The Porsche 915/62 5-Speed Transaxle (911 SC 1980–1983) is a five-speed manual transaxle.
First gear is 3.182:1 and 5th is 0.786:1. Divide one by the other and the overall spread is 4.05:1. That is a middling spread for a five-speed: enough first gear to leave a junction without drama, and a top gear that is not only for motorways.
5th is an overdrive at 0.786:1: the output shaft turns faster than the input shaft. 4th is direct drive at 1.00:1.
The biggest step is 1st to 2nd, where the ratio drops 42%. The smallest is 3rd to 4th at 21%. A drop that size lands the engine well down the rev range after the change, so it pays to know where the torque lives.
It is a transaxle, so the final drive sits in the same case: 3.875:1 as listed here. The speed table uses that figure and nothing else. With a 3.875 final drive and 225/45R17 tyres, 5th at 6,000 rpm works out at 236 km/h (146 mph), and first runs out at 58 km/h (36 mph). At 70 mph in 5th the engine turns about 2,870 rpm.
Ratios
Porsche 915/62 5-Speed Transaxle (911 SC 1980–1983): ratio per gear, and how far the ratio drops from the gear before.
Gear
Ratio
Step
1st
3.182:1
-
2nd
1.833:1
42.4%
3rd
1.261:1
31.2%
4th
1.00:1
20.7%
5th
0.786:1
21.4%
Final drive
3.875:1
Road speed at 6,000 rpm
Road speed at 6,000 rpm on a 225/45R17 tyre (1.993 m rolling circumference), with the listed 3.875 final drive. No slip, no drivetrain losses.
Results are theoretical: stock ratio specs, your inputs, no drivetrain losses. Verify with your drivetrain supplier before spending real money.
Swapping the engine as well? StandaloneHQ counts the inputs and outputs the new engine needs and shortlists the wire-in ECUs that fit: ECU Pinout Planner.
BoxSelector crunches gear ratios, tyre sizes, and RPM figures to give you useful speed and cruise data for your build. The results are theoretical, based on stock ratio specs and your inputs, and assume perfect mechanical conditions, zero drivetrain losses, and a world without speed bumps, traction limits, or your right foot getting sweaty. Real-world results will vary. Probably a lot for automatics.
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If you fit a TH400 and wonder why there's no 5th gear, that's on you. BoxSelector and its author accept no responsibility for mechanical failures, unsuitable gearbox choices, blown differentials, bent input shafts, voided warranties, shed fires, or any other consequences arising from using this tool. You're a grown adult building a car. Act like it.
Gear ratios may have changed, and manufacturers love to quietly update things.
We've done our best to source accurate ratio data from factory specs and reputable technical references. Some ratios (particularly on production variants with running changes) may differ from the figures listed here. Always verify critical specs against your actual unit or a current manufacturer source before committing to anything expensive.
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