New with Canon 40 + 70-300mm zoom

I've been looking on the forum and I've just bought a 75-300mm lens for my 40d thinking this would be Ok for shooting motocross. IU see no mention of it and am worried that it's not OK. Should i have bought one with a lower F than 4/5.6?
I am totally new to photography so any help would be good!

The 75-300, while cheap, isn’t exactly the best choice for a telephoto in general and even less so for sports shooting particularly. Image quality is quite good in the 75-150mm range, but beyond that it drops quickly and suffers quite a bit in the 200-300mm range.
Mechanical quality is poor and autofocus is comparably slow.

The 70-300 IS has good image quality throughout its range, but AF is not fast too.

Both mentioned lenses have slow apertures, that could be a problem for action shots if the weather is bad. The best choice at a still moderate price point would probably be the EF 70-200/4 L.

motoxmad wrote in post #6028671
I've been looking on the forum and I've just bought a 75-300mm lens for my 40d thinking this would be Ok for shooting motocross. IU see no mention of it and am worried that it's not OK. Should i have bought one with a lower F than 4/5.6?
I am totally new to photography so any help would be good!

Here are a couple of supercross images with the 75-300mm standard lens (no image stabilization) taken with a Digital Rebel 300d a couple of years ago. Available light used with each image at an indoor stadium:

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