When kids visit the Cold War Air Museum and see the MiG-23 for the first time they invariably ask, "How fast is it?" While the short answer is "Mach 2.35," that doesn't really give you a feel for how insanely, crazy fast that really is. So we've taken to explaining that at Mach 2.35, the MiG could travel from Dallas to Houston (about 240 miles = 4 hours by car) in just over 9 minutes. And the answer to the second question, "How much gas does it burn?" is (of course), "ALL OF IT!"
But that's not the full answer, which has a lot more to do with how high you want to fly and how much Jet-A you want to burn.
The long answer is this chart, relating altitude and maximum indicated airspeed at the various wing sweep angles of 16, 45, and 72 degrees.
There are a couple of things to read from this chart.
First, maximum airspeed at full forward sweep of 16 degrees is Mach 0.8. With the intermediate sweep of 45 degrees and the belly tank, Mach 1.6 is the limit. And with any external stores 1.8 is as fast as you can go. Moreover, at 45 degrees you're not going faster than 1,200 km/hr (648 KIAS) because that's the maximum design airspeed for that wing sweep. Maximum indicated airspeed is 1,350 km/hr (729 KIAS).
Second, altitude is restricted by airspeed. If you have to stay under Mach 1 (and, alas, you do) then the static ceiling is about 13.5km (about 44,000 feet).
Assuming the FAA would consider letting you make a "BOOM!" stretching from Dallas to Houston, indicated airspeed is still limited, so you can't really get zorching until you're over 10km (about 33,000 feet).
Finally, temperature limits maximum altitude. On an ISA day (15 degrees Celsius), the MiG-23 will top out at 17.5km (about 57,000 feet). On a colder day you can go higher, but on a day that's ISA+15C the ceiling will be 12.5km (about 41,000 feet) and you won't be able to make Mach 2.35 since you'll run into the 1,350 indicated airspeed limit.
Still, at 50,000 feet, cruising along at 1,500 mph sure would beat taking the bus... that is, if you don't mind that you're burning two gallons of gas per second. :-)