Formula Used
Overall ratio
Overall Ratio = Transmission Gear Ratio × Rear End Ratio × Transfer Case Ratio
Engine RPM from road speed
RPM = MPH × Overall Ratio × 336 × Slip Factor ÷ Tire Diameter
Road speed from engine RPM
MPH = RPM × Tire Diameter ÷ (Overall Ratio × 336 × Slip Factor)
Required rear gear ratio
Rear Ratio = Target RPM × Tire Diameter ÷ (Target MPH × 336 × Transmission Ratio × Transfer Ratio × Slip Factor)
Metric tire diameter
Diameter = Rim Size + 2 × ((Width × Aspect Ratio ÷ 100) ÷ 25.4)
Ring and pinion ratio
Gear Ratio = Ring Gear Teeth ÷ Pinion Teeth
How to Use This Calculator
- Choose direct tire diameter or enter metric tire size.
- Enter the transmission gear ratio for the selected gear.
- Enter the rear end ratio used in the axle.
- Add converter slip if the setup has automatic transmission loss.
- Enter speed and RPM values for the main calculation.
- Use target RPM and target speed to estimate a better rear gear.
- Review the comparison table before choosing a final gear set.
- Download the CSV or PDF file for records.
Example Data Table
| Vehicle |
Tire |
Transmission |
Rear Gear |
Speed |
Approx RPM |
| Street cruiser |
28 in |
1.00 |
3.08 |
70 mph |
2587 |
| Balanced build |
27 in |
1.00 |
3.73 |
60 mph |
2785 |
| Drag setup |
29 in |
1.00 |
4.56 |
60 mph |
3170 |
| Overdrive road car |
28 in |
0.70 |
4.10 |
70 mph |
2411 |
Transmission Rear End Gear Ratio Guide
Why Gear Ratio Matters
A transmission rear end gear ratio affects speed, engine rpm, launch feel, cruising comfort, and fuel use.
The rear axle multiplies torque before it reaches the tires. A higher numerical ratio gives stronger
acceleration. It also raises engine speed at highway pace. A lower numerical ratio reduces rpm.
It can improve quiet cruising. It may also make the vehicle feel slower from a stop.
Physics Behind the Calculation
The calculator links tire circumference, engine rotation, transmission gearing, and axle reduction.
Tire diameter controls how far the vehicle moves per wheel revolution. Gear ratios control how many
engine turns are needed for one tire turn. The constant 336 converts miles per hour, inches, and minutes
into a practical automotive formula. Slip is added when a converter or clutch does not transfer rotation perfectly.
Choosing a Better Gear
First, decide the purpose of the vehicle. A drag car often needs strong launch multiplication.
A highway car often needs lower cruise rpm. A towing vehicle needs balance. Tire size changes the final
result, so always include the real tire diameter. Tall tires reduce rpm. Short tires raise rpm.
Overdrive gears can allow steeper rear gears without excessive highway speed.
Reading the Output
The calculated rpm shows engine speed at the entered road speed. The calculated speed shows expected
road speed at the entered rpm. The required rear ratio estimates the axle gear needed for your target.
The comparison table helps compare common gear sets. The wheel torque estimate shows multiplication
after drivetrain loss. Use it as a planning value, not a chassis dyno result.
Practical Advice
Check real tire diameter under load when accuracy matters. Race tires can grow at speed.
Automatic transmissions may have converter slip. Manual transmissions may still have clutch loss during launch.
Final gear choice should consider traction, engine power band, fuel range, noise, and intended road use.
FAQs
1. What is a rear end gear ratio?
It is the axle reduction between the driveshaft and wheels. A 3.73 ratio means the driveshaft turns about 3.73 times for one wheel turn.
2. Does a higher gear ratio improve acceleration?
Usually yes. A higher numerical ratio multiplies torque more strongly. It helps launch and low speed pull, but it also raises cruising rpm.
3. Why does tire diameter matter?
Tire diameter changes distance traveled per wheel revolution. Taller tires lower rpm at the same speed. Shorter tires raise rpm and feel like steeper gearing.
4. What does converter slip mean?
Converter slip is rotational loss in an automatic transmission. More slip raises engine rpm for the same road speed. Locked converters usually have very low slip.
5. Can I use this for overdrive gears?
Yes. Enter the overdrive ratio as the transmission gear ratio. For example, use 0.70 for a common overdrive gear calculation.
6. How is ring and pinion ratio calculated?
Divide ring gear teeth by pinion teeth. For example, 41 ring teeth and 11 pinion teeth create a 3.727 ratio.
7. Is wheel torque exact?
No. It is an estimate based on engine torque, gearing, and efficiency. Real wheel torque changes with engine curve, traction, losses, and converter behavior.
8. Which ratio is best for highway use?
The best ratio depends on tire size, overdrive, target rpm, and engine power band. Lower numerical ratios often reduce highway rpm and noise.