Hi,
I would take care when using a extension lead with a 12v tyre inflater. My Halfords pump works fine when connected direct to the 12v socket on the car.However when used with a extension cable the pump power drops off,the plug gets hot and after 30 secs the fuse blows.
I admit the extension is not one from Maplins but it is 3m long and rated at 5amps and of good quality.
Regards
Nidge