EMoto Gearing Planner

Choosing a Rear Sprocket Size

The trade is one sentence: more teeth means more pull and less speed. Everything else on this page is detail about where that trade lands for the riding you actually do.

The trade in detail

Adding teeth to the rear sprocket raises the total reduction. Three things follow, and they follow together — you cannot have one without the others.

Wheel torque goes up, in direct proportion. Go from 48 to 58 teeth and the wheel sees about 21 % more torque at the same motor torque. That is what makes a bike climb, and what makes it tractable at walking pace in rocks.

Top speed goes down, by the same proportion. The same 48 to 58 change costs you about 17 % of your speed at the rpm ceiling. There is no way to keep the climbing and not pay this.

Chain speed goes down, which is the one people forget and the one that is quietly good news: the chain runs slower over the countershaft at the same motor rpm, which means less wear and less noise. The rear sprocket is downstream of the countershaft, so chain speed responds to the front sprocket and the rpm ceiling, not to the rear. Gearing down for torque is free on this axis.

What Charged Cycle Works recommends

Presented as their recommendation, not ours. Charged Cycle Works, retrieved 2026-08-19, suggest:

That last one is worth reading carefully, because it is a recommendation about two things at once. The 16/19 setup is a smaller rear wheel, and a smaller rear wheel acts like adding teeth. Their 52T or 54T recommendation is calibrated to that wheel, and lifting the tooth count out of it and bolting it to a 19 inch rear will not give you what they were describing.

What riders report

On the electricbike.com Sur-Ron thread, riders report 42 mph on GPS with the stock 48T, 35 mph on a 60T, and 38 mph on a 58T at 240 lb rider weight. Retrieved 2026-08-19.

Treat these as what they are: individual GPS runs, on different bikes, at different rider weights, on different surfaces, at unknown states of charge. They are not a controlled comparison and they cannot be lined up against each other as one. What they are good for is showing the spread — the range of real outcomes behind any single published figure.

Why rider weight and terrain move the answer more than the chart does

A tooth-count chart implicitly assumes a rider. A 240 lb rider on a loose climb is asking for something a 140 lb rider on a fire road is not, and the gap between those two is larger than the gap between adjacent sprocket sizes.

Terrain does the same thing. Top speed is only a constraint if you spend time at it. A rider who is at full speed for thirty seconds of a two-hour ride is paying for a ceiling they touch twice, while giving up torque they use continuously. The honest question is not “what is the best sprocket”, it is “how much of my riding happens near the ceiling”. Most people, asked that plainly, gear down.

The interaction with wheel size

A 16 inch rear wheel lowers the rolling radius, and lower rolling radius acts exactly like adding teeth: more torque at the wheel, less speed at the ceiling. The two changes stack, and they stack multiplicatively.

This is the most common way people end up with a bike that feels wrong. They fit a smaller wheel for the tyre choice, fit a bigger sprocket for the climbing, and find the bike geared so low it is uncomfortable everywhere else — each change was reasonable, and together they were too much.

The fix is to model both at once. Enter your current setup as Setup A in the gearing calculator and the proposed wheel and sprocket together as Setup B. The delta row will tell you in plain words what the combination costs before you have bought either part.

The interaction with tyre width

Charged Cycle Works state a maximum recommended rear tyre width of around 4.3 inches for this class of bike, and note that wider tyres may require dishing the rear wheel away from the chain. Retrieved 2026-08-19.

That makes sprocket choice and tyre choice non-independent. A sprocket sits on the same side of the wheel as the chain, and a wide tyre pushed towards the chain by a wide rim is competing for the same space. Deciding tooth count in isolation and discovering the clearance problem at fitting time is a wasted afternoon and sometimes a wasted part.

The practical order of operations

  1. Decide what terrain you actually ride, honestly, by hours rather than by aspiration.
  2. Pick a candidate tooth count from that.
  3. Run it through the gearing calculator against your current setup, with any wheel or tyre change entered at the same time.
  4. Measure your centre distance at both ends of the axle slot — how to measure centre distance covers it.
  5. Run those figures through the chain length calculator to get your link count range.
  6. Buy the sprocket and the chain together. A sprocket that arrives without the chain it needs is a bike on a stand.

Sources