I have a inflatable kayak in the back of my car (mitsubishi mirage) and plan on adding a foldable bike soon. If I don’t carry them in the trunk I’ll shave about 55 lbs of weight. Is this weight savings enough to make an impact on my gas mileage?

It’s not going to make a massive difference, but any extra weight in the car will use more fuel to pull around, especially when accelerating. It’s prudent to remove anything unnecessary from your car if you’re even slightly bothered about the amount of fuel it uses.

It’s going to make a small difference, but exactly how big that difference is will depend on a lot of things, like the speeds you travel at, how much you accelerate and decelerate (mostly highway driving will be at a constant speed, city driving will be a lot more stop and go), how heavy your car is, etc.

I did a quick and dirty calculation based on the assumption that mpg and weight have a roughly linear relationship. Assuming a 4,000 lb car gets 15 mpg and a 2000 lb car gets 30 mpg, dropping 55 lb from a 2,000 lb car results in the mileage increasing from 30.0 mpg to 30.5 mpg. The real world doesn’t exactly work that way, but that should give you a rough ballpark idea of the magnitude of your mpg increase.

The key to this is the amount of braking you do. The heavier car will use more fuel to accelerate to a given speed, but will also coast further as it dissipates that speed. Whereas if you use the brakes, energy of motion simply become heat.

IOW, if you can avoid braking, the extra weight has little effect on accelerate/decelerate efficiency. In the real world, some braking is of course unavoidable.

The other loss is from the extra drag attributable to the extra weight. This is mainly due to increased distortion of the tires, which means more force is needed to maintain a given speed.

Here’s a useful article: How does weight affect a vehicle’s efficiency? It gives an EPA rule of thumb that decreasing weight by 100 lbs gives a 1 - 2% improvement in fuel efficiency. It also has a table that gives detailed numbers.