Got that? OK. Things get a bit more complex.
Let's compare three different cars, using the
previously mentioned LS6 El Camino as an example.
The first is an automatic transmission Stocker,
the second is a stick shift Stocker and the
third is an automaticequipped Super Stocker.
The Stockers are set up to use a 30inch tall
slick while the Super Stock car includes a 14
X 33inch slick. We'll take a preliminary guess
(guess definitely is the operative word) at
picking a set of gears for the Super Stock and
both Stockers. The following are the basic tire
and gear ratio specifications:

Stock
Automatic 
Stock
Stick 
Super
Stock 
Axle
Ratio 
4.56 
4.88 
5.43 
Tire
Diameter 
30.00" 
30.00" 
33.00" 
Next, we'll compute the vehicle speeds at a
specific rpm number. The formulas are as follows:
Loaded
Radius Of Tire: 
9X30"
slick 
14X33"
slick 
Loaded radius
= Tire
Diameter ÷
2 
30.00
÷ 2 
33.33
÷ 2 

=
15 
=
16.5 
Next, we have to determine the tire revolutions
per mile for each car. The following formula
is used for the calculations:
Tire
Revolutions Per Mile: 
9X30"
slick 
14X33"
slick 
Tire Revolutions
=
10084 ÷
loaded radius of tire 
10084÷
15 
10084÷
16.5 

=
672.2666 
=
611.1515 
The above figures are obviously meaningless,
unless they are plugged into the next formula
 maximum speed. For this formula, we have
to use the previously mentioned peak horsepower
rpm numbers I arrived at earlier. There are
two schools of thought when it comes to gearing
contemporary drag racecars. One is to set the
car up so that it crosses the finish stripe
at the rpm at which peak power is made while
the other conviction is to have the engine turning
approximately 10 percent over the power peak
at the finish line. We'll look at the math for
the first concept (cross the finish line at
peak power rpm) first:
Maximum
Speed: 
Maximum
Speed =
(rpm X
60) ÷
(Gear Ratio
X Tire
Revolutions Per Mile) 
9X30"
slick (auto) 
9X30"
slick (stick) 
14X33"
slick 
(6,250
X 60) ÷
(4.56 X 672.266) 
(6,250
X 60) ÷
(4.88 X 672.266) 
(6.700
X 60) ÷
(5.43 X 611.151) 
=
(375000) ÷ (3065.5356) 
=
(375000) ÷ (3280.661) 
=
(402000) ÷ (3318.5526) 
=
122.32 mph 
=
114.30 mph 
=
121.13 mph 
If the respective combinations were set up
to run for the 10 percent over peak rpm theory,
then the results would be as follows (we won't
redo the math  I've simply increased the
rpm by 10 percent in each case):
Maximum
Speed (10% over Maximum rpm): 
9X30"
slick (auto) 
9X30"
slick (stick) 
14X33"
slick 
=
134.56 mph 
=
134.56 mph 
=
134.56 mph 
There's quite a difference in speeds between
the three car combinations at a given rpm level.
The next thing to do is to consider one of the
very first notions pointed out in this article.
Automatics tend to slip as much as seven percent
in a Stock/Super Stock application while tires
tend to grow three percent for every 100 mph.
Let's assume there is no clutch slippage for
the stick shift car (although there could be
some). This means the automaticequipped stocker
could lose as much as 9.4 mph while the Super
Stocker could lose 9.3 mph. Meanwhile, each
of the combinations (Stock and Super Stock)
could gain roughly 3.754 mph with tire growth,
given the three percent per 100 mph concept
(keep in mind that we're not taking radial slicks
into account for this exercise). In the end,
the final speed for the three combinations works
out as follows:
Maximum
Speed (Revised): 
9X30"
slick (auto) 
9X30"
slick (stick) 
14X33"
slick 
=
128.89 mph 
=
129.48 mph 
=
127.67 mph 
When I'm crunching these numbers, the next
thing I do is cross check the final (ultimate)
speeds with NHRA national records to see if
I'm on the right track. For example, both of
the Stock Eliminator El Caminos fit into "A"
(the last time I checked, the Classification
Guide factored the Stock combination as 460
HP with a Factor of 8.07) while the Super Stocker
fit into SS/EA (465 HP with a Factor of 7.98).
At the time of this writing, the A/SA record
is 131.81 mph. The A/S record is 135.72. The
SS/EA record is 140.40 mph. As you can see,
we're reasonably close with the gear ratio selection
for the pair of Stockers, but light years away
with the SS combination. What happened? I guessed
wrong. I did the math again, this time with
a 5.00:1 final gear for the SS combination.
This time, the car "ran" 144.71 mph
using the 10 percent over maximum engine speed
rule. Taking converter slippage into consideration,
then adding speed for tire growth, the final
speed across the finish line worked out to 138.33
mph. As you can see, this is very close to the
speed record. It's probably the right gear for
the car (although in certain cases, a 4.86:1
or a 5.14 might work too). Just remember that
aerodynamics comes into play with final speeds.
The El Camino is basically a barn door, and
it might not quite run as fast as something
like a Corvette.
When all is said and done, the idea is to select
the axle ratio for your car only after you've
considered all of the variables  and that
includes the tire diameter, converter slippage,
tire growth and of course, engine power peak
characteristics. With the right axle ratio combo
(one that's matched to the torque characteristics
of your engine), your car will flat fly. If
you don't have the right combination, it won't
fall out of a tree. Use your calculator. It
works.
