let $a/k$ be an abelian variety over a global field $k$. for each place $v$ of $k$, one associates an integer $c(v)$ called the tamagawa number of the place, using the reduction of the abelian variety at $v$. let $c$ denote the product of the $c(v)'s$. let $t$ denote the order of the torsion subgroup of mordell-weil group $a(k)$. the ratio $c/t$ is a factor in the leading term of the l-function of $a/k$ at $s=1$ predicted by the conjecture of birch and swinnerton-dyer. we investigate in this talk possible cancellations in the ratio $c/t$. for elliptic curves over $q$. the smallest ratio $c/t$ is $1/5$, obtained only by the modular curve $x_1(11)$.