On a bench, square means square to the vice, and the pipe can be turned to whatever position suits the saw. Under a car the pipe is fixed, the saw is above your head or beside your shoulder, and the only reference you have is the tube itself. That is where most out-of-square cuts come from.
The instinct is to work from the floor, because the floor is flat and the eye trusts it. The floor has nothing to do with the pipe axis. A car sitting on ramps is rarely level, and a cut that follows the ground leaves a face that is square to the driveway and skewed to the tube.
Everything has to be taken from the tube. Sight along it, find a straight length, and use the seam or a mark on the surface as a datum. If the section carries a weld seam down its length, that line is a reliable reference for rotation, though not for squareness.
For squareness, the face has to be checked against the axis rather than against anything else in the tunnel. A wrap of tape or a strip of card cut to the circumference does this better than any measurement, because it transfers a true line all the way round in one go.
Wrap the tape around the pipe and bring the free end back to meet its own start, keeping the edge flat against the surface. If the edge meets itself cleanly all the way round, the line is square. If one side rides up, the wrap has drifted and needs to come off and start again.
Mark along the edge with a fine pen, then take the tape off and look at the line from several positions before cutting. A line that looks right from underneath can be visibly out when viewed from the side of the car, and the eye catches that better than a rule does.
A hacksaw with a fresh blade will follow a line accurately if there is room to swing it, and a reciprocating saw is faster where there is not. Both reward a light hand at the start: the first stroke decides where the blade sits, and the rest of the cut follows it.
A chain or roller cutter is quicker still and leaves a clean face on thin wall tube, but it distorts the end slightly as it closes. That is acceptable when the distorted end will be cut back or dressed, and less so when the face is going straight into a butt joint.
With the cut made, hold a square against the outside of the pipe at several points around the circumference and look at the gap between blade and face. It should read the same all the way round. A face that is out shows as a wedge of light on one side and nothing on the other.
Where the section is supplied as exhaust tube with a consistent wall, a file will true a slightly out face in a few passes. Take the file around the high side only, check again, and repeat rather than trying to bring the whole face down at once.
Sometimes the cut lands where no file will reach and no square will sit. In that case the honest answer is to take the section off and cut it again, because a skewed face under the car becomes a skewed joint, and a skewed joint pulls the run out of line all the way to the tip.
If the section cannot come off at all, the next best option is to dress the face in place with a small abrasive disc and check it against a fresh wrap of tape. Failing that, it comes off. Replacing a short length with new straight exhaust pipe costs less than living with a joint that never sat flat.
