Quarter Mile Gear Ratio Calculator
Find your optimal final drive ratio using real physics and empirical data
Vehicle Setup
Engine Characteristics
Drivetrain
Tire Setup
Track Conditions
Performance Prediction
Estimates based on Fox (1973) empirical formulas. Actual results depend on traction, launch, and driver skill.
Optimal Final Drive Ratio
Finish line RPM: —
This is —% of peak HP
RPM Through the Quarter Mile
Shows engine RPM vs speed with shift points marked
Common Ratio Options
Optimal Shift Points
How Quarter Mile Gear Calculations Work
RPM at Any Speed
This fundamental formula determines your engine speed at any given MPH. The constant 336 converts between tire rotations and vehicle speed.
Ideal Final Drive Ratio
Calculate the mathematically optimal gear ratio to hit your target RPM at the finish line.
Real-World Example
A 450hp small-block Camaro running 113 MPH with 28″ tires and peak power at 6200 RPM:
- Target finish RPM: 6450 (104% of peak)
- Ideal ratio calculation: (6450 × 28) ÷ (113 × 336 × 1.0) = 4.76
- Closest common ratio: 4.56:1
- Actual finish RPM with 4.56: 6173 RPM
Drag Tire Growth Reference Chart
| Tire Type | Growth at 120 MPH | Growth at 150 MPH | Effective Ratio Change |
|---|---|---|---|
| Bias-Ply Slicks | 1.0-1.25″ | 1.25-1.5″ | ~3-5% taller |
| Radial Slicks | < 0.1″ | 0.1-0.15″ | < 0.5% change |
| DOT Drag Radials | ~0.25″ | 0.3-0.4″ | ~1% taller |
Pro Tip: Bias-ply tires can grow 3% or more in diameter at high speeds, effectively making your gearing taller. If running bias-plys, consider a slightly shorter gear to compensate.
Popular Final Drive Ratios by Application
Street/Strip (2.73-3.55)
- 2.73 – Highway cruising, overdrive trans
- 3.08 – Balanced street performance
- 3.23 – Common GM 10-bolt ratio
- 3.42 – Popular Ford 8.8″ option
- 3.55 – Good all-around performance
Performance (3.73-4.30)
- 3.73 – Most common performance ratio
- 3.90 – Strong acceleration, decent highway
- 4.10 – Classic muscle car ratio
- 4.11 – Ford 9″ variant
- 4.30 – Aggressive street/strip
Race Only (4.56+)
- 4.56 – Entry-level race ratio
- 4.88 – Popular bracket racing
- 5.13 – Serious drag racing
- 5.38 – Small tire/light cars
- 5.57+ – Specialized applications
Finish Line RPM Strategy by Power Curve
Peaky Power (Small Cam, Turbo)
Stay at or just below peak power. These engines fall off sharply after peak, so conservative gearing prevents over-revving. Typical of stock-ish engines, small turbos, and mild builds.
Moderate Power (Typical Performance)
Run slightly past peak for best average power throughout the run. Most aftermarket cams and bolt-on combinations fall here.
Broad Power (Big Cam, Large Displacement)
These engines maintain power well past peak. Aggressive cams, large cubes, and centrifugal superchargers benefit from deeper gearing.
Quarter Mile ET Prediction Using Fox Formula
The calculator uses the empirical Fox formula (1973), which has proven remarkably accurate for naturally aspirated combinations:
These formulas assume good traction and driver skill. Real-world results vary based on:
- 60-foot times (launch efficiency)
- Shift points and technique
- Track conditions and prep
- Weather (density altitude effects)
Transmission Gear Ratios Quick Reference
| Transmission | 1st | 2nd | 3rd | 4th | 5th | 6th |
|---|---|---|---|---|---|---|
| Powerglide | 1.76 | 1.00 | – | – | – | – |
| TH400 | 2.48 | 1.48 | 1.00 | – | – | – |
| 4L80E | 2.48 | 1.48 | 1.00 | 0.75 | – | – |
| Muncie M22 | 2.20 | 1.64 | 1.28 | 1.00 | – | – |
| T56 6-Speed | 2.97 | 2.07 | 1.43 | 1.00 | 0.80 | 0.62 |
Close-ratio transmissions with 6+ speeds can be worth a couple tenths in the quarter-mile over a typical three-speed automatic by keeping the engine in its powerband.
Gear Ratio Troubleshooting
Hitting Rev Limiter Early
Problem: Engine reaches redline before the finish line
Solution: Your gear is too short (numerically high). Need a taller gear or smaller tire.
Example: Going from 4.88 to 4.56 gives ~7% more top end
Low Trap Speed
Problem: MPH lower than expected for your power
Solution: If your engine RPM is well below peak power at the finish line, your ratio is too tall. Consider a shorter gear.
Example: Finishing at 85% of peak HP costs significant trap speed
Poor 60-Foot Times
Problem: Slow launches despite good power
Solution: May need more gear multiplication. However, Using excessively short gearing to chase high trap speeds often results in poor launches and slower overall ETs.
Weather Correction Factors
Density altitude combines temperature, humidity, and elevation to show how “thick” the air is. Higher DA means less oxygen for combustion:
Sea Level, 60°F
Denver, 90°F Summer
Typical Hot Day
Rule of thumb: Air density significantly affects engine power. Hot, humid, high-altitude conditions can reduce power by 10-15%. Expect to lose roughly 3% power per 1000 feet of density altitude.
Case Studies: Optimal Gearing in Action
11-Second Street/Strip Chevelle
Setup: 468 BBC, TH400, 3600 lbs, 28″ tires
Performance: 11.20 @ 120 MPH
Gearing: 4.10:1 final drive
Switching to 30″ tires dropped effective ratio to 3.83:1. ET stayed nearly the same but gained consistency. The taller tire improved 60-foot times despite “less gear.”
9-Second Turbo LS
Setup: Twin 67mm turbos, 4L80E, 3200 lbs, 28″ radials
Performance: 9.40 @ 145 MPH
Gearing: 3.73:1 final drive
Turbo cars don’t use 4.30 gears is a myth. This combo could use more gear but traction limitations prevent it. The 0.75:1 overdrive helps on the big end.
Pro Tuning Secrets
Converter Slip Compensation
High-stall converters can slip 8-12% at high speed. Factor this into your calculations or you’ll be under-geared. A 4500 RPM converter might flash to 5000+ but still slip 6% at the top end.
The 1-2 Shift
On a Powerglide, the massive 1.76:1 to 1.00:1 drop means careful gear selection. Too much final drive and you’ll bog on the shift. Many run 4.30-4.56 gears to keep the engine singing.
Track-Specific Gearing
If there are no standing starts or pit stops, you can be less concerned with a low first gear. Some racers run different gears at different tracks based on shutdown room and surface quality.
Weight Transfer Effects
Heavier cars with good weight transfer can often run more gear than the math suggests. The load on the tires helps them hook despite aggressive ratios.