No thats not quite right.
A fully charged lead-acid battery will have something like 12.8V (divide all these numbers in half if you are using 6V batts!)
As you use up your charge, the voltage will drop, but half a charge is NOT going to be 6 volts, it's going to be just over 12.
This image if from
THE 12V SIDE OF LIFE, which explains how power systems on RVs work.
So, with half a pack of power, your car may not be able to have the same top speed, but it's not going to be something weird like only HALF your top speed.
For example, when I converted my motorcycle from 36 to 48 volts, the new top speed was 45 mph. I think I could get it to 47 mph on the flat with a fresh charge. I ran the cycle hard for 20 miles - that's basically the whole pack. Then I could only get to maybe 35 mph on the flat. It really felt like the bike was tired and trying too hard.
Now 10 miles in to the ride (half the pack) I could still go 44 mph no problem.
If your EV is going really slow compared to normal, you either have pushed the batteries too far and need to charge right away, or you have some other problem.
PS: Keep in mind it's dangerous to get on the freeway just before running out of gas too!
