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If your cruising at a high altitude your cruise speed is mach .74 if I'm correct. What should the speed be on a short flight. For example Newark to Boston I cruise at 18,000ft but if I set the speed to mach .74 it overspeeds. I now you don't use mach below a certain altitude. But for short flights when your cruising at a low altitude is 340 kias to much? Thanks.

the short answer is yes ..if you are overspeed your going to fast for the alt..keep it 20 to 30 knots slower than your overspeed..

If your cruising at a high altitude your cruise speed is mach .74 if I'm correct. What should the speed be on a short flight. For example Newark to Boston I cruise at 18,000ft but if I set the speed to mach .74 it overspeeds. I now you don't use mach below a certain altitude. But for short flights when your cruising at a low altitude is 340 kias to much? Thanks.

Well, as I'm sure you're aware, there are a lot of variables involved with cruise speeds at different altitudes, so your cruise speed could be more or less of a mach number, depending specifically on the altitude, and the type of aircraft.
Besides that, I don't know how it is in other countries, but in the United States, in the vast majority of spaces, there is a "speed limit" of 250 knots indicated, below 10,000 feet msl. (Some specific regions can also be slower than this, such as in major terminal areas.)

Also, in the US, 18,000 feet is the transition altitude for setting your altimeter, at and above this altitude, it has to be on 29.92 inches. (1013 mb). Also, at and above this, up to FL600, you are in Class A airspace. (IFR only)
Below this, the altimeter has to correspond with the local altimeter setting given by ATC.
Keeping all this in mind, you may want to cruise at an altitude 1000 feet above or below that, since in FS, the constant transitioning could screw with the built in ATC code variables.
And finally, remember, when flying under IFR, when flying East, fly on odd altitudes, such as 15,000....17,0000....19,000, etc.
And when flying West use even altitudes, such as 14,0000....16,000 etc.
(There are some RVSM specifics above FL270 or 280 I believe, but I don't know much about RVSM yet.)

Also, in the US, VFR flying is prohibited at and above 18,000, and when below that, when flying VFR, still use the east-west rule, but add 500 feet of altitude to that, such as 15,500....or 16,500, etc.

Back to your airspeed question, it is best to look in the operating manual of the specific aircraft to see what recommended power settings and airspeeds you should be achieving when cruising at a certain altitude. (Don't run the engine on any aircraft at full power all the time!)

-SeanTK

But for short flights when your cruising at a low altitude is 340 kias to much?

chris1060, the Air Speed Indicator works on air pressure and therefore 'under-reads' as you get higher; the Mach speed is calculated to allow for thinner air and is therefore 'true airspeed.'

A sort of round-figure 'rule of thumb' is that, at 30,000 feet and Mach 0.75, your 'Indicated Airspeed' will be 300 knots and your true airspeed will be 450.

You can cross-check the speeds in either 'denomination' at any time by checking the VSI (main left-hand panel in most airliners). That usually shows airspeed on a scale or dial and the Mach speed in figures.

As SeanTK suggests, a sensible every-day principle is '250 Indicated' up to 10,000 feet. Beyond that, say 275 in the climb; Mach 0.65 above 20,000 feet. Cruising below 20,000 feet, 275-300 knots should be plenty. Best to switch to Mach speed at that height anyway; if using autopilot, change the 'speed hold' to Mach speed and it will automatically 'convert' the autopilot reading to tell you your Mach speed. which I would expect to be about 0.62 Mach at that height.