The BMW Getrag 420G 6-Speed (E39 M5 / E46 M3) is a six-speed manual gearbox.
First gear is 4.23:1 and 6th is 0.83:1. Divide one by the other and the overall spread is 5.10:1. That is a wide spread: a short first to get the car moving and a tall top to bring cruise revs down.
6th is an overdrive at 0.83:1: the output shaft turns faster than the input shaft. 5th is direct drive at 1.00:1.
The biggest step is 1st to 2nd, where the ratio drops 40%. The smallest is 5th to 6th at 17%. On each upshift the revs drop by the same percentage as the ratio.
The final drive lives in a separate diff, so the speed table shows three common ratios: 3.42, 3.73 and 4.10. With a 3.73 final drive and 225/45R17 tyres, 6th at 6,000 rpm works out at 232 km/h (144 mph), and first runs out at 45 km/h (28 mph). At 70 mph in 6th the engine turns about 2,920 rpm.
Ratios
BMW Getrag 420G 6-Speed (E39 M5 / E46 M3): ratio per gear, and how far the ratio drops from the gear before.
Gear
Ratio
Step
1st
4.23:1
-
2nd
2.53:1
40.2%
3rd
1.67:1
34.0%
4th
1.23:1
26.3%
5th
1.00:1
18.7%
6th
0.83:1
17.0%
Road speed at 6,000 rpm
Road speed at 6,000 rpm on a 225/45R17 tyre (1.993 m rolling circumference), by 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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