gre tunnel mru

RouterOS General
1

how can I put on gre mru?
mtu put without problems.
there are problems with http download


[xxx@MikroTik] /ip firewall> /interface gre print
Flags: X - disabled, R - running
1  R name="gre-2" mtu=1420 l2mtu=65535 local-address=8.8.8.8
      remote-address=9.9.9.9 dscp=0

[xxx@MikroTik] /ip firewall mangle> print
Flags: X - disabled, I - invalid, D - dynamic
 0   chain=forward action=change-mss new-mss=1420 passthrough=yes tcp-flags=syn
     protocol=tcp tcp-mss=1421-65535
2

is it possible?
support at mikrotik none. 2nd time I face, cases yourself shut, do not say anything