09/10/2026
This is the legendary Twin-Stick - Spicer 5x4 Duplex / SST 20 Speed.
In the 60s-80s it was king in Peterbilt 359, Kenworth W900, etc. It's not one gearbox, it's TWO boxes in series:
- Main box: 5 speeds [short stick or right stick] - the big gaps
- Auxiliary / Brownie box: 4 speeds [long stick or left stick] - 1=Deep Under, 2=Under, 3=Direct, 4=Overdrive
5 x 4 = 20 forward combinations. With reverse on the main you also get 4 reverse speeds. The plate you show is the progressive shift sequence to avoid duplicate ratios. In reality you never use all 20, you use about 12-13 progressive ratios.
▪︎2. How to change it
It's non-synchronized, so you must double-clutch everything and match RPMs. Rule from the old drivers:
> Upshift the box you want to go UP first, downshift the box you want to go DOWN first. Never get both boxes in neutral at the same time rolling heavy.
▪︎Basic progressive upshift - loaded:
•1. Start: Main 1 + Aux 1 - clutch, engage, let out easy
•2. Aux 1 -> 2 -> 3 -> 4 [just lift, clutch, shift aux, rev slightly] - this is `1-1, 1-2, 1-3, 1-4`
•3. Now split: Main to N, Aux to N almost together, Main into 2, raise revs ∼300 rpm, then Aux into 2. Now you're in 2-2
•4. Aux 2 -> 3 -> 4 [2-3, 2-4]
•5. Split again: Main 3 + Aux 2 -> Aux 3 -> 4 [3-2, 3-3, 3-4]
•6. Split: Main 4 + Aux 2 -> 3 -> 4 -> Direct [4-2, 4-3, 4-4] - 4-4 is usually your 1:1
•7. Split: Main 5 + Aux 3 -> 4 [5-3, 5-4] - 5-4 is double overdrive, your highway gear.
Bobtail / empty you can skip: 1-2, 1-4, 2-2, 2-4, 3-4, 4-4, 5-4 is enough.
Downshift is reverse: Drop the Aux first. Clutch -> Aux N -> bump throttle up ∼400 rpm -> Aux into lower gear. If you need to drop the Main, you need to go up in RPMs first.
▪︎Tips the driver in your video is doing:
- He never floors it, he feathers to ∼1600 rpm to sync. If you slam to governor you will grind.
- On a hill, don't try to shift Aux 1->2 with Main in 1, you'll stop. Put Aux in 1 and shift the Main 1->2->3 instead, then get Aux out of Deep Under when you have momentum.
▪︎3. Speed and RPMs per gear:
RPM depends on engine - this era was mostly Detroit 8V71 / 8V92 or Cummins NTC 350. All were governed around 2100 RPM.
▪︎Target:
- Upshift at: 1800-2000 RPM
- Drop to after shift: 1250-1350 RPM
- Never lug under 1100 RPM heavy, never over-rev past governor
▪︎Speed depends totally on rear axle ratio and tire size, but with a common 4.11 rears and 11R22.5, progressive speed looks like this:
Combo Usable name Approx MPH at 1800 rpm When to use
1-1 Deep Deep 2-3 mph Off-road / steepest hill start only
1-2 1st 4-5 mph Heavy start
1-4 2nd 8-9 mph
2-2 3rd 12 mph
2-4 4th 16 mph
3-2 5th 20 mph
3-4 6th 26 mph
4-2 7th 32 mph City
4-3 8th 38 mph
4-4 9th Direct 45 mph 1:1 workhorse
5-3 10th 55 mph
5-4 11th Overdrive 65-75 mph Highway
You don't need to be precise on speed - you drive by sound. If the engine drops below ∼1200 and starts shaking, you are in too high a gear for that combination. If it's screaming at 2000+ and you're not accelerating, shift up on the Aux.
▪︎How to use it on the road:
▪︎To drive it smooth:
•1. M = Main (right stick, 5-speed), A = Aux (left stick, 4-speed)
•2. Empty/bobtail just run this: `M1+A2 > M1+A4 > M2+A4 > M3+A4 > M4+A4 > M5+A4`
•3. Heavy: run the full 12-step progressive list on the card.
•4. Split shift = two sticks moving at almost same time. Clutch -> both to N for a split second -> Main to next, rev +300rpm -> Aux to 2.
Keep that RPM rule on the bottom — if you try to force it without the rev match, it will grind and pop out. Old Spicers like a light touch.
KM/H card for Cummins NTC 400 + 3.70 rears + 11R22.5 — same gearbox, but now calculated for real road speeds.
▪︎What this means for you in KM/H:
For a Cummins 400 you live between 1300-1900 rpm. Upshift at 1900, it drops to ∼1300 in the next gear — that's the sweet spot.
- City / loaded:
M1+A2 = 5-8 km/h start
M3+A4 = 38-56 km/h
M4+A4 = 66 km/h @1300 / 97 km/h @1900 — this is your main 1:1 direct, use it for hills heavy
- Highway cruise with 3.70:
M5+A4 (double over) = 90 km/h @ 1350 rpm / 100 km/h @ 1500 rpm / 110 km/h @ 1650 rpm — perfect for that Cummins, not screaming
> With a 3.70 you are faster than the old 4.11 trucks. Your 5th + 4th will do 150+ km/h at governor, but you never need it. Keep it at 100 km/h and 1500 rpm — fuel and bearings will thank you.
▪︎Quick reminder on your video: That driver is doing it right — aux first. When he moves his left hand (aux) then right hand (main), that's a SPLIT shift marked red on the card. When only one hand moves, that's just aux 2->3->4.