Q.

I'm going to buy a 2014 Ram Laramie pickup. I have a choice of 17-inch or 20-inch tires. I don't plan to drive off-road much, if at all. I do plan to drive several thousand miles around town and then perhaps 10,000 miles towing a travel trailer that weighs about 7,500 pounds. My thinking is that the 17-inch tires would weigh a lot less and so would provide better mileage around town. They also might be quieter rolling down the road. Another nice feature is that my wife and dog have short legs, so getting into and out of the truck will be easier. What I'm wondering is: What effect will the smaller tires have on mileage on the highway as I tow the trailer long distances at 65 mph?

TOM: You'll get better mileage on the highway, too. I think you're right to lean toward the 17-inch wheels. We're generally opposed to people supersizing their wheels. Or their french fries, for that matter.

RAY: Smaller wheel-tire combinations provide better mileage (city and highway), better acceleration and a quieter, more comfortable ride.

TOM: So why, you ask, would anyone give up all those things and pay extra to get ginormous wheels?

RAY: 'Cause they look cool! Have you seen those 20-inch bad boys on the Ram? I was grunting and growing a forehead ridge after just a few hours of driving around with those.

TOM: The other reason people opt for larger wheels is that, up to a point, they can improve handling. Usually as a wheel gets larger, the tire's sidewall (or aspect ratio) gets smaller, so the total diameter of the wheel-tire combination stays about the same.

RAY: This is so the speedometer stays accurate and, more importantly, so the wheel-tire combination fits inside the wheel well and doesn't scrape!

TOM: And by the way, since the wheel-tire combination usually ends up being about the same size, your dog and wife might not get much help from the smaller wheels in terms of getting into the truck. Definitely get the running boards so that they have a step!

RAY: Or mount a large slingshot in your garage and launch them into the vehicle.

TOM: But in terms of cornering, when a tire has a shorter sidewall, it's stiffer, so you get less flexing from the tires on turns. That's how larger wheels improve handling.

RAY: But that same stiffness is what makes your overall ride harsher.

TOM: And the extra weight of the larger wheels is what cuts into your acceleration and fuel economy.

RAY: And here's one more strike against fancy, colossal wheels: Because of the shorter sidewalls of their tires, the rims are closer to the pavement, so they get bent and damaged more easily by potholes and curbstones.

TOM: The replacement cost for these larger wheels tends to send their poor owners into shock.

RAY: That's why we keep smelling salts next to every lift at the shop.

TOM: So my advice would be to have a look at the truck in both configurations. Sometimes, very small or very large wheels can look out of scale on a vehicle.

RAY: But if you're content with how the 17-inch wheels look on the Ram, that's what I'd go for. Happy travels.

Q.

I manage an RV park but know nothing about diesel engines. Why does everyone who has a diesel start and idle it for 30 minutes before leaving? Even people towing nothing but a small trailer do that every morning before they leave. Why?

TOM: Because they're inconsiderate knuckleheads.

RAY: There's no reason to idle a diesel engine in an RV for 30 minutes before hitting the road.

TOM: In all weather but extreme cold, most diesel-engine manufacturers recommend idling the engine for 10 to 15 seconds before driving away gently. That's SECONDS!

RAY: Now, some RVs have air brakes and need to build up pressure in the braking system before driving away. But that also takes no more than about five minutes. Not half an hour.

TOM: And lots of those RVs have (or should have) what's called an "auxiliary air compressor" on board, which instantly gives the brake system enough pressure and eliminates the need for idling.

RAY: Even if someone left his or her lights on, or otherwise drained the battery, it's still better for the engine to drive the vehicle for an hour than to let it idle for an hour.

TOM: Cummins, for instance (one of the major diesel engine manufacturers), warns in its owner's manuals that excessive idling of the engine can cause carbon to build up on the pistons, piston rings, injector tips, valves and more. Which leads to expensive repairs and shortens the life of the engine.

RAY: So, not only is it not necessary, it's actually harmful!

TOM: So if early risers at the RV park don't care that they're throwing away $4-a-gallon diesel fuel, polluting the pristine nature they've driven a long way to enjoy and annoying their formerly sleeping neighbors, perhaps they'll be motivated to stop this dumb practice by being alerted to the fact that they're actually harming their engines, not helping them. Pass the word.

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