You need to figure out MPG at different speeds.
Odds are if you drop back to 6-8mph, you will have upwards of 2mpg but you need to confirm. That would give you over a 300 mile range
Once you know you mpg assume maybe a 1.5 mph current
on both rivers (conservative).
Lets say 2mpg at 8mph:
- Mississippi: You will actually be making 9.5mph for the 160 mile, that works out to around 17 hrs and you will be burning 4gal/hr, so figure 68gallons
- Ohio/Tenn: You will be making 6.5mph against the current
for 60 miles, that works out to around 9hrs at 4 gal/hr, so around 36gallons.
Total around 104gallons burnt.
While you are probably fine, MAKE SURE TO CHECK YOUR ACTUAL MPG and update the calculations and make sure you have a buffer.