Go kart gear ratio and top speed calculator
Gear ratio is the axle sprocket teeth divided by the driver teeth. Enter your engine rpm, sprockets and rear tire height to get the ratio, the top speed on paper, and what a few teeth up or down on the axle sprocket would do. Or pick one of our karts to load its stock setup.
How the calculator works
- Divide the axle sprocket teeth by the driver teeth to get the gear ratio. A 60T axle sprocket and a 10T clutch give 6.0:1, so the engine turns six times for each turn of the axle.
- Axle rpm is the engine rpm divided by the gear ratio. With a torque converter, divide again by the converter's high ratio.
- Tire circumference is the tire height times 3.1416. A 13 in. tire rolls about 40.8 in. per turn.
- Top speed in mph is axle rpm times circumference times 60, divided by 63,360 (the inches in a mile). 3,600 rpm through 6.0:1 on a 13 in. tire: 600 x 40.8 x 60 / 63,360 = 23.2 mph.
From the parts lists
Stock gearing on SpiderCarts karts
The sprockets, tires and drive each plan calls for, with the top speed on paper at 3,600 rpm.
| Kart | Drive | Sprockets | Ratio | Rear tire | Top speed |
|---|---|---|---|---|---|
| Arachnid | Torque converter | 10T* / 60T #41 | 6.0:1 | 18 in. | 35.7 mph |
| Black Widow | Clutch | 10T / 54T #41 | 5.4:1 | 14 in. | 27.8 mph |
| GrandDaddy | Torque converter | 10T* / 60T #41 | 6.0:1 | 18 in. | 35.7 mph |
| Itzy Bitzy | Clutch | 12T / 60T #35 | 5.0:1 | 4.10/3.50-6 | Measure tire |
| Recluse | Clutch | 10T / 54T #41 | 5.4:1 | 11 in. | 21.8 mph |
| Scorpion | Clutch | 10T / 54T #41 | 5.4:1 | 12 in. | 23.8 mph |
| Tarantula | Torque converter | 10T* / 60T #41 | 6.0:1 | 18 in. | 35.7 mph |
* The parts lists do not give the torque converter's driver size, so these rows assume 10T and the converter at about 0.9:1. The Itzy Bitzy list gives a tire size but no height. Load any kart in the calculator and change what yours has.
What gear ratio does to your kart
Gear ratio is a trade between pull and top speed. More teeth on the axle sprocket, or fewer on the driver, raise the ratio. The engine turns more times for each turn of the axle, so the kart pulls harder off the line and up hills, and tops out slower. Fewer teeth on the axle sprocket do the opposite.
Tire height changes the gearing too. A taller tire covers more ground per turn, so it acts like a smaller axle sprocket. Go from 13 in. to 18 in. tires on the same sprockets and the top speed on paper goes up by about 38 percent. The engine needs that much more pull to get there.
If you gear too tall, the engine bogs. It never reaches the rpm the math assumes, so you lose speed instead of gaining it, and a centrifugal clutch slips and gets hot. If your kart struggles off the line or on a slope, add a few teeth to the axle sprocket before you change anything else.
Yard karts and fast karts
The Recluse and the Scorpion run a 10T clutch and a 54T axle sprocket, 5.4:1, on 11 in. and 12 in. rear slicks. That works out to about 22 and 24 mph on paper. The Black Widow uses the same sprockets on 14 in. tires, about 28 mph.
The Arachnid and GrandDaddy are bigger karts with bigger engines. They run a 60T axle sprocket and a torque converter on 18 in. tires. With a 10T driver that works out to about 36 mph at full shift, and the converter's low range gives them the pull to get there.
For a kart that carries kids or runs on grass, lean toward more teeth on the axle. On a light kart on pavement, fewer teeth can work if the engine can pull them. Start from the gearing in your plans and change one thing at a time.
Chain size: #35 vs #40 and #41
#35 chain has a 3/8 in. pitch. It is light and suits small engines, like the 79cc on the Itzy Bitzy.
#40 and #41 both have a 1/2 in. pitch. #41 is narrower and lighter, and kart parts are often sold as #40/41. Every SpiderCarts plan except the Itzy Bitzy uses #41.
The chain, the driver and the axle sprocket must all match. A #35 chain will not run on a #41 sprocket, and #41 chain can bind on the wider teeth of a #40 sprocket. Pick the chain size first, then shop for tooth counts.
When a torque converter beats a clutch
A centrifugal clutch gives you one fixed ratio. What you set with the sprockets is what you get. It is cheap and simple, and it suits a light kart with a 6 or 7 hp engine, like the Recluse, Scorpion and Black Widow.
A torque converter runs a belt between two pulleys that change diameter, so the ratio shifts as you go. A 30 Series starts near 2.7:1 and shifts up to about 0.9:1. Off the line the kart pulls as if it had a much bigger axle sprocket, and at the top end it still runs faster than the sprockets alone would allow. That is why the Arachnid, GrandDaddy and Tarantula call for one.
Robert Dicken explains the choice in centrifugal clutch and torque converters, and shows the setup in the torque converter overview and the centrifugal clutch overview.
Governors, rpm and safety
Most small engines leave the factory with a governor that holds them near 3,600 rpm. The calculator starts at 3,600 for that reason.
Pulling the governor raises rpm and top speed. A stock cast flywheel can break apart at high rpm, and the stock connecting rod can fail. Fit a billet flywheel and rod before you remove the governor. The brakes, frame and steering on your kart were sized for the speed the plans call for, so if you gear for more speed, make sure the brakes can stop the kart. Wear a helmet, and keep the kart off public roads.
Questions
Gear ratio FAQ
What is a good gear ratio for a go kart?
About 5.4:1 for a 6 hp kart with a centrifugal clutch on 11 to 14 inch rear tires. That is what the Recluse, Scorpion and Black Widow run (a 10T clutch and a 54T axle sprocket). The Itzy Bitzy, a 79cc kid's kart, runs 5.0:1 (12T and 60T on #35 chain). If the kart bogs off the line or on hills, add teeth to the axle sprocket.
How do I calculate go kart top speed?
Top speed in mph = engine rpm / gear ratio x tire height x 3.1416 x 60 / 63,360. For example, 3,600 rpm through 6.0:1 on a 13 inch tire works out to about 23.2 mph. With a torque converter, first multiply the gear ratio by the converter's high ratio, about 0.9.
Will a bigger axle sprocket make my kart faster?
Not faster at the top end. More teeth on the axle sprocket raise the ratio, so the kart pulls harder off the line and up hills but tops out slower. For more top speed, use fewer teeth on the axle sprocket, more teeth on the driver, or a taller tire, as long as the engine can still pull it.
Why is my kart slower than the calculator says?
The calculator assumes the engine reaches full rpm and the tires never slip. Rider weight, grass, hills, wind, chain drag, a slipping clutch or a torque converter that never fully shifts all take speed away. If the engine never reaches full rpm, the kart is geared too tall for its load.
Gearing included
Every plan lists the drivetrain
Each SpiderCarts plan comes with a parts list that names the clutch or converter, sprockets, chain and tires, along with the cut list and the assembly steps.