My gas gauge doesnt work, do you think its dangerous to keep a 1 gallon gasoline canister in my trunk with gas?

I tried to rely on my trip odometer but it proved to be very unreliable as the mpg on my car varies widely between highway, city and if I use air cond. or not etc. So its impossible to rely on that as I already ran out of gas trying to do so. So do you think keeping a gallon or half a gallon gasoline in my trunk is a solution, or is it too dangerous it these hot summer days? (I have a special container for gasoline does that make any difference? )