Starting Line Ratio Calculator
Calculation Result
Starting Line Ratio (SLR):
What is a Starting Line Ratio?
In drag racing and performance tuning, the Starting Line Ratio (SLR) is the total mechanical advantage your vehicle has the moment it leaves the line. It is the product of your transmission's first gear ratio and your rear axle gear ratio. This number determines how much torque is multiplied from the engine to the wheels during the launch.
The Importance of the Starting Line Ratio
Finding the perfect SLR is a balancing act between traction and acceleration. If the ratio is too high, you may experience excessive wheel spin or even "blow the tires off" the car. If the ratio is too low, the engine might "bog" or fail to get into its power band quickly enough, resulting in a poor 60-foot time.
Starting Line Ratio Formulas
The math behind the SLR is straightforward:
SLR = First Gear Ratio × Rear Axle Ratio
Example Calculation
Let's look at two common setups to see how the SLR changes:
- Setup A: A GM TH400 transmission (2.48 first gear) with a 3.73 rear gear.
Calculation: 2.48 × 3.73 = 9.25 SLR - Setup B: A Powerglide transmission (1.76 first gear) with a 4.56 rear gear.
Calculation: 1.76 × 4.56 = 8.02 SLR
Typical Target Ranges
While every car is different based on weight, tire size, and horsepower, here are general guidelines used by racers:
- Heavy Cars / Small Engines: Usually require a higher SLR (10.0 to 12.0) to get the mass moving.
- Lightweight Cars / Big Engines: Can run a lower SLR (8.0 to 9.5) because they have enough torque to overcome inertia without massive gear reduction.
- Pro Street / General Performance: Often targets a range between 9.0 and 11.0 for a balance of streetability and track performance.
Note: Tire diameter also plays a significant role in "Effective Ratio." A larger diameter tire will effectively lower your ratio, while a smaller tire will increase it. This calculator focuses on the mechanical gear ratios of the drivetrain components.