1991 Ford Festiva suddenly using twice as much fuel

i have had my 1991 ford festiva for about 8 years when i first bought it i filled the tank
and got 450 kms before the petrol light came on i used that as a gauge to measure
how much fuel i needed to get around for the week...over the years the cost of fuel has
gone up massively but now fuel is coming down i have noticed for the economy
doesn't seem to be there so about 3 weeks ago i filled the tank to see how many kms i
got the petrol light came on after just 240 kms

i have replaced the air filter ,spark plugs,distributor and nothing changed so i then did a
DIY check to make sure of no vacuum leaks, there isn't any

i had a mate look at it and he suggested the float in the carburetor so we took it to
some flat ground and checked the float (only by eye luckily the carburetor has a glass
window to see) it looked fine so we gave the carburetor a clean out since we have the
filter etc off

another mate checked the choke was turning off and did an adjustment of the fuel
mixture

after all this nothing has changed the fuel usage

as for the running of the car there is no noticeable lag or loss of power the engine
sounds the same as it always has starts easy etc

it isn't making any sense  

any advise would be appreciated