I am averaging about 10 to 11 mpg, and my driving is about 70/30 freeway to street.

I drive slow and steady, my friends refer to me as grandma when I drive, so I know that isn't the problem.

I do have 33" tires on my Rover, but I can't believe that is the cause. There appears to be a crack in the top of my gas tank, as whenever I top it off, gas spills out. But still, I wouldn't think it would be enough to drop my mileage that low.

Anyone have any ideas? No smoke or anything from the exhaust, just changed the oil and no sludge buildup (that I could tell). I plan on checking the spark plugs for replacement/regap.
Is there anything else I should look for?

 

i get 12-13mpg on stock sized tires. that number sounds pretty reasonable to me.
you may check your filler hose for cracks and leaks.

 

Wow. My old neighbor got 15ish mpg in his Rover, and had 31" tires. Never mind then, if 11mpg is what I'm gonna get, that's what I'm gonna get.

 

Yes the gas leak will will cause a drop in mpg as the gas leaks/evaporates.
There was a recall on the gas tanks.
Second, are you using premium gas? If n ot you will loose about 2 mpg.
Second, when was your last tune up?
How old are your oxygen sensors?

 

If I can't afford Premium I use Plus. If I can't afford Plus I don't drive my Rover. Filled up with Regular once, got a noticable drop in power (not so much mpg, maybe only 1 or 2).

Last tune up, no idea I bought it less than 2 months ago but am out of the job so haven't had the funds to take it to a mechanic.
Same answer with the oxygen sensors, no idea.

First thing on my list is replacing/re-gaping the spark plugs, since those are cheap and easy. Second might be to cut off my two mufflers to see if that eases backpressure.

 

Removing the mufflers wont do anything but make it loud, and then you'll get a ticket for being to loud.
Spark plugs are cheap, under $2 each.
Also check your air filter to see if its clogged.