If you are building a new Environmental Management System and one of your high impacts is the use of vehicles, I highly recommend against using the reduction of petrol consumption as an environmental objective.
It's a pain in the butt to measure and it can also be majorly effected by the growth of your business.
As an example if you run a small manufacturing business and do a lot of deliveries, you may have vehicle use as one of your aspects. You could set reduction of petrol consumption as an objective but if you have a high growth period and need to do more deliveries you aren't going to meet those objectives.
you're better off setting reduction of unecessary travel as an objective which will give you the same result but allow for growth.
The way I have structured this with one of my clients is by printing the most efficient route to each of their major clients using the www.whereis.com website. This tells you exactly how far it is from the office to the client. The driver then writes down their start and end km's on their delivery docket and it can be matched to how far they should have travelled. Set the objective that no trips should exceed 2km's from the designated route.
This way you can be sure there is no unecessary travel and keep fuel consumption to essential travel only.