Susy -- Static gutters using calc() https://github.com/ericam/susy/issues/536#issuecomment-153795940

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@import 'susy';

$susy: (
  columns: 12,
  column-width: 5em,
);

$gutter: gutters(static);

@mixin calc-span($span) {
  $span: append($span, 'wide');
  float: left;
  width: calc(#{span($span)} - #{$gutter});
  margin-right: $gutter;
}

.container {
  @include container(show);
}

.test {
  @include calc-span(3);
  background: rgba(gray, .5);
}

.test2 {
  @include calc-span(4);
  background: rgba(gray, .5);
}

Susy-static-gutter.html

<div class="container">
     <div class="test">hello</div>
     <div class="test2">hello</div>
     <div class="test">hello</div>
</div>