I don't know if I agree that running it off the wheels will give meaningful information about range. Power = Volts x Amps. It will take many more amps to move a 3500 lb car 50 miles than it will take just spinning the wheels the same number of turns.
Right now I don't have the batteries enclosed or insulated, don't have them heated (Which I doubt I'll do), and it's 18 degrees outside! I won't get nearly the range that I will in the summer. Furthermore, Lead Acid batteries need to be exercised before they meet their max capacity. From what I've read, it takes about 30 to 50 charge / discharge cycles for them to get to max. After that they gradually degrade over time.
I don't want to ever kill the batteries to the extent the car won't go. There is a trade-off between how deeply discharged they go and how long they last. Probably never want to go below about 50%. The controller has a lower limit of battery voltage, under which it disables the drive anyway. Too deep of a drain really kills battery life.
My plan is to start out with short hops around the neighborhood and never go more than 3-4 miles away. I'll start out putting no more than 20-30 miles on it and monitor the battery voltage. As it starts to drop to anywhere near the limit I'll head home for a recharge. As soon as I'm confident, I'll start driving it to work - I can also recharge it there. It's about 17 miles each way. When the weather starts to warm, I'll get more range and the batteries will begin to be broken in. Based on conversations with people that have the same system on similar cars, I expect 60 to 100 miles.
Another factor is where / how the car is driven. I live in a rural area near a couple of rivers. So there's a good bit of up & down terrain. That will be different than straight & level. I expect range will be different at 65 mph on a highway than at 35 mph on country roads too.
I will try to set up the GPS to record trips so I get an accurate distance measurement and speeds.