How to Calculate Speed in Each Gear
Gear speed calculations look more intimidating than they are.
Read ArticleTHE 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.
How It Works
01
Rolling Circumference
Computed from your inputs to find the actual distance covered per wheel revolution:
- Sidewalls = [Tyre Width × Aspect Ratio (%) × 2] / 25.4
- 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 ÷ unit 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.
A 3.27 first gear is the “send it” option: short first, big shove off the line, made for lumpy V8s that just want to light the tyres. A 2.87 first is the calm one, with longer legs for the road. The older TKO-500 and TKO-600 each came with one overdrive (0.68 and 0.64). The newer TKX gives you a choice of overdrive, so each version has its own entry in the list. Pick the one on your invoice, not the one on the forum.
The TR6070 is a TR6060 that discovered cruise control. The first six gears follow the familiar TR6060 sets, then it adds a tall 7th: 0.42 to 0.48 depending on which Corvette it came from. On the motorway the engine is barely awake and you start wondering if it’s 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.
Not always the same gearing. The early LS T56 runs a 2.66 first. TREMEC sells the Magnum with a 2.66 or a 2.97 first and a choice of overdrives, so check which one you have. The Magnum has also been to the gym: bigger bearings, beefier shafts and a much higher torque rating. Early T56 works fine for mild LS swaps; the Magnum is 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.
Subaru's spec sheets go by model year, so the list does too. The 2004 to 2006 box runs a shorter 2nd, 3rd and 4th (2.375, 1.761, 1.346). From 2007 they got taller (2.235, 1.521, 1.137), and 2019 to 2021 cars got a revised 3rd (1.590). First and top gear are the same on all of them. Both are strong boxes. Pick by the year on your STI's paperwork.
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.
Yes. BMW's own M3 CSL press kit lists the standard M3 gear set, starting at 4.227. The CSL's gains came from weight and power, not gearing. The 4.35 first gear people remember belongs to the Z4 M Coupe, which has its own entry in the list.
It's the manual behind the S65 V8. BMW's M3 technical data gives 4.055 first, 1.000 fifth and a 0.872 sixth. Plenty of lists carry 0.83 for sixth, which is wrong, and it matters: the motorway cruise is where you notice it.
The later 8HP75 is the wider box: a shorter 5.000 first for launches and a taller 0.640 eighth for cruising. The early 8HP70 runs 4.714 to 0.667, so it is a touch tighter at both ends. Same family. The 75 just stretched in both directions.
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.
The final drive (diff ratio) multiplies every gear in the box. A 3.90 and a 4.30 with the same gearbox are two different cars: the 4.30 pulls harder in every gear and sits higher at cruise. Because it touches all the gears at once, it is the cheapest way to re-gear a car and the one people forget to check. The calculator lets you set the diff per gearbox slot so you can compare a 3.46 against a 3.90 on the same box before you buy either.
Crawl ratio is first gear multiplied by the transfer case low range multiplied by the diff ratio. A 4.0 first, 2.72 low range and 3.73 diff gives 40.6:1. Below about 20:1 is road manners; 40 to 60:1 is proper slow-speed control off road; past 60:1 you are moving at geological pace. Switch the calculator to 4WD, pick a transfer case, and the crawl ratio is worked out for you, with commentary.
If the gearbox was never bolted to your engine at the factory, yes: either a bellhousing made for that engine-and-box pairing, or an adapter plate that sits between the two, usually with a spacer, a matching flywheel and a clutch or converter to suit. That is the part of a swap the ratio maths does not cover, so the Adaptor Search tab builds a search for your engine family and each gearbox you have selected, and the TC Adaptor tab does the same for gearbox-to-transfer-case plates. It opens Google in a new tab. We are not affiliated with any supplier it finds.