I have an Orbea Onix from 2005 which was a demo bike that I bought from the LBS. I am having a lot of problems keeping up with people at speeds above 19 mph sustained on flat surfaces and I was wondering if it's got anything to do with the chainring and cassette that I have. I am a fairly new cyclist but in good shape and was wondering if there was anything unusual about the parts that I have.

The chainring says FSA 130mm BCD 53/39 (no, I don't have a granny gear). And the cassette is a Shimano Ultegra 12, 13, 14, 15, 16, 17, 19, 21, 23, 25.

Don't know the crank size...assuming it's 170mm...it's whatever came with the bike. Would appreciate any thoughts and links to sites that'll help me make some sense.

Thanks!

Nothing wrong with that gearing unless you are having problems climbing steep hills. With that gearing you can go at least 34 mph if you can turn the pedals faster.
Train more, pump your tires up before each ride, be as aerodynamic as possible. Learn to draft safely. Drafting can conserve 20 - 25% of your energy as well as increase your speed.
Nothing wrong with the equipment.

Al

Yes, with that combination you shouldn't pedal-out until about 35mph. If you can't keep-up at 19mph then you are not at the same conditioning level as the other riders.

Dont know your age or ability ,but my exprience is just hours in the saddle . local group rides at differant levels plus intervals on your own , just keep riding , your gearing sounds fine to me.

As others noted, your gearing is about the most common provided with a stock bike. Nothing wrong with it.

To ride at 19mph all it takes is a 39/16 at a cadence of 100 rpm or a 53/19 at 87 rpm. Your top gear of 53/12 wii produce 35 mph at only 100 rpm. Experienced riders can pedal at 120-130 rpm for brief periods and achieve a top speed in the range of 42-45 mph with a 53/12 (a good tail wind and a downhill slope help too).

A common mistake among beginners is pedaling much too slow. Your cadence should be at least 80 and 90-100 is even better. I see a lot of beginners mashing at 60 rpm and not shifting to lower gears when needed. It's a bad habit to get into.

Not that it matters, but most crank arms are stamped on the inside on th earm, with their length.

"I am a fairly new cyclist but in good shape and was wondering if there was anything unusual about the parts that I have."
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The reason that other cyclists leave you in the dust is because they are more fit than you are. It is not the bike's fault (unless the tires are flat)
Once you become fit, the gears that you have should allow you to acheive a top speed of over 40 mph, and you should be able to climb just about any hill with the 25.