They might have had the heated seats ona quick google shows the mpg of a ww2 Sherman is .7mpg and an modern M1A2 is .5mpg
(i assume all modern MBT's will have similar mpg)
the Russians were expecting to roflstomp Ukraine, they appear unprepared for a sustained conflict and/or Ukrainian interference on the supply lines
a t-90 should have a range of 500km assuming ideal conditions, they should'nt be running out of fuel on a drive from Belarus to Kyiv even if going off-road, involved in fighting, etc