Use your tyre size and engine RPM to compare gearboxes and diff ratios. Check the mechanical top speeds, optimal cruise gear, and more across 35+ gearboxes for your swap, or enter your own ratios for a custom-built transmission.
Vehicle Parameters
2WD/4WD
km/h/mph
Transmission Slots
Set your parameters above and hit Calculate
Engine Family
Enter your engine above and select transmissions on the Input tab. Links below will update automatically and open Google search in a new tab.
Transfer Case Adaptor Search
Select transmissions and transfer cases on the Input tab (4WD mode). Links below search for adaptor plates between your chosen gearbox and transfer case and will open Google in a new tab.
THE MOST COMMON QUESTION
Can i buy you a drink?
Sure thing, good lookin'! But don't think I'm that easy... Seriously, thanks nonetheless, especially if you're buying me 10mm sockets. I'm a fucking idiot and will lose them immediately anyway.
Computed from your inputs to find the actual distance covered per wheel revolution:
Sidewalls = [Tyre Width × Aspect Ratio (%) × 2]
Rolling Circumference = [Sidewalls + Wheel Diameter] × π
02
Top Speed Per Gear
Calculating the speed at redline for each gear. Transaxle diffs are pre-filled but editable.
Top Speed = (Redline RPM ÷ Gear Ratio ÷ Final Drive) × Rolling Circumference × (60 ÷ conversion scalar)
03
Cruise RPM
Engine RPM in the highest gear at 50–150 km/h or 30-90 mph, benchmarked against your ideal cruise RPM target line on the chart.
04
Custom Built Trans
Select "Custom Built" in any slot, set your gear count (max 10 — and yes, we will judge you for 10), and dial in each ratio.
Common Questions
A transaxle is just the gearbox and diff living together in one unit - super common in rear-mid-engine cars, and even stuff like C5–C7 Corvettes trying Maserati/Ferrari tricks. Because the diff’s built in, you’re generally stuck with the factory ratio, so BoxSelector pre-fills it… unless you’ve swapped gears anyway, you rich bastard.
All three run the exact same ratios, so on paper they’re identical. The 700R4 is the old-school one, the 4L60E added electronics, and the 4L65E just beefed everything up internally. So they’ll all drive the same, you just get progressively more tolerance for bad decisions.
The 6L50E is the lightweight: slightly shorter gearing, happier in smaller cars that don’t need to bench press a house. The 6L80E and 6L90E? Basically the same ratios and same behaviour, just that the 6L90E’s been on a bulk and can handle more torque thanks to some extra clutch packs. So they’ll drive the same… right up until you start throwing real power at them.
The TKX/TKO short ratio (3.27) is the “send it” option: short first gear, big shove off the line, perfect for lumpy V8s that just want to light the tyres. The long ratio (2.87) is the chilled one: longer legs and a deeper 0.64 overdrive, so it cruises quietly instead of yelling at you on the motorway. Same box, same torque rating, just depends if you want chaos or calm.
The TR6070 is just a TR6060 that discovered cruise control. Same first six gears, then it adds a ridiculously tall 7th (0.57) so on the motorway the engine’s barely awake and you start wondering if it’s even still running.
The T56 and TR6060 are basically the same bloke, but the TR6060’s been hitting the gym and can handle more torque. The real difference? The T56 bolts the shifter straight on like it means it, while the TR6060 runs a remote shifter with linkages doing a full sightseeing tour underneath the car.
Same wide-ratio gearing, but the Magnum's been to the gym—bigger bearings, beefier shafts, and double the torque rating. Early T56 works fine for mild LS swaps; Magnum's for when you stop pretending it'll be gentle.
Same first three gears, but the 4L80E sneaks in overdrive for highway chill while the TH400 stays direct (no OD). TH400's lighter and drag-focused; 4L80E's the electronic truck brute that pretends to care about fuel.
T5's the lightweight Fox Body classic (300 lb-ft max), TKX/TKO doubles that to 600 lb-ft with better synchros and guts. If your Mustang's still on street tyres, T5's fine; anything serious, TKX laughs last.
TY856 (04-08) has taller 2nd/3rd for rally vibes; TY85 (09+) shortens them for street snap and bigger R180 diff. Both bombproof, but pick by your STI's mood—wild or refined.
915's the raw 70s dogleg-shifter (lighter car feel); G50's heavier with synchro gates and more modern bite. 915 for purists who like guiding shifts; G50 for when vague became uncool.
Same guts, but CSL's first gear is shorter (4.35 vs 4.23) for track snap. E39 M5/E46 M3 spread it out for road mercy; CSL says "lap times or GTFO."
Bulletproof 6-speed built for the S65's torque—triple-cone synchros and magnesium case. It's the E46 box's angry evolution that laughs at 500hp wheelspin.
8HP70 (early F) is punchier first gear; 8HP75 (G-chassis) goes taller overall for efficiency. Same family, but 75 pretends it's a daily driver instead of a sleeper.
Short (E-chassis) stacks gears tight for track aggression while long (F-chassis) stretches them for autobahn calm. Pick your poison - circuit or cruise control.
