Kawasaki H2 Gear Ratios & Gearbox Data
Ratios for every variant of the supercharged H2 gearbox, the method for turning them into real road speeds once the engine is in a car, and an honest account of which figures are solid and which are still disputed.
Why the Gearbox Changes When the Engine Leaves the Bike
In a motorcycle the H2 gearbox is only part of the transmission. Behind it sits a chain final drive that contributes another 2.3:1 to 2.6:1 depending on model, and that reduction is trivially changed by swapping a sprocket. Overall gearing is therefore something the rider adjusts rather than something the gearbox dictates.
In a car that flexibility usually disappears. Most installations couple the gearbox output shaft to a propshaft and a fixed differential, deleting the chain drive entirely. The gearbox internals and the primary reduction then become fixed constants, and the differential is the only ratio left to choose. Choosing it correctly means knowing exactly what sits upstream of it — which is where most calculations go wrong.
These figures are compiled from Kawasaki published specifications, parts catalogues and owner's manuals, and cross-checked against each other and against our own installations. Public data on this family is inconsistent and in places contradictory. Where sources disagree we say so below rather than quietly choosing one. This is the best position we can reach from what is publicly available — not a guarantee. Confirm the ratios for your specific engine against the parts catalogue for its model and year before a differential is ordered.
The Two Ratio Sets
The supercharged H2 family uses two distinct gearsets, and they follow the same split as the two engine specifications. Third through sixth gear are identical across every variant — only the primary reduction and the two lowest gears differ.
For reference, the factory chain final drive figures are 2.333 (42/18) on the Ninja H2R, 2.444 (44/18) on the Ninja H2 and H2 SX, and 2.556 (46/18) on the Z H2. In a direct-coupled car installation none of these apply — the differential replaces them entirely.
Kawasaki's 2018 Ninja H2 SX specification and the Z H2 specifications give 2.471 (42/17). A later Ninja H2 SX owner's manual gives 2.429 (34/14). Both are Kawasaki figures, so this is unlikely to be a simple error in one of them. The probable explanation is that second gear changed during H2 SX production while the Z H2 kept the original. We have a 2022 SX SE manual showing 34/14, so the change had happened by then, but we have not established the exact model year and will not guess at one. Until it is confirmed against the parts catalogue for the specific engine, treat second gear on any Ninja H2 SX as unresolved between those two figures.
Nothing else here depends on it. Second gear does not affect the primary reduction, top gear or differential selection, all of which rest on ratios common to every variant. On the confidence of the rest: the primary reductions and third to sixth gear are corroborated across independent sources, and the arithmetic closes against Kawasaki's own published overall top-gear ratios, so they can be relied on. First gear is consistent everywhere we have looked. All of it is nevertheless drawn from published data rather than from teeth we have counted ourselves, and we would rather say so than imply otherwise.
The Primary Reduction Is Not Optional
This is the single most common error we see in H2 gearing calculations, and it produces answers that are wrong by more than half.
Builders who have deleted the output sprocket often conclude that there is no reduction between the engine and the gearbox output, and calculate using the gear ratio and differential alone. The primary reduction is not the sprocket. It is the gear pair between the crankshaft and the clutch basket, inside the crankcases, and it cannot be removed without dismantling the engine. It multiplies every gear.
The correct expression for a direct-coupled installation is therefore:
Primary reduction × Gear ratio × Differential ratio
Omitting the primary understates the overall ratio by around 48% on a Z H2 and 55% on a Ninja H2, which in practice means a top speed prediction roughly half as high as reality and a differential chosen on the strength of it.
Identifying Which Gearbox You Have
The ZXT00NE engine number prefix runs across the entire supercharged family and serial numbers are sequential across all of them, so the engine number alone will not tell you which gearset is inside. Neither will the supercharger housing colour, which is not a specification code.
The frame number does answer it. Any online Kawasaki parts catalogue with a VIN search will return the model, year and full model code from a chassis number. We have used this route to confirm engines in our own cars and for customers assessing engines before purchase, and it takes a couple of minutes. Prefixes we have seen returned include JKAZRT00 for a Z H2 and JKAZXT02 for a Ninja H2 SX, with ZX1002 appearing as a Ninja H2 model code from 2019 on. VIN construction varies by market, though, so run the lookup rather than reading a prefix and assuming.
Where an engine has been separated from its motorcycle and no frame number survives, the tooth counts remain definitive — first gear is 51/16 on a Ninja H2 or H2R against 40/13 on a Ninja H2 SX or Z H2. Every variant in the family uses a dog-ring gearbox, so on any of them this means stripping the shaft assemblies. It is a definitive answer rather than a quick one.
The distinction matters for two reasons. The primary reduction differs, so every calculated speed shifts by around 5%. And because compression ratio follows the same split — 8.5:1 on the Ninja H2 against 11.2:1 on the Z H2 and SX — identifying the gearbox also confirms which calibration the engine needs.
Calculating Road Speed
Once the overall ratio is established, road speed depends only on tyre rolling circumference and engine speed.
The most reliable route is through the tyre manufacturer's revolutions per mile figure, which most motorsport tyre data sheets publish directly:
(Engine rpm × 60) ÷ (Overall ratio × Revs per mile)
Where only a rolling circumference is given, revolutions per mile is 1609.34 divided by the circumference in metres. A 1.740 m circumference gives 925 revs per mile, for example.
One caveat is worth stating because it affects how the answers should be read. Published revolutions-per-mile figures are normally zero-load values. Under vertical load the rolling radius is reduced, so real-world speed typically sits 2–3% below the calculated figure at moderate speeds, before centrifugal tyre growth begins to pull it back the other way near the top end. These calculations are a sound basis for choosing a differential; they are not a substitute for a measured speed once the car runs.
A Worked Example
A Z H2 specification engine in a lightweight sports car, direct coupled through a propshaft to a 3.38 differential, running 230/550R13 rear tyres at 925 revolutions per mile.
Reading the Table
The two columns are not alternatives. The lower figure is a working shift point chosen for mechanical sympathy; the higher is what the gear will reach if taken to the limiter. Because top speed is set entirely by sixth gear, both are available in the same car — shifting at 12,750 through the lower gears costs nothing at the top end.
Both columns sit above the factory engine speed limit for this variant. The limiter here is set by our own calibration, on standalone management with knock control and logging, not by Kawasaki. Do not read these as stock figures, and do not raise a limiter on a high-compression engine without the instrumentation to see what it is doing.
What matters more than either column is where each upshift lands. With this gearset a 12,750 rpm shift drops the engine to between 10,239 and 11,278 rpm, and measured torque on this engine peaks at 10,190 rpm. Every upshift therefore lands at or just above the torque peak, and the engine never falls below it in any gear. That is an unusually good match, and it comes from the close ratio steps — 1.13 to 1.25 — rather than from anything the builder chose.
Choosing the Differential
The instinct is to gear for the highest top speed the engine could theoretically reach. On a car with no aerodynamic assistance that is almost always the wrong answer, because drag will stop the car long before the gearing does, and every unusable mph in sixth is torque multiplication given away in all six gears.
The better test is where peak power falls in top gear. On this installation, measured peak power at 13,830 rpm arrives at 133 mph in sixth, with around 400 rpm still in reserve to the limiter. For a car whose body runs out of breath in the low-to-mid 130s, that is the correct relationship: the engine reaches its best power at the speed the car can actually sustain, with headroom rather than a wall. A shorter differential would have brought peak power forward to a speed the car exceeds, leaving the limiter — not drag — as the constraint, with no remedy short of another differential.
Sixth gear here works out at roughly 104 rpm per mph, placing 70 mph at around 7,280 rpm. That is normal for a bike-engined car but worth confirming against intended use before a differential is ordered, particularly where the car will cover road miles rather than only event miles.
What a Z H2 Gearbox Costs to Repair
Worth establishing before an engine is bought, because it is not what most builders assume.
Kawasaki does not list individual gears for the Z H2. The transmission section of the parts catalogue contains exactly two gear part numbers, both of them complete shaft assemblies:
13341-0640 — Input shaft assembly
13342-0678 — Output shaft assembly
At the time of writing these list at roughly £950 and £2,450 respectively, so a gearbox wanting attention at both ends is approaching £3,400 in parts before any labour. There is no route through the factory parts system to replace a single damaged gear.
This follows from the dog-ring design rather than being an arbitrary catalogue decision. We have confirmed it on the Z H2 catalogue; since every variant in the family uses a dog-ring gearbox, expect the same across the range, but check the catalogue for your own model before assuming it. Either way it changes what a used engine is worth. One offered cheaply with an unknown or noisy gearbox is not a bargain. Where the original gearbox is retained in a car installation, as it usually is, its condition deserves the same scrutiny as compression and supercharger condition — and it is far better established before purchase than after.
Gearing Is a Specification Decision
Ratio tables answer the arithmetic. They do not answer whether a given differential suits the car, the circuit, the driver or the way the engine has actually been calibrated — and a differential is an expensive thing to get wrong twice.
We are happy to work through final drive selection for an H2 installation alongside the rest of the conversion. The inputs we need are the engine specification, the differential options available to you, tyre data, and an honest description of how the car will be used.
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