So, I got help from Bank of Mom and got a new set of Cooper tires on my 84 240 yesterday.
I opted for the taller 195/70/14 instead of the 185/70/14's it had on it. I also went from BALD - maybe 1/16th or less of tread on one tire and maybe 3 or 4 /32nds of tread on the others up to 10/32nds of an inch on the new tires.

Anyway, I thought I wouldn't notice a difference, but I do. The car seems to accelerate more slowly and it's easier to lose speed on hills = more downshifting. Since the speedometer is driven off the transmission, not off the tires, it's basically reading the speed of the gears, not the speed relative to the road.

I think that would mean I wouldn't see any change in mileage based on my odometer. I got slightly better mileage, but it was all highway and it was about .75-1.0 mpg difference, so it could just be random variation.

Goatman - if you're reading this - my 1st gear no longer seems as excessively low.

All in all I think I would be happier with the stock 185/70/14's, if only because it wouldn't feel as slow as it really is :P

Or maybe it just feels slower because the old tires were barely keeping me on the road haha.

Happy Bricking