Here’s the latest reader question, along with my reply!
AO writes: I have been considering a new BMW 740i sedan (which comes with a compact spare). I want to carry a full size spare (on alloy wheel) in the trunk (I believe it’s 19 cu. ft.). I heard the D-rings in the trunk floor might not be strong enough to secure it safely. Also, I was told in a rear crash it could force the tire into the passenger compartment or inertia could make it dangerous, too. I saw your old 2018 post on this subject, but didn’t see where you’d written on it since. Any updated information or opinion you may offer would be much appreciated as I can’t seem to find reliable guidance on whether it’s truly risky to do or not. Thank you for any help you may offer and your fine website.
My reply: There are pros and cons to consider. The obvious con is that you will lose some trunk space if you use the trunk to carry a full-sized spare. The current (2025) BMW 7 comes standard with 20 inch wheels. These are big wheels. Before you come to a decision about whether to carry around a full-size replacement wheel for this car, I suggest test-fitting a full-size wheel to see what you’ll be left with, in terms of space.
The next issue is the one you’ve already mentioned – i.e., properly securing the wheel. You don’t want a wheel that big (and heavy) loose in the trunk. I would not worry much about the tire being forced into the passenger compartment (as a result of a rear-end impact) because that’s pretty unlikely. The impact would need to be extreme for such a thing to happen. But I would worry about the wheel/tire freewheeling – so to speak – inside the trunk as you corner. The weight transfer would be noticeable in terms of handling (as well as the sound) and you might end up with damaged rear quarter panels and so on. The metal skins of new cars are pretty thin and so pretty easy to dent. Very expensive to repair that.
I can’t recall offhand whether the 7 has a well deep enough (and wide enough) to accommodate a full-size spare. If it does, that ought to keep it from moving around. If not, you’ll need to find a way to cinch it down, to assure it does not. You will likely need to do some finagling to get it secured. But – assuming you don’t mind the loss of trunk space – you will never have to worry about being stranded by a sidewall puncture (which inflator kits can’t fix) or having to limp home at 50 MPH on a donut spare!
Hope this helps!
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The other day I did a spare tire test run on my two pre-2010 cars. I could get the rim off the wheel on either car. I am of overage strength, but I wonder how most people are going to change a tire when the rim bolts are so tight?
Another consideration: Make sure that the full size spare is a “factory” wheel if at all possible, the tire pressure sensor is properly “mated” to your car, and that you check its air pressure regularly, lest you get that annoying tire pressure warning light.