This question can probably only be asked for a narrower taxonomic unit (genus, maybe maybe family). Otherwise it's really too dependent on the taxon, because wing shape, body shape/size, and flight style all affect speed and don't correlate well with wingspan across all species. Longer wings create more lift, but how much of the lift is used to go up versus forward? Depends on the bird. Also wider wings will create more drag. A bird with the same wings can flap them with higher frequency and reach higher velocity. And so on.
There is no single equation that accurately describes the relationship between the flight speed of a bird and its wingspan. The flight speed of a bird is influenced by a variety of factors, including the size and shape of its wings, the efficiency of its flapping motion, and the density of the air it is flying through.
However, there are some general patterns that can be observed in the relationship between a bird's wingspan and its flight speed. In general, birds with larger wingspans tend to be able to fly faster than those with smaller wingspans, as their larger wings provide more lift and allow them to generate more thrust.
Additionally, the shape of a bird's wings can also affect its flight speed. Birds with long, narrow wings tend to be able to fly faster than those with short, wide wings, as the narrow wings produce less drag and allow the bird to cut through the air more efficiently.
It is important to note that the relationship between a bird's wingspan and flight speed is not always straightforward, and other factors such as the bird's weight and body shape can also affect its flight performance.