Maybe a hundred miles if I'm lucky. Lithium iron phosphate isn't as good as, say, modern Tesla batteries in terms of energy density, but they're a good choice for conversions because they're much less likely to catch on fire if something goes wrong.

I could get better range with more batteries, but I also didn't want to increase the total weight of the vehicle by more than a few hundred pounds, just to stay within design tolerances.

(The RX-8 is about 3,000 pounds normally, which is pretty light for that sort of car. A Miata would probably be an even better choice, as it's around 2,000 pounds. It's hard to find newer Miatas for a reasonable price, though. RX-8's can be had pretty cheap because the rotary engine is easy to destroy if you don't maintain it properly and even in the best case usually needs to be rebuilt every 100,000 miles or so to replace worn apex seals. So, there are a lot of used RX-8's on the market that need engine work.)