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Old 07 December 2010, 07:10 PM
dewey dewey is offline
 
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Default Highway miles are better for a car than city miles?

I have always heard it said that highway miles were better for a car than city miles. So if a used car was 2 years old and had 50,000 miles would it be in better shape than a 2 year old car with 25000 miles. Or is the premise just an urban legend made up by auto salespersons.

dewey
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Old 07 December 2010, 07:29 PM
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I think it's a factor, but not as cut and dried as all that. Someone can mess a car up with rough driving and a lot of highway miles. The excessive starting and stopping with city driving definitely wears the engine and transmission faster though.

Just an anecdote, but I put a lot of miles on my cars, almost all of them city. My daughters car (which used to be mine) is well over 300,000. Mine is well over 200,000. Both are more than ten years old and still in remarkable condition. They are both Toyotas too.
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  #3  
Old 07 December 2010, 07:36 PM
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There are two underlying theories,AFAIK. One is that it is stops and starts that puts stresses on a car - and most anything else. It's kind of like how hard 'high-impact' exercise can be on a body - running rather than brisk walking. All that stopping and starting in city or rush hour traffic flexes the joints and weakens a tiny bit every time. The stopping puts wear and tear on the brakes directly.

The second point is that running a car when it is cold causes a little moisture to condense inside the engine, which can cause rust. If you drive it enough to thoroughly heat the engine, the moisture will be evaporated. Either city driving or highway driving will thoroughly warm the engine though, if the engine runs long enough to get hot. It is very short drives, less than a couple of jmiles or so, that allow moisture to form and remain.
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  #4  
Old 07 December 2010, 08:09 PM
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I think the point is, in theory, a 2 year old car that had 50,000 highway miles would be in better shape than a 2 year old car with 50,000 city miles.
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  #5  
Old 07 December 2010, 08:48 PM
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Quote:
Originally Posted by A Turtle Named Mack View Post
It is very short drives, less than a couple of jmiles or so, that allow moisture to form and remain.
The moisture tends to condense in the oil as well. Plus, on older cars, the engines would get carbon buildup. It's one of the reasons that the low mileage car "owned by a little old lady who only drove it to the corner store, church, and bingo on Tuesdays" is often a bad buy. The engines are all carboned up.

-RB
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Old 07 December 2010, 09:07 PM
dewey dewey is offline
 
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Quote:
Originally Posted by Stan The Man View Post
I think the point is, in theory, a 2 year old car that had 50,000 highway miles would be in better shape than a 2 year old car with 50,000 city miles.
Yes, but that is not the choice I am giving you. Assuming all else is the same would you buy a car with 25000 city miles or 50000 highway miles. I realise that it is not that simple but I am setting up the theoretical parameters.

The business of the moisture condensing in the engine is quite interesting. I was not aware of that.

dewey
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Old 07 December 2010, 09:12 PM
overyonder overyonder is offline
 
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With fuel injection, the "carboning up" of engines is a thing of the past.

OY

Quote:
Originally Posted by Roadsterboy View Post
The moisture tends to condense in the oil as well. Plus, on older cars, the engines would get carbon buildup. It's one of the reasons that the low mileage car "owned by a little old lady who only drove it to the corner store, church, and bingo on Tuesdays" is often a bad buy. The engines are all carboned up.

-RB
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  #8  
Old 07 December 2010, 09:14 PM
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To make that choice, I would have to know that the highway miles were "gentle" meaning done at mostly legal speeds on easy roads and not up the AlCan highway.

Assuming that the operating hours of each car were about the same, I would probably take the highway car even with the higher mileage.

ETA: Re-thinking that. The mileages are too close for the city car to have been much abused. I would go with the city car and the lower mileage.

ETAA (edited to add again ) I would probably go with the lower cost vehicle.
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  #9  
Old 07 December 2010, 09:16 PM
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Quote:
Originally Posted by Roadsterboy View Post
The moisture tends to condense in the oil as well. Plus, on older cars, the engines would get carbon buildup. It's one of the reasons that the low mileage car "owned by a little old lady who only drove it to the corner store, church, and bingo on Tuesdays" is often a bad buy. The engines are all carboned up.

-RB
I would think this is less a problem in modern cars that can automatically adjust the mixture levels depending on the conditions. Also, modern lubricants will stick to the moving parts a lot longer than in the days of yore. It used to be that when the oil was cold, it drained into the sump and took a few minutes to get round the engine again on start-up. Modern oils will take a lot longer to drain away.
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  #10  
Old 07 December 2010, 09:19 PM
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How can you guarantee the way miles are put on a vehicle?
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  #11  
Old 07 December 2010, 09:24 PM
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Assuming all else the same, I think I'd opt for the 25,000 city miles.
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  #12  
Old 07 December 2010, 09:28 PM
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Chloe, a look at the tires would help. Lots of city driving would cause them to be worn down more, or be new. Less worn tires could indicate more highway miles and less city miles.
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  #13  
Old 07 December 2010, 09:29 PM
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The salesperson is always going to claim that high mileage cars have mostly 'highway miles'.

Cannon 'Yellow primer?' Fodder
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  #14  
Old 07 December 2010, 09:30 PM
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My Dad told me the most efficient speed for a car is 70mph. I think I saw someone do some maths that proved it.

Anyway, highway miles: little break wear, few gear changes. (Eta: and your steering rack and ball joints will have less wear too)

I'd still pick the lower milage car though, as I can't believe that it has that much of an impact.
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  #15  
Old 07 December 2010, 09:40 PM
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Quote:
Originally Posted by redspider View Post
My Dad told me the most efficient speed for a car is 70mph. I think I saw someone do some maths that proved it.
By observing the gas usage gauge in my car, the most efficient speed is 65 on a level road. It's the speed at which it travels by maintaining lowest RPM at the highest gear. I can drive at 40-50 mpg if I can stick to 65

Of course, that speed changes if I'm going down slope or up slope, because the car can travel faster on a down slope using the same gas.
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  #16  
Old 07 December 2010, 10:02 PM
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My car is the same way. 65-70 is its sweet spot, and it likes 80 more than 50, and the mileage shows it. In classic stop/go city driving, I get 21-22mpg, while in highway driving I get a steady 30-31.

The city driving problem worsens in winter, when there are the same short trips compounded by a cold engine for the start- there, 16-18 is normal enough.
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  #17  
Old 07 December 2010, 10:17 PM
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Also, cold oil is supposed to be bad for an engine (at least according to Castrol adverts) so short trips (as implied by city miles) are worse than longer ones.
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  #18  
Old 08 December 2010, 04:21 AM
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Well, cold oil* is thicker than oil at it's operating temperature, so it's simply not flowing as well. Being gentle for a couple of minutes after starting is-in my opinion at least-a good idea. But it's the same thing if you only drive very short trips-the oil temp never gets to operating temperature (you can't judge from the temp gauge in your car, as it's usually based on water temp.).

-RB

*by cold I mean at ambient temperature, and not necessarily "cold" as in it's ten degrees F outside, although obviously oil flows even worse when the temperatures are extremely cold.
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  #19  
Old 08 December 2010, 06:07 AM
Troberg Troberg is offline
 
 
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Well, highway miles are better for the car, especially the transmission, brakes and suspension, but it's not important enough to be a major factor in the decision of which car to buy. If a used car has enough mileage on it for it to make a difference, other factors are bound to have developed that are more important, such as bumps, worn upholstery, non-working central locks, quality of paint, rust and other issues typical to used cars.

Quote:
Originally Posted by redspider View Post
My Dad told me the most efficient speed for a car is 70mph. I think I saw someone do some maths that proved it.
It depends on the car, of course. Engine, aerodynamics, gearbox and so on.

As a rule of thumb, a four cylinder engine is most comfortable working at around 2000 rpm. An eight cylinder likes 1000 rpm and so on. For efficiency, the highest gear is the best. So, drive at the optimum rpm in the highest gear and see what speed that makes.

This is all for four stroke engines. Two stroke engines are very different, as are more exotic designs such as the Wankel or Stirling.

Quote:
Originally Posted by Mad Jay View Post
By observing the gas usage gauge in my car, the most efficient speed is 65 on a level road. It's the speed at which it travels by maintaining lowest RPM at the highest gear. I can drive at 40-50 mpg if I can stick to 65
Don't trust the gas usage gauge to have any kind of precision. It doesn't measure the actual gas usage, instead it measures a secondary parameter (intake manifold pressure) which has a pretty good correlation with fuel usage, but not under all circumstances. For example, when starting cold so that the engine has to use choke, the value is way off.

It's an OK rough estimate, but you need to understand what it measures and how that relates to what it shows.
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  #20  
Old 08 December 2010, 09:23 PM
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It's absolutely true that "highway miles" are easier on a car than city miles. Consider this.

- for every mile of city driving, there will be more stoplights or stop signs, meaning more use of the brakes, which means more wear of brake pads and brake drums

- because of more stoplights and stop signs, there will be more acceleration and deceleration - and that means more shift operations for the transmission

- more acceleration/deceleration means more time that the engine spends at higher revs, which creates more heat and more wear than at lower revolutions

- "city miles" are at a lower speed, plus time stopped at lights at idle, means more hours of engine operation for a certain number of miles travelled - some engine auxiliary components such as the alternator, fuel pump, water pump, power steering pump, and cooling fan will all wear out because of hours of operation, and not just mileage - the car can be at idle and travel zero miles, but the engine and its components still suffer from wear

Consider driving for 20 minutes - on a highway with a speed limit of, say, 60mph, you'd travel 20 miles, or maybe 18 miles with the time it takes to get to the highway at lower speeds. You'd maybe travel a third of that or less in the city. True enough, tires and suspension components don't wear with less travel, but the engine does, and that's the real "age" issue on cars.
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