You can't. NiMhs will always have more voltage and will always be faster. To get more speed from the 1S you need more power, so either put a little timing into the motor via the speedo, or use a 10.5.
Favourite would be to put a little timing into the motor via the speedo until you are the same speed as them, and then hope they will not be upset at you not running blinky. If you're careful with it, you should match their speed and not be too obvious about it. Hopefully they'll accept that and you can all race together. HTH
