Answers.com - SpeedStep

If your have a core 2 duo CPU that is not running at full stock speed, the reason is Intels SpeedStep

So for example:

E4400 stock speed is 2ghz.

200x10= 2ghz <--stock
200x6= 1.2ghz <-- speed step reducing power at idle

E6600 stock speed is 2.4ghz
266x9= 2.394ghz or 2.4ghz <-- stock speed
266x6= 1.596ghz or 1.6ghz <--speed step reducing power at idle

When your PC is at idle, speed step kicks in to reduce the mulit, and vcore.  This allows lower power consumption, and heat.  Its basically the same as AMD's Cool-n-quiet.  When you put your system on a full load, speed step should put the stock multiplier back.

To better understand how to figure out the default or stock multiplier, you will need to know what your CPU's rated FSB is.  For example:

All Allendales (E4xxx) or C2D Pentiums (E2xxx) are rated at 800mhz.  They are Quad Pumped so you divide by 4.
800/4=200mhz.

All the older Conroes (E6300/6400/6320/6420/6600) are rated at 1066mhz.
1066/4=266mhz.

Newer C2D (E6550/6750/ect) are rated at 1333mhz.
1333/4=333.25mhz.

So to determine your actual or default multiplier for 4 examples:

E4400 2ghz
2000/200=10 <-multi

E2140 1.6ghz
1600/200=8 <-multi

E6400 2.13ghz
2130/266=8 <-multi

E6750 2.66ghz
2660/333=7.9 or 8 <-multi (this would be rounded)

You can use a handy tool to monitor your multiplier with CPU-Z.  To simulate a load, try using Orthos Stress Prime 2004.  That is one easy program to load dual or quad core CPUs.  In running that program, it should force your multiplier back to its normal stock setting.  Its also a handy tool to help you determine if you have a stable system, thermal and CPU/Memory wise.

And here are examples of other threads created, Core 2 Duos that seem to confuse people:

E6750 showing at 2.0 GHZ

Kentsfield Q6600 Core Speed only 1.86 Ghz

Can someone tell me why the 1.87GHz is next to the 2.66Ghz?

e6750 with a mind of it's own...

Newb builder, why is my CPU not running at its speed?

Bios Info:

Speed Step is also known by C1E and EIST (Enhanced Intel SpeedStep Technology).  You can find these setting in your bios.  Not all bios are the same, but usually have both C1E and EIST as separte functions.
 
To disable speed step completely, you will need to disable both C1E & EIST.  Also, when EIST is disabled, you should be able to adjust the CPU ratio or multiplier higher or lower, which will force it to stay at that specified multiplier, and specified vcore.  So on a CPU that has a multi of 10, the choices will be 6-10.  You will not be able to go higher then the original stock multiplier, unless you brought the Extreme Edition Intel CPU.
 
I've done some tests that Zorg has offerred to me to try.  Now with C1E disabled, and EIST enabled, I can go into windows and bring up CPU-Z to see what is going on.  When you bring up Power Options from the control panel, and switch it to "Always On", CPU-Z should show the stock multiplier even at idle and the vcore should be at stock voltage (1.325v).  And when you switch it to "Minimal Power Management" then CPU-Z will drop the Multiplier to its lowest, and the vcore should drop below stock voltage.  I've tried it with my system OC'd and it still works fine.

Screen Caps on my Dad's OC (2.4ghz) E4300 (1.8ghz Stock Speed) with Speed Step Enabled:

At Idle - 1.6ghz (x6 lowest available multiplier)

Load - 2.4ghz (x9 stock or default)

If you check out the hyperlinks, you will find AMD Cool-n-Quiet links to check out.

<<<<<<<<<EDIT BY MODERATOR: ADDED LINK TO CompuTronix's POST C2D/C2Q TEMP GUIDE>>>>>>>>>

Core 2 Quad and Duo Temperature Guide

To make this work I had to change the power profile in windows.  I usually have it at "always on" but that will prevent C&Q from working.  I have to change it to "minimal power management" if I want C&Q to work.  
 
One other thing.  After I enabled this, I loved the lower power usage resulting in a cooler chip.  I liked this feature during the summer.  I eventually had to disable this as it was causing issue while gaming.  At first I couldn't figure out why Team Fortress 2 was crashing so often.  After I disabled this, the problem was gone.  You might want to mention that if you have any stability issues, put it back the way it was.

Since I don't have an AMD chip that uses Cool-n-quiet, it would be prolly best for someone who uses it to make another thread on it (requesting a sticky), with links to the software or what not.

All I know is that SpeedStep is similar to C&Q, so I have no known experience using it.

The main purpose of this thread, is to explain why C2D may be seen as running below the stock speed, instead of seeing new post on the same thing over and over.  I did use the search and found quite a few.  I thought the ones I listed was enough.  

I pulled this from my other post. Tests made on a Gigabyte GA-P35-DQ6.
 
BIOS Setting: EIST Enabled C1E Disabled
In XP you need to set Control Panel -> Power Management ->  Minimal Power Requirements to enable EIST.
EIST only works with the stock frequency - no overclock.
It will adjust the multiplier and Vcore.
It will adjust only the multiplier if the Vcore is set to manual in the BIOS.
Right click My Computer and click Properties will give a current snapshot of the CPU frequency under the General Tab.
 
BIOS Setting: C1E Enabled EIST Disabled
No settings in XP are required.
It will work at stock frequency and overclock frequencies.
It will adjust the multiplier and Vcore at the stock frequency.
It will adjust only the multiplier if overclocked or the Vcore is set to manual in the BIOS.
Right click My Computer and click Properties will NOT give a current snapshot of the CPU frequency under the General Tab.

Anyone tell me why this didnt get stickied? had another thread opened
http://www.tomshardware.com/forum/247093-28-speed
 
That this sticky worth may have helped eliminate.

Well, it's impossible to get rid of it all. Just like the C2d overclocking guide, many noobs still post questions on how to overclock. Reducing the amount of question is better than nothing.

Anyhow just one more suggestion...add "CPU-Z showing incorrect speed" to the title or something like that. The link that chookman provided gave me the idea.

I vote for sticky!

EDIT: One final suggestion, I promise Add this to the overclocking section and get it sticked there too. Some noobs might post there without looking here.