Posted by The Tire God on October 18, 1999 at 06:20:02:
In Reply to: Re: Superior tire assistance for everyone. posted by Ronny Jepsen on October 05, 1999 at 16:06:05:
The manufacturer tests your vehicle and has already determined the optimum balance of performance, ride comfort and economy. It is typically best not to alter the tire design to much without because of the adverse impact on suspension and electromechanical equipment such as odometers.
In your situation the 235/75 are 176.25 in profile and the 255/70 are 178.50. This is a 1.27% difference in profile and will not adversely affect your odometer equipment and will likley give you better traction but poorer fuel economy. If you haul things or go offroad then use LT tires.
All three of your tire choices are very good.
Since you appear very inquisitive it would be in your best interest to give this site a quick look.
Excerpted from:
http://www.michelin.com/us/eng/tire/guide/lesson4.html
Just about everything you need to know about a tire can be found on its
sidewall. If you check out the sides of any tire, you find an
alphanumeric code that may look like this: 205/55R16 88V. Each letter
and number conveys important information, like whether a particular tire
will be compatible with your vehicle.
Some size designations are preceded by a letter indicating what type of
vehicle the tire is intended for. "P," which stands for "passenger," is
the most common. "LT," which stands for "light truck," is also common.
The first number (205 in this example) is a three-digit number which
refers to the overall width of the tire, in millimeters.
The second number (55) refers to the aspect ratio, which is the
relationship between the tire's height and it's width. In this example,
the sidewall's height is about 55% of the tire's width.
The letter following the aspect ratio is usually an "R," standing for
"radial."
The next number indicates the diameter of the wheel rim on which the
tire will fit (16 inches, in this example).
The final number and letter represent the service description, or load
index and speed rating. The load index is an assigned number ranging
from 0 to 279 that corresponds with the load carrying capacity of the
tire; that is, how much weight it is certified to carry at maximum
inflation pressure. The rating can be matched against a load index chart
to determine corresponding maximum weights. A load index rating of 88
indicates a maximum load of 1,235 lbs.
The speed rating is a letter which indicates the range of speeds at
which a tire is certified to carry a load (see the speed symbols chart
to determine your tire's maximum speed). Each tire is assigned a rating
from A (lowest) to Z (highest). There is one notable exception: the "H"
rating falls out of sequence between "U" and "V," and is used for tires
certified for speeds up to 130 mph. The Q rating is the lowest commonly
used for passenger cars. A "V" -rated tire is certified up to 240 km/h
or 149 mph.
Just to recap. A tire marked 205/55R16 88V would have a width of 205
millimeters, an aspect ratio of 55, would be a radial tire sized for a
rim with a 16 inch diameter, feature a load index of 88 (capable of
carrying a maximum load of 1,235 pounds) and a speed rating of "V"
(indicating it is certified up to 149 miles per hour).
It's important to note that markings do not always appear exactly as
they do in this example, as tires can be certified under a variety of
designations.
Uniform Tire Quality Grading Standards
Also on the sidewall is The Uniform Tire Quality Grade (UTGQ), which
offers three more key pieces of information about a tire:
Treadwear:
Treadwear is a critical indicator of how much longer your tire will
last. In the life of every tire, tread eventually degrades to a degree
where the tire can no longer be used safely.
Each type of tire is tested under controlled conditions on a government
test course and assigned a treadwear grade which theoretically indicates
the useful life of the tread. It's important to remember that this is a
theoretical figure and cannot be linked to projected tire mileage, as
factors like road surface quality, driving habits, inflation, wheel
alignment and rotation come into play.
Treadwear grades typically range from 60 to 620 in 20 point increments.
The higher the grade, the longer the tread life as measured under
government conditions.
Traction:
Traction grades indicate a tire's braking performance. Traction is
tested in a straight ahead motion on wet pavement. A grade from "A" to
"C" is assigned, with "A" signifying the best traction.
Temperature:
Temperature grades represent a tire's ability to withstand heat under
test conditions. Since tires are made of rubber and other materials
which are degraded by high levels of heat, determining their ability to
withstand heat is very important. Temperature grades are also assigned
A-C with A signifying the most resistance to heat.
Other Markings
Maximum Load, Maximum inflation
For passenger tires, the maximum load and maximum inflation markings
indicate the maximum load that can be carried at the maximum pressure.
For light truck tires, it is stated in direct relationship to the
maximum laod capacity and inflation pressure.
DOT Markings
Essentially the DOT marking serves as the tire's fingerprint. DOT
signifies that the tire complies with U.S. Department of Transportation
Tire Safety Standards, and is permitted for highway use. For example
refer to the following markings: DOT M5H3 459X 064. The first letter and
number following DOT designate the tire's manufacturer and plant code.
The third, fourth and fifth number and letter, 59X, is the tire size
code which is an optional manufacturers code that identifies the tire
size and other specific characteristics of the tire. The final three
numbers denote when the tire was produced, with the first two indicating
the week, and the last number indicating the year (064 indicates the
tire was built in the 6th week of 1994).
Inflation Pressure
The level of air in your tires affects your vehicle's overall
performance. Not even the highest quality tire will perform well if it's
not inflated properly. The correct pressure varies from vehicle to
vehicle and depends in part upon driver preference. Each vehicle has a
recommended inflation pressure, usually found on a placard on the door
section, door post, glove door, or fuel door.