KPH - What's a recommended speed?

I have a Giant XTC 2 2005 model hardtail mountain bike.

When I got this bike i used to ride at about 20kph consistantly.

I now maintain a speed of just above 30kph according to my computer that has recently been calibrated by my LBS.

What is a good speed to sit at for becoming a stronger bike rider? Is this a good pace?

Um not relevant? MTB raceing is generally on offroad. So you need to change your perspective. Think of your training in terms of percentage of effort. On bituman i can do 32kph for 4 hours, but in an xc 100km event id be lucky to do an avarage of 16kph. So to train on bituman i try to match the effort and do lots of hills and hill sprints. Read some of the other posts about anerobic training and hill work, cause im no expert.

People reference one's heart rate for exercise benefit. You should aim to push yourself to around 80% max HR for best benefit rather than a specific kph number as the kph number is independently influenced by the type of bike.

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Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

dunno if this is much good to you, but it'll give you a comparison.
i commute home after dirt crits on my hardtail mtb each week.
the trip is almost 19kms, however, about 12 of them are a consistant uphill climb.
on the first flat section i'm cruising at around the 30kmh mark, but come back to around 24-27kmh after that.
as insaneclimber mentions, offroad is a very different story and much slower.