Dear Tom and Ray: Say I have an electric car (one that runs on nothing but electricity) that can be charged by plugging into any outlet. But for cross-country trips or long trips outside my car’s range, I carry a 120-volt Honda generator and a gas can in the trunk with me. Could I just pull over when I need to and charge up the car? Would that work? How much charging from a gas generator would it take to charge an electric car? – John

TOM: John, you’ve just invented the plug-in hybrid! Again!

RAY: This is exactly the idea behind parallel plug-in hybrids, like the Chevy Volt. They run on electricity until the electricity runs out. Then they use a gasoline engine to “generate” more electricity to recharge the batteries or move the wheels.

TOM: So your idea is not far-fetched. However, its execution is. If you do this yourself rather than let General Motors do it, you lose both convenience and efficiency.

RAY: In the Volt, you don’t have to think about it. When the batteries run down, the engine automatically starts up, and you can just keep right on driving – for hundreds of miles – on gasoline power.

TOM: With your plan, you’d have to find a place to stop and run the generator while the car is parked. And you’d have to always carry a loaded gasoline can in your trunk, which is not recommended.

RAY: Plus, your plan isn’t practical right now. The charge time at 120 volts, for an all-electric Nissan Leaf, for example (which goes about 85 miles on a charge), is about 20 hours.

TOM: That means on a cross-country trip, you’d drive for about an hour and a half, then have to stay overnight and run your generator to charge up your car. The hotel bills alone will kill you! That is, if your fellow guests don’t revolt against the noise from the generator.

RAY: You could get a bigger, higher-wattage generator, and cut your recharge time down a bit using a 220-volt outlet. But then you might have to tow that generator behind your car, and that would cut your range to about 50 miles per charge. So there’s no great homemade solution for long-distance electric driving here.

TOM: In time, there will be faster charging systems. And quicker-charging batteries. Or even battery swaps, where you leave your old one at a “filling station” to be recharged for the next guy, and take a fully charged one with you.

RAY: But for now, your idea is on the impractical side, John. But keep thinking!

Rule of thumb about synthetic oil has no truth

Dear Tom and Ray: I was buying oil for an oil change and was planning to use a coupon from the manufacturer. The auto-parts store was out of stock of the partial synthetic blend I use. So I decided to go with the full synthetic, because with the coupon, the price was almost the same. As I was checking out, the sales associate said (and I paraphrase): “Now, remember, once you use full synthetic oil, you have to always use full synthetic oil.”

I looked at him and said, “Why is that?”

He replied, “Well – that’s what they say.”

Personally, I think he was just trying to make sure I spend $10 a quart from now on, instead of half that for the synthetic blend. But who knows? Maybe he’s right! Is he right? – Richard

TOM: I don’t think so, Richard. We heard the same kinds of warnings when synthetic oil first came on the market – not to mix it with conventional (dinosaur-based) oil, or something terrible would happen. But we never saw any hard evidence to back that up.

RAY: And then the manufacturers started mixing the two themselves! What do you think the “synthetic blend” you usually buy is? It’s a blend of synthetic oil and conventional oil in the same container!

TOM: So if the manufacturers are blending it together, I don’t see any reason why you can’t do the same thing in your crankcase if you want to.

RAY: So you certainly can go back to the synthetic blend next time. Or, you may want to stay with the full synthetic. It’s great stuff. It is more expensive. But because it lubricates so well and doesn’t break down as quickly as conventional oil, you don’t have to change your oil as often.

TOM: So that means we have fewer quarts of used oil to recycle or dispose of, fewer empty oil containers in our landfills and, not incidentally, less foreign oil we have to import.

RAY: And if you spend $40 on four quarts of synthetic and change it after 10,000 miles, or $20 on four quarts of a blend and change it every 5,000 miles, you end up spending the same amount – on the oil. But you save money on the filter and what you pay Pokey Lube for the labor. And you can skip the tailpipe polishing they inevitably sell you once they’ve got your car up on the lift.

Send your comments or questions for Tom and Ray to: Car Talk Plaza, P.O. Box 3500 Harvard Square, Cambridge, MA 02238. Listen to them Saturdays at 10 a.m. on 91.3 FM or 88.5 FM. Visit them on the Internet at www.cartalk.com.