4iiii Precision vs Favero Assioma

This comparison turns on one sentence that both brands write in the same way and mean differently. 4iiii claims ±1%. Favero claims ±1%. The 4iiii figure describes the torque measured at your left crank arm; your total power is that measurement doubled. The Assioma Duo figure describes power measured at both pedals. Same words, different statements.

What the ±1% claim actually covers

On a single-sided meter — the 4iiii Precision 3+, the Assioma Uno, most budget crank meters — the instrument reads one leg and multiplies by two. If your legs are perfectly balanced, your total is accurate to roughly the claimed figure. If you produce, say, 52% on the left, your total reads about 4% high, every ride, consistently.

That consistency is why single-sided meters are perfectly usable training tools: FTP, interval targets and season-on-season comparisons are all measures of change, and a consistent offset does not corrupt a change. What it does corrupt is any comparison against someone else's numbers, and any attempt to diagnose an imbalance. Single vs dual-sided power meters goes through this properly, and how accurate are power meters covers what the claims mean generally.

The battery difference is bigger than the accuracy difference

4iiii claims up to 800 hours from a CR2032. That is the best battery story in power meters, and for a pain cave it is a real behavioral advantage: nothing to charge, nothing to remember, no session canceled because you forgot. Favero's pedals are rechargeable with a magnetic connector, which is well implemented but is still a habit you have to keep.

If you ride mostly indoors, a coin cell is worth more than a fast charger. If you ride long unsupported days outdoors, it is worth even more, because a spare CR2032 lives in a saddle bag and a USB cable does not help you at hour nine.

The compatibility homework is the real cost of the 4iiii

A pedal fits any bike with standard pedal threads. A bonded crank arm has to be the correct arm for your exact crankset model and your crank length, and crank length is a fit decision rather than a shopping preference. If your crankset is unusual, old, or simply not on the supported list, this purchase does not work — which is exactly when the Magene P505, as a complete crankset, becomes the sensible alternative.

The flip side is protection. A crank-arm meter sits inboard, sheltered from the crashes and rock strikes that a pedal takes. On a road bike that barely matters; on a gravel or mountain bike it matters a lot, which is why our gravel power meter page weighs it differently.

So which one?

Two bikes, or a dedicated trainer bike plus a road bike: pedals, because the meter can be the same instrument in both places and you avoid two sets of numbers.

One bike, settled pedals, tight budget: 4iiii, and spend the difference on something that makes you ride more — a decent fan does more for session completion than a second power sensor.

You want to see balance: Assioma Duo. It is the only pick here that actually measures both legs rather than estimating or doubling.

Your crankset is not supported: Magene P505, or go back to pedals.