What does the RPM gadge measue?

The exhaust manifold has nothing to do with the RPM gauge. The RPM gauge merely measures how many "rotations per minute" your engine makes. Most cars dont even have them. It will cause no harn to your car if the gauge doesnt work.

the more you let the crack go the more significant damage your doing to your engine. with a leak in the manifold your engine is losing back pressure. and without sufficient back pressure u can plan on having a lot more problems

The large crack in the exhaust manifold could have caused some further damage to your engine, but I doubt that it would overtly effect the RPM gauge after it is fixed. I think that there are just two problems that your going to have to look into.

That would have been a super opportunity to upgrade to a set of headers rather than the factory manifold.

if you want to fix the problem get under the hood and check out the distributor. the tach wire on my camaro attatched to the back of my distributor cap. its possible it came undo in your car also. although it may be different in mustangs

no rpm guage tells the number of rotations your crank makes per minute not tire rotation.

furthermore, if your tach wire is attached properly then you probably have a short somewhere. use an ohm meter to find out if the wire is grounded out or not. if the wire isnt grounded out and it is attached properly to the distributor then you might possibly have a broken guage. like others have said it wont hurt your car if it doesnt work properly