* Tow (or haul) more than the maximum-rated capacity
Ugly things can happen if you do. Examples include hitches/mounting points actually coming apart - and your load coming loose at 65 mph. Or overloaded brakes failing as you make your way down a steep grade. Beds/trunks weighed down by whatever you've crammed in there can compress the rear suspension dangerously - creating an evil-handling, ready-to-wreck ride that you'll regret ever having set in motion. And even if nothing immediately awful happens, you're still placing extreme stress on your vehicle that will result in rapid wear and tear of parts, which means they'll likely need to be replaced sooner than they otherwise might have. Burning up a clutch, ruining an automatic transmission or blowing out your shocks/struts can end up costing you a lot more than what you would have spent to rent/borrow a vehicle with the capacity you needed. Or just use yours - and make two trips with two smaller/lighter loads.
* Keep your 4WD in 4WD on dry, paved roads
If you have a truck or SUV with a part-time 4WD system, the majority of your on-street driving should be with the selector in 2WD. The other ranges - typically, 4WD High and 4WD Low - are not intended for use on dry pavement. Driving on smooth surfaces with 4WD engaged (and especially going around corners on dry, smooth surfaces with 4WD engaged ) can damage your 4WD system and leave you facing costly repairs. 4WD High range should only be engaged when there is snow on the road - or you've left paved roads entirely. 4WD Low should never be used on paved roads unless you are trying to make your way through very deep, unplowed snow. Otherwise, save it for driving off-road, at low speed, through deep mud, sand or on a very rugged dirt trail. 2WD should be re-engaged as soon as you're back on dry pavement/smooth roads. If your pick-up or SUV has a locking differential(s), be certain you only use it when the vehicle is moving forward or reversing - and not going around a corner. When cornering, the inside and outside wheels turn at different rates; the differential allows them do so without binding up. But when the differential's locked, the axles can bind - even break - if the stresses are severe enough. If that happens, you'll be stuck - and looking at a very big bill.
* Use a different weight (or grade) of oil than specified by the manufacturer
This is especially important with modern engines, because they are built with much closer tolerances than the engines of the past. Running a heavy oil (say, 20W-50) in an engine designed to use 5W-30 can increase friction inside the engine, reduce fuel efficiency - and possibly even lead to more serious problems that may not be covered by your new car warranty. The same also goes for American Petroleum Institute (API) ratings. If your vehicle's manufacturer says to only use oil with at least an SM rating (for improved oxidation resistance, protection against deposits and better low-temperature performance) and you cheap out by using a lower-performance SH/SG oil (for 1996 and older engines) you found on sale at Budget Auto Supply, you not only risk possible engine trouble - you risk losing warranty coverage for any oil-related failure that occurs. (See http://www.carmild.com)
* Use your transmission (manual or automatic) to "park" the car
The proper way to park your vehicle is to engage the parking brake before releasing the foot brake or putting the vehicle in "Park" (automatic-equipped vehicles) or releasing the clutch with the transmission in gear (manual-equipped cars). This way, the parking brake holds the weight of the car - not the transmission. If you don't do it this way, you risk breaking parts inside the transmission (expensive) or (less expensive, but annoying) finding it's hard to get the shift lever out of "Park" when you want to get going again. People have become stuck this way - and had to call AAA. Or had to arm-wrestle with the shift linkage for awhile before they were able to free the car. It's something that should never happen - because it's totally preventable.
* Drive fast (or far) on a space-saver spare
Many new cars come with temporary use only "mini-spares" designed for just that - temporary use only. They are not designed to let you continue driving as you were before the flat; they're designed only to let you keep on driving - at reduced speed and extra carefully - to the nearest service station. Most "minis" have warning labels that caution you not to exceed 55 mph or drive more than 100 miles (at the outside). The warning label will also usually tell you that your car may handle differently (read: less well) and that braking performance will be reduced (read: you will need more time to stop safely). The idea is to limp along to the tire store - grateful for any forward motion at all. Don't push your luck - or expect a temporary-use-only "mini" to do what a normal tire can do.
* Use tap water to top off your radiator (or battery)
Tap water can have impurities in it that you may not be able to see with your naked eye but which are nonetheless bad news for your car's cooling system. Unwanted chemical reactions can take place inside expensive-to-replace radiators and alloy engine parts. If you need to top off your car's cooling system, you should use distilled water - mixed in a 50-50 ratio with the appropriate anti-freeze (standard "green" or long-life "orange"). Never exceed 70 percent anti-freeze in the system. Top off the overflow reservoir - not the radiator itself (unless the vehicle has been sitting for at least a few hours and had time to cool down). If you do remove the radiator cap, always use a large towel to avoid being scalded by escaping steam/coolant. If you're stuck in the middle of nowhere and distilled water's not available, look for bottled water - it's at least filtered and should be more pure than ordinary tap water. Don't ever use bubbly mineral water under any circumstances.
