The same way I save for everything . . . put my money into investments at the asset allocations I've decided are appropriate. When I need the money I sell some of those investments at the same asset allocations.
Putting money into a savings account is most likely to lose you money because you aren't earning a return.
I think that the conventional wisdom is that savings accounts can be useful vehicles for large (inflexible) purchases that are required in the short term.
The trade-off here is stability vs. long term appreciation. But if you need the money in the short term, the long term appreciation doesn't matter as much.
I think that historical data says that you usually finish ahead in the stock market if your time horizon is greater than ~10 years. If you're thinking about a 5 year horizon or less, savings accounts might be an attractive choice, no?
Depending on average annual mileage and expected lifetime, the van with 160k miles on it may not have another 5 years in it.
Also, if you live in a place that salts the roads in the winter, 12 years might be getting close to the age at which your frame will start to show considerable rust. That might mean the car with 85k miles might have less life than you'd guess based on mileage alone.
Ultimately, it would suck if that $10k went up 5% this year, then down 25% next year right when your van died.
I think it's best to focus on savings rate here and put money earmarked for large short term investments in a high yield savings account (even though you'll just get ~ inflation).