You can get LiPo batteries with voltages above 7.4. Try a 3S. LiPos have longer run times than NiMH of the same mAh capacity and the charge doesn't fall off as fast as NiMH. Once you go LiPo you won't go back.
Get one Gens Ace LiPo and try it. My 2S (7.4V), 5000 mAh Gens Ace LiPo makes my stock SC10RS do wheelies on asphalt!
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