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Free Drivetrain Planning Tool
Gearbox Ratio Calculator
Plan your transmission setup with precision. Calculate gear speeds, cruising RPM and driveline characteristics for your engine swap or performance build — before you buy a single part.
20+
Preset Gearboxes
10
Gear Ratios Supported
Instant
Live Calculations
Free
No Account Needed
Match Your Power
See exactly how each gear pulls from idle to redline with your specific tyre size and diff ratio. Understand where your engine sits in the powerband at any given speed.
Cruise Comfortably
Enter a target speed and instantly see your cruising RPM in top gear. Critical for UK motorway builds where you need low RPM at 70 mph for a quiet, efficient drive.
Compare Gearboxes
Switch between Tremec T-56 close and wide ratios, GM 8L90-E autos, 4L65-E and more. See how different ratio spreads affect your real-world performance.
Why Gear Ratios Matter
Your gearbox ratios, combined with the final drive and tyre diameter, determine the relationship between engine RPM and road speed in every gear. Getting this right is essential for both performance and driveability — whether you're building a track car that needs tight ratio spacing through the gears, or a road-going restomod that needs to cruise quietly at motorway speeds.
This calculator lets you model the entire driveline before committing to a purchase. Swap between presets for popular transmissions from Tremec and GM — or enter fully custom ratios for any gearbox. Combined with your tyre size and diff ratio, you'll see the actual speed at redline in each gear, and the RPM at any target cruising speed.
Typical Use Cases
- LS and LT engine swaps into classic chassis — matching a Tremec T-56 or 6L80-E to your diff ratio
- Track builds — ensuring tight ratio spacing keeps you in the powerband through every corner
- Road cars and restomods — confirming low RPM at 70 mph for a comfortable motorway cruise
- Comparing close-ratio versus wide-ratio sixth gear options before purchase
How to use the calculator:
01
Enter Tyre Size
Wheel diameter, width and profile — rolling circumference is calculated automatically.
02
Set Diff Ratio
Enter the final drive ratio from your rear axle differential.
03
Pick a Gearbox
Choose from popular presets or enter custom gear ratios manually.
04
See Results
Instant speed-per-gear at redline, cruising RPM, and a visual chart of your driveline.
Calculate Your Setup
Enter your vehicle details below. All fields update in real time.
Vehicle Setup
Enter your tyre size (e.g., 245/45R17)
Rear axle differential ratio
Maximum safe engine RPM
Select a preset or enter custom ratios below
Manual Gear Ratio Override
Enter custom ratios to override selected gearbox
Calculate RPM at this speed in every gear
Speed & RPM Results
All calculations use the tyre circumference and final drive ratio provided above.
Speed vs RPM by Gear
Visualise how each gear pulls from low RPM to redline with your current setup.
Ready to Buy Your Gearbox?
Browse our complete range of manual and automatic transmissions, bellhousings, clutches, converters and more — all in stock and ready to ship across the UK.