Good question, I copied this response from Quora, but looks legit to me, feel free to do more digging as required. The equation doesn’t copy well here, but you can Google it and find the answer with the equation intact.

Let’s assume that you have a 1500 kg car including baggage, driver, etc. You want then to add another 65 kg passenger. Since fuel consumption is highly dependent on driving style and on the routes you take, let’s also assume that your average fuel consumption is 6.0 L/100 km, considering no extra passengers. The last assumption I’d make is that fuel consumption increases linearly with car weight, if the weight deviations are low.

We can use a simple proportion to estimate the fuel consumption with the added weight, considering all of the above assumptions:

(Removed equation would be here)

That is a 4.33% increase on fuel consumption.

These assumptions may not be very accurate, but personally I believe that this estimation is a worst-case scenario.