From the Federal reserve Bank of San Francisco
http://www.frbsf.org/education/activities/drecon/2002/0206.html Look at chart 2 and the explanation.
They have a much more in depth article which, I will look for that explains this in greater detail.
There is no fixed spread between the ten year note and the 30 year fixed mortgage. The term premium will expand and shrink based on many factors.
And More,
http://library.hsh.com/read_article-hsh.asp?row_id=85
Mortgages are priced for sale to attract investors who seek fixed income investments. There are many kinds of bonds available, and mortgage rates (yields) rise and fall with those competing investments to a greater or lesser degree.
But how to price them? Fixed mortgage rates, like other bonds, track US Treasury bonds quite well. Since Treasury obligations are backed by the "full faith and credit" of the United States, they are the benchmark for many other bonds.
There is no specific "lockstep" relationship between Treasuries of any term and fixed mortgage rates. Given enough data points, a relationship could be established against many different financial instruments. However, as a 30-year fixed rate mortgage rarely lasts longer than about 10 years before being paid off or refinanced, the closest instrument which has similar (though lesser) risks is the ten-year Treasury Constant Maturity. Because of this, the ten-year year Treasury makes an excellent tool to track mortgage rates.
Here's an oversimplification of the relationships of mortgages to Treasuries:
As we mentioned, intermediate term bonds and long-term mortgages (more properly, Mortgage-Backed Securities, or MBS) compete for the same fixed-income investor dollar. Treasury issues are 100% guaranteed to be repaid, but mortgages are not; therefore mortgages carry more risk of default or early repayment, which could potentially disturb the return on the investment. Therefore, mortgage rates must be priced higher to compensate for that risk.
But how much higher are mortgages priced? In a normal market, the average "spread" or markup above the 100% secured Treasury is about 170 basis points, or 1.7%. That markup -- the spread relationship -- widens and contracts with a range of market conditions, investor appetites and supply of available product -- as well as the presence of competing investment opportunities, like corporate bonds or domestic (or foreign) equity markets. Professional money managers, and investment and retirement funds constantly strive to obtain high-yielding instruments at a given level of risk. Money shuffles from place to place in search of this -- from bond to bond, and market to market.
As we mentioned, the relationship isn't a fixed one, but one that changes with market conditions. Recently, for example, ten-year Treasuries rose from a low of 4.22% to 5.01% over a three-week period -- about 80 basis points, altogether. At the same time, the average 30-year fixed mortgage rate rose from about 6.59% to 7.21%, a rise of only 62 basis points. Over time, there are any number of examples where Treasury yields have risen faster than mortgage rates, as well as times when mortgage rates rose faster than Treasury yields. Consequently, the spread between the two expands and narrows appreciably, which is why you can't simply take the ten-year yield, add 1.7% to it and know exactly what today's rate is.
And More,
http://www.investopedia.com/articles/pf/07/mortgage_rate.asp
Fixed Interest Rate Mortgages
The interest rate on a fixed-rate mortgage is fixed for the life of the mortgage. However, on average, 30-year fixed-rate mortgages have a lifespan of only about seven years. This is because homeowners frequently move or refinance their mortgages. (To read more about refinancing your mortgage, see The True Economics Of Refinancing A Mortgage and Mortgages: The ABCs Of Refinancing.)
Mortgage-backed security prices are highly correlated with the prices of U.S. Treasury bonds. This means the price of a mortgage-backed security backed by 30-year mortgages will move with the price of the U.S. Treasury five-year note or the U.S. Treasury 10-year bond based on a financial principal known as duration. (In practice, a 30-year mortgage's duration is closer to the five-year note, but the market tends to use the 10-year bond as a benchmark.) This also means that the interest rate on 30-year fixed-rate mortgages offered to consumers should move up or down with the yield of the U.S. Treasury 10-year bond. (A bond's yield is a function of its coupon rate and price.)
More from the Federal Reserve,
http://stlouisfed.org/publications/re/2004/b/pages/fed_said.html
Mortgage interest rates dipped to record lows early last summer, providing homeowners with a refinancing bonanza. This decline in mortgage interest rates mirrored a fall in the 10-year Treasury bond yield (the interest rate on the bond), as shown in the top panel of Figure 1. In fact?as shown in the bottom panel of Figure 1?mortgage interest rates almost always mirror the yields on long-term Treasury bonds because they respond to the same forces.