Ok so I have mavic 317 rims, and normally I ride size 26x2.25 tyres. But in November I got a good deal on schwalbe fat alberts...but they are size 2.35..they look fat on my rims. They are great for downhill mud n rocky stuff , but so ****in heavy to ride uphill. I like climbs but not with these tyres. Can't believe how much weight they have added to my bike...for my size rims shud I really stick to the 2.25 tyres.
Pump them up a bit more - maybe that will lower the rolling resistance enough. If not [i]pretend you are training for the olympics[/i], or, [i]pretend you are trying to hypnotise yourself with the drone of the knobblies[/i]. 😉
Maybe you could shove [i]some goo in there and run them tubeless[/i] (save a few grams [2xtubes = ~400gms]). If that is a no go, as a last resort maybe you could [i]fill them with helium[/i]? 🙄
I put 2.25 mountain king 2's on my trailbike. They have almost the same grip as my schwalbe fat alberts but uhhh are harder to pedal up hills despite being smaller and lighter.
What 2.25's are you running? Just train harder, I caught some cyclocrossers on a 10 minute climb riding my 32lb Mega once. The look on their faces! 
you could try putting one of your normal size (or even a 2.1) on the back.
What andyl said. I'm going to run 2.3/4 nobbly nics on the front this summer with a 2.2 rocket ron on the back for general hooning around local trails and trail centres.
Train harder...I'm brill at climbing but fat alberts are heavy. I'm only small so it very sluggish going uphill. There's loads,of climbs in the calderdale valley where I live but these tyres are holding me back. I hate that. Lol...I'm no weakling but fat alberts are a no no for climbing. I love the mountain king tyres for summer for me they are fast rolling too. Think I mite go back to those tyres . A lot lighter on my bike too. I have smaller rims cos my bike is only 14 inch frame...