge to edge is 1.0" greater that
the stated wheel width, making a 6.0" rim actually measure 7.0" wide at it's widest point.
(Outside face of wheel is towards your Right)
Backspacing refers to the distance from the mounting surface of the wheel (where it contacts the hub) to the
back side (inwards-facing) edge of the rim. The easiest way to measure this is by placing a bare wheel on the
floor upside down. Place a straight-edge across the wheel, then use a ruler to measure the distance from the
bottom of the straight edge to the mounting surface of the rim. Stock fifth generation El Camino wheels have
about 4" of backspacing, and this is a good measurement to shoot for. You can try to measure backspacing
with a tire mounted on the wheel, but you will have to adjust your measurement if the tire sidewall extends
beyond the rim.
Offset refers to how much the hub mounting surface of the wheel is offset from the wheel's center, usually
given in millimeters. Wheels can be positive or negative offset. To get this measurement requires a bit of basic
math. First measure overall wheel width from the outside edges of the wheel lips, then divide this
measurement in half. Next measure the backspacing as above. A 7" wide wheel is actually 8" wide from wheel
lip to wheel lip. If the hub mounting surface were exactly in the center of the wheel, it would have zero offset.
It would also have 4" backspacing. Many of the 13" x 7" American Racing Libre wheels are just like this. If
the backspacing on this wheel were 3", then the wheel offset would be +25mm (1 inch = 25.4 mm). As wheel
diameter and width increase, you can usually get away with a bit more positive offset, as the 15" and 16"
Panasports are +22mm offset. Most FWD cars use wheels that range between +30mm to +45mm offset.
Note: Using improperly spaced wheels may require the use of wheel spacers between the hub and wheel
mounting surface for proper fitment, but keep in mind that this may require longer wheel studs. Furthermore,
wheels that have significantly less backspacing or negative offset than stock (i.e. deep-dish wheels that "stick
out") can put extreme stresses on wheel bearings and suspension components. These wheels may also
increase steering wheel kick-back. Of course it is always better to get wheels that fit correctly, and not have
to use spacers at all.
Deciphering Tire Sidewalls:
Before buying new tires, take a minute to learn what all those numbers and letters on the tire sidewall mean:
Tire Size: Let's say, for example, we have a P185/60-R13 tire. The "P" indicates this is a passenger car tire. Many times this is omitted (LT = Light Truck tire). The first
number is the Section Width of the tire in millimeters, measured from sidewall to sidewall of a fully-inflated tire without any load placed on it. For a 185/60-13 tire, the
width is 185mm or 7.28" (to convert to inches, divide by 25.4). It is important to note that this is not the tread width of the tire at the road surface. The second number
is the Aspect Ratio, a ratio of sidewall height to tire width (a percentage of the section width). The tire above is 7.28" wide, so we multiply by the aspect ratio to find the
height of one sidewall. In this case, 185 x 0.60 = 111mm (or 7.28" x 0.60 = 4.36"). The "R" means this is a radial tire. The last number is the Diameter of the wheel in
inches. When switching to larger diameter wheels, try to keep the tire outside diameter the same as the car's stock tire to avoid problems with speedometer and
odometer calibration and changing overall gear ratios.
To calculate the outside diameter of a tire: Take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, one above the wheel and
the one below the wheel) and add the diameter of the wheel to get your answer.
For example: 185/60-14: 185mm x .60 = 111mm x 2 = 222mm + 355.6mm (14") = 577.6mm or 22.74" diameter.
Note: When mounted on a wider than designed wheel, tire section width increases by 0.2"(5mm) for every 0.5" increase in rim width. This would make a 185mm wide
tire mounted on a 7.5" rim have a section width of 190mm.
A general rule of thumb for a high-performance tire is a rim width that is 85-90% as wide as the tread of the tire (not tire section width).
Speed Rating: The maximum safe top speed of a tire under perfect conditions is given as a letter in part of the tire size designation. If the tire says 185/60R14 85H or
185/60HR14 on the sidewall, the R indicates it is a radial tire, and the H means it is speed rated up to 130mph. Common speed ratings are:
Q=99 MPH, 160km/h U=124 MPH, 200km/h W=168 MPH, 270km/h
S=112 MPH, 180km/h H=130 MPH, 210km/h Y=186 MPH, 300km/h
T=118 MPH, 190km/h V=149 MPH, 240km/h Z=149+ MPH, 240+ km/h
Treadwear, Traction, Temperature, and Load: The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade
(UTQG) labeling system to establish ratings for treadwear, traction, and temperature resistance. These tests are conducted independently by each manufacturer
following government guidelines to assign values that represent a comparison between the tested tire and a control tire. This is designed to tell the consumer how long a
tire will last, the distance needed to stop, and how quickly the tire heats up. Since each tire manufacturer interprets their tests in their own manner, this makes
comparing two tires from different manufacturers virtually impossible, so take these ratings as only a very rough guide. While traction and temperature resistance
ratings represent specific performance levels, the treadwear ratings are assigned by the manufacturers following field testing and are most accurate when comparing
tires of the same brand.
Treadwear: Treadwear receives a comparative rating based on wear rate of the the tire in field testing following a government specified course. For example, a tire grade
of 150 wears 1.5 times longer than a tire graded 100. The tested tires are only worn down part way, so this rating can't really say anything about total tire tread life.
Actual performance of the tire can vary significantly depending on conditions, driving habits, care, road characteristics, and climate, so take these ratings with a grain of
salt. Ratings are valid for comparison of tires made by the same manufacturer, but not for tires made by different manufacturers. DOT-approved auto-x competition
tires typically have treadwear ratings from 0 to 60. Ultra sticky high performance street tires have ratings in the 120 to 180 range. Touring tires can have treadwear
ratings above 400.
Traction: Straight-line wet braking traction is represented by a grade of A, B, or C, with A being the highest rating. In 1997 a new top rating of "AA" was introduced to
indicate even greater wet braking traction. However, due to its newness, this grade will probably be applied initially to new tire lines as they are introduced and later to
existing lines which excel in wet braking, but had been limited to the previous top grade of "A". Traction grades do not indicate wet cornering ability.
Temperature: Temperature resistance is graded A, B or C, with A being the highest rating. It represents the tire's resistance to the heat generated by running at high
speed. Grade C is the minimum level of performance for all passenger car tires as set under Federal Motor Vehicle Safety Standards. This grade is established for a tire
that is properly inflated and not overloaded. Some competition auto-x tires are designed to have a B or C rating so that they heat up enough to reach full operating
temperature early on in an auto-x course.
Load Ratings: Many tires come with a service description added on the end of the tire size. These service descriptions contain a number, which is the Load Index, and a
letter which indicates the speed rating. The load index represents the maximum load each tire is designed to support. Because the maximum tire load capacity is branded
on the tire's sidewall, the Load Index is used as a quick reference. Multiply the tire load rating by 4 to get the maximum carrying capacity of the tires. Never use a load
rating below that specified by the car's manufacturer. See the tires' Load Chart for specifics.
Date Code: Every tire has string of numbers and letters molded into the sidewall that tell the tire serial number, DOT compliance code, and when the tire was made. This
information is usually found near the rim, and often has a flat-head screw imprint both before and after it, as the information is stamped into a plate that is screwed into
the tire mold. Look at the last three numbers in the code to get the week of manufacture and the year. If the last three numbers were 127, this would mean the tire was
made the in the 12th week of 1997 (or 1987, for that matter). Why would you want to know this? Well, if you're buying a car and the owner says they just put on new
tires, but they don't look that new, check the date code. Be aware that you can buy new tires that may have been sitting in a warehouse for a few years, but the date
code is usually less than 2 years old for new tires.
Competition Tires: For those interested in a street-legal tire that gives near-racing slick grip, consider a set of DOT legal competition tires. These are not for winter use,
may not work well in the rain, and often wear out in 5,000 - 10,000 miles (or less), but their performance capabilities will make you smile. Treadwear ratings (for what
they're worth) will range from 0 to 60. Choices include both road-racing and auto-x tires, with the auto-x tires often having a softer compound and a carcass that heats
up faster than a road racing tire. Popular choices among autocrossers and road-racers include the BFG R1 (and upcoming BFG G-force), Yokohama A008RSII,
Yokohama AS032, Toyo Proxes, Kumho V-700, and Hoosier AS302/RS302. To makes these tires last longer, it is important to heat them up to full operating
temperature the FIRST time they are used, and then let them cool and rest for at least 24 hours to "cure" the rubber compound. Tires treated this way can last twice as
long as non-heat cured tires. The Yokohama A008RS-II tires (but not the A008RS) can also be flipped on the rim when the outside tread starts to wear as a way to
increase the useful life of the tire. Looks funny, but works.
Fenders -- To Flare or Not to Flare:
Stock-bodied El Caminos will accept a huge variety of wheels, but sometimes a little extra clearance is needed, especially if your Elky suspension has been severely
lowered. The most common interference is the rear wheel well inner fender lips. An easy solution is to "roll" these fender lips upward to give an extra 1/2" of clearance.
This is easily accomplished with a baseball bat and a friend. One person wedges the baseball bat between the rear tire and the fender lip at either the front or rear of the
wheel well while the other person SLOWLY drives the car a few feet forward or backward, causing the baseball bat to be pulled upward between the wheel opening and
the tire. By varying both the angle of the bat and the thickness of the bat, you can vary the amount of flattening. It's best to take it slow, carefully bending the inner
fender lip up so that after 5 or 10 passes, the inner lip is bent up against the inside of the rear fender. Repeat the process on the other side. Done properly, you should
have no change in the outward appearance of your fenders.
One last note about your tires! Tires are manufactured by bonding layers to make the final product. All bonding agents have different properties and over
time can brake down. Old Tires are dangerous tires! Even if the tread is in good shape. UV is another factor with old tires, UV exposure, especially for the
sidewalls, that is significant enough can cause the compounds to brake down and dry out leaving very tiny cracks in the walls. These conditions have been
proven to cause tread separation and sidewall shredding! Inspect your tires very well often and once they are over 5 years old regardless of the condition of
the tread replace them if your going to drive it!
Transmission and Drive Line Info
Before deciding on a differential gear ratio, let's look at a few facts. The GM 700R4 four-speed's final drive ratio (forth gear) is 0.70:1 (20 percent) of that on a 400 or
350 Thd. Installing 4.10:1 gears would provide the same effective final drive ratio as a 2.87:1 ring-and-pinion. This is much better than the 3.11:1 and 3.08:1 final ratios
without an overdrive. The stock GM gearing is suitable for street use given EPA constraints and Gas prices at the time of manufacturing.
If we calculate the rpm at freeway speeds with 4.10s, we have to consider speed, tire size, transmission final drive ratio, and ring-and-pinion ratio. For our calculations,
we can use the following formula: mph x ring-and-pinion ratio x transmission - final drive ratio x 336/tire diameter = rpm. For example, 60 (mph) x 4.10:1 (differential
gear ratio) x 0.70 (overdrive ratio) x 336 (constant)/26 (tire diameter) = 2,225.4 (rpm). Applying the same formula for 4.56:1 gears gives us 2,475 rpm at 60 mph.
Applying the same formula for a Thd 350 or 400 with 4.10:1 gears gives us 3,179 rpm at 60 mph. The 700R4 has a first gear ratio of 3.06:1 and about 40% reduction to
a 1.63 Second and third is 1:1. With proper calibration the 700R4 can be a good overall transmission. It provides a good economy on the highway and good out of the
hole times for a fun street machine without being particularly hard on the wallet.
