My grandparents in Toronto, Canada merely drive in the GTA, and don't need their spare tire in their car's trunk, as they don't know how to replace a tire and would merely call roadside assistance if a tire is marred. So they're contemplating sacrificing their spare tire to protect the environment and use less fuel.
I mentioned in a comment that this question isn't really answerable, and I have to say that I stand by this - we can't give a meaningfully accurate answer to this question as there's too many variables to consider.
But hey, it's a Friday and I can provide an approximation - along with a calculation that the OP can carry out themselves if they want to see how pointless this idea is.
For this example I'm going to use the following assumed figures:
Current fuel economy = 10 l/100km
Current weight of the vehicle = 3,000lb (typical small-midsize car would be about this)
Current weight of the spare tire = 25lb (probably about right for a space-save "donut" or a small-midsize "full" spare)
An approximation for calculating this is to use the following formula:
(MPG2/MPG1) = (WT1/WT2)^0.72
- MPG1 = current fuel economy in MPG
- MPG2 = the "new" fuel economy in MPG
- WT1 = the current weight of the car
- WT2 = the "new" weight of the car
So, converting the current economy to MPG gives us 28.25mpg, and taking the weight of the spare off the current total weight of the car gives us 2,975lb.
Our equation now looks like this:
(MPG2/28.25) = (3000/2975)^0.72
which means that:
(MPG2/28.25) = 1.0060433274267437981760681519655
MPG2 = 1.0060433274267437981760681519655*28.25
MPG2 = 28.420723999805512298473925293027
Let's convert that back to l/100km... and we get 9.94 l/100km. Which is a truly spectacular saving of 0.06 l/100km.
Or, in technical terms - naff all.
Even if we had accurate figures for the particular car in question this would still only be an approximation - although hopefully it's illustrative of the fact that this really, really isn't worth it. Why not? you might ask - "every little helps and all that!" but that ignores the fact that removing the spare and just relying on roadside assistance is a stupid thing to do
Roadside assistance don't carry around suitable spares for all types of vehicle so typically they will swap a spare on where the driver of the vehicle is unable to do so. In the event that you don't have a spare then you're stuck either a) getting the car recovered to a garage for a new tire or b) paying what is usually a substantial premium to have a new tire fitted at the roadside.
It happens, I know - best will in the world sometimes you just get caught out without a suitable spare (been there, done that - having to wait beside the motorway in snow and sub-zero temperatures for a few hours while they tracked down a suitable replacement tire and mobile fitting van was so much fun). Heck, some cars these days don't even come with a spare, but intentionally putting yourself in that position when you've already got a perfectly good spare and are just trying to save a minuscule amount if fuel? Pure idiocy.