up hill struggle

[westy425]

Had to replace rear wheel on my mtb used spare I had off my old bike obviously made sure it was same size but it feels like I'm fighting to cycle on flat roads and harder on the up hill sections of my commute any suggestions what my cause this?

 

[RecordAceFromNew]

Numerous possible causes, including:
1) brake rubbing - can the wheel be spun freely when off the ground? Do new and old hubs have the same distance between locknuts? Are the rim widths the same?
2) bearings overtight - having taken wheel off bike, can you feel a lot of friction spinning the axle with your hand?
3) different gearing - is the cassette/freewheel size different to old?
etc.

 

[westy425]

Thanks for reply can rule out some of them brakes are free and cassette wad swapped due to old being 5 speed and needing a 6 very helpful thanks