In an ideal road, yes. However, there are stops and starts, places where you have to change speed, bumps, etc. While driving technique can help negate the ill effects of extra weight, removing weight cannot hurt. At least, you might not have to downshift as often on steep incline.

My driving is also mostly hilly city driving, so it would have a more significant effect.

Also, gas will be saved on those occasions where you cannot drive efficiently, such as stop and go traffic.

Agreed, those conditions describe when weight can have an effect -- though even then, the amount must be significant to produce measurable results. Removing a 40 pound spare tire and 30 pounds of backseat (commonly discussed items when someone considers weight removal) from a 2,500 pound vehicle won't do the job.

I'm actually going to add weight to my car. Mostly it needs more aero, but I need to increase the coastability. There's a couple downhills I'm used to linking and having to restarted the engine sucks. As long as I don't ever use my brakes though, I feel like extra weight isn't hurting me because I'm regaining that inertia somewhere.

How much weight are you going to add? It would probably have to be a significant amount to help your coast distance as much as you need, and you'll pay in acceleration. It could still help if you really are as good at avoiding brake usage as you suggest; it's probably more efficient to run the engine harder while accelerating and store that energy in ballast than it is to start and run the engine twice when you could have done it once.

It should be easy to test. Take a drive and record the numbers. Throw some heavy stuff in the car and drive it again. Record the numbers.

In my experience, the weight difference between a full tank and an empty one calculates to about 1% per 1% weight reduced. This is averaged from over 800 daily commute data points.

Reducing the weight is going to increase mpg if you keep your driving style the same. It will also lessen the wear on you ball joints/brakes/tires so you reap other benefits as well. It can even help cornering so that will help hypermiling technique of carrying speed thru corners...higher rate.

Look at it this way, less weight means you get up to target speed quicker, and can thus cut your engine earlier and glide. ;)

it also means you will slow down faster.

Only because your car is storing less energy, and thus has less to give up to aerodynamic drag. If you're driving slow enough that aero drag isn't the primary loss, I'd say that less weight definately wins.

And, less weight means less drag from the bearings, which is a loss of energy.

in theory it is less. but i believe the difference is negligible compared to other causes of friction like the tires and aerodynamics even at 35mph.

people always say that aerodynamics dont really take effect until higher speeds, but that is not necissarily true. ever ride a bicycle at 20mph? i believe people confuse the generation of downforce with the generation of drag.