Good point, Trev, and one I didn't factor in to my list above. No, there isn't a BR speedo you'll find that works below 4.8v without a booster or receiver pack.
Like I said, it all depends on what car you have. If you've got an early link car, or T-bar car, with the slots for four cells, the easiest option is to buy some NiMh cells and make them up to use in your car. If you don't, then use a 1S LiPo and a receiver pack or booster. If you need more help, come back on here. HTH
