Then again 40mpg at 70-75 doesn't sound so bad for the age of the car, with a carb and the close-set gearing. It's possibly the carburettor that's making your consumption say consistently "low" (try comparing it to an american 4x4, i suppose
(this is why commuting by road is so inefficient, costly, and polluting a lot of the time, if you're not going far... and why widespread hybridisation / adoption of electric power could be a very cool thing)
(I dread to think what my economy is on the morning home->station dash in the astra, even though it's injection... coming back all chilled and not even exceeding 30mph probably isnt much better. It helps that it's only about a mile if not less..... so even if it's all of 10mpg, that's only about a gallon a week, which beats being late - and even though its 100% cold all the way, its not going far, so it wont be hurting much)