Do you think I would take a performance hit if I replace my steel wheels with stock 16” alloy wheels from a Golf SE? Seems like the alloys may be lighter than the steel wheels.
1. Mustang guys don't care about mpgs, we shouldn't be worried that much about performance.
2. The performance difference between steel and alloy wheels isn't going to change destiny. In a speed competition the only difference between alloy and steel wheels of the same size is that the slightly increased rotational inertia of the steel wheels will have to be accelerated up to speed one time. Accelerating the weight of the car dominates the task of accelerating the wheels to speed.
Put another way, the aluminum wheels might possibly make a 250 rwhp car .1 second quicker in the quarter mile. Maybe.
3. The main selling points of aluminum alloy wheels:
* they look sweet. This helps to sell cars.
* they reduce the cars "unsprung weight", thus improving the ride.
* yes, the car will be slightly faster.
4. As has been said, comparing the performance of a larger diameter or wider wheel isn't meaningful. LOL @ the boy racers who don their ride with larger diameter wheels thinking that it will increase acceleration or top speed: they just attached a boat anchor to their car. Wider tires will also slow acceleration due to increased rotational inertia. Both larger diameter and wider tire/wheel combos will have increased rotational inertia, but it will be more pronounced than the aluminum vs steel wheels, since the increased mass of the wider/larger wheels will be at the outside diameter.
5. For increased acceleration and better ride, investigate what the minimum width for safety is for a tire. (Tires have load carrying limits, so while a bicycle tire will give us great ride and top speed, it's going to pop quickly and it would melt the first time we accelerated hard.) Tires are marked with a max weight, and this needs to meet or exceed your gross vehicle weight.
So if you're going to buy some new wheels anyway, find some aluminum wheels 16" dia, make sure the offset and lug pattern is the same, and I'm guessing 185 mm bead would be a good point to start your investigation.
This would be a good time to research tires as well. If you reduce the outside diameter (OD) of your tire, you will slightly improve acceleration, but then your speedo and odo will be off. If you go with a narrower wheel/tire, to keep the tire's OD the same you will need a numerically higher profile. Once you figure all that out, make sure that the tire you need isn't a rare size that has to be $pecial ordered.
Hope this helps.