I chuckle when some people suggest buying a second car is saving them money overall (by spending less on gas). This is the same logic my wife uses when she gets a 20% coupon in the mail but only if you spend $200 do you qualify "I saved $40 honey", "um yeah but you spent $160! haha"...
In other words, you're not saving money by buying another car, you're spending money. Likely more than or equal to the same amount as if you just drove the 4Runner with whatever gas prices (unless of course you have some ridiculous 100 mile commute each way to work).
That's not entirely true, it depends on how much you drive and how much you spend.
I currently have a DD that is a Mazda B2300 pickup I bought for $2000 in 2013 (basically a 4 cyl Ford Ranger with a nicer interior and a slightly different grille.) It gets between 24 - 26 MPG driving to work (about a 25 mile round trip.)
I'll be honest and admit that it doesn't necessarily "save money" by the time you factor in the cost to buy, to maintain, register, insure, etc a second car.
However, there are some tangible benefits that can save money, for example, the tires on my DD cost $48 each. The BFGs that I put on my 4runner cost $213 each. Having a DD not only saves me a bit on gas (in Winter my MPG tends to drop to around 15 vs 25 for the DD) but it also keeps me from grinding my expensive tires down going to and from work.
Also I can have the 4runner torn down for work or at the shop and not have to worry about borrowing or renting a 2nd vehicle.
Having a small truck as the 2nd vehicle means that if I need to pick up some lumber, cement, fertilizer, wood chips, move furniture, etc, I can use my beater Mazda instead of my nice 4runner. And if the Mazda gets a dent in it, so what (believe me, it's got a bunch in it now!)
Finally, by having a DD I'm not running up the mileage on my 4runner to get to and from work, and that means that when it comes time to sell, I'll be able to get more for it because most people would pay more for a vehicle with 140,000 miles on it than they would on a vehicle that has 210,000.
Previously my DD was a little 4 door sedan, a Protégé that had 208,000 miles on it when I bought it for $900. Drove that for 2 1/2 years and although I ended up having to put an engine in it (at a cost of about $2k) it was still running great when I sold it in 2013. And it got 35 MPG on a pretty regular basis.
I realize the 2 vehicle solution doesn't work for everybody. If you live in an apartment where parking space is tight, or if you live in an area where rust eats cars alive and anything more than 10 years old is a rusty pile of crap, then the "buy a cheap beater" plan doesn't work. But it works for me and I enjoy both saving $$ at the pump and not putting excessive miles on my 4runner just getting to and from work. :shrug:
EDITED TO ADD: Forgot the other nice thing about having the DD. It has a manual transmission, more fun to drive.

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