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  #21  
Old 09 December 2010, 01:12 AM
jimmy101_again jimmy101_again is offline
 
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Originally Posted by Troberg View Post
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.
Most of the ones that come on new cars are considerably more complex than that. I believe they use the fuel injector on time and the cars speed to calculate the MPG. There's a crap-load of compute power on a modern car and about a zillion engine and vehicle operating parameters measured every second so there's more than enough data to get a reasonable value for the instantaneous MPG. No need to depend just on something as qualitative (and inaccurate) as the intake manifold pressure.
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  #22  
Old 09 December 2010, 02:46 AM
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erwins erwins is offline
 
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With a 2-year-old car, I would take the one with fewer miles, all else being equal. If there is a price difference, it would depend on what the amount was. I don't think 2 years of city driving--with just an average number of miles per year--would be so detrimental to the car that it would offset the fact that there are twice as many miles on the other car.

Certainly the blue book value of the higher mileage car would be negatively affected, vs. no affect on the blue book value of the other car. (Usually blue book values figure 12k miles per year or so--25k in two years will be about average and most likely won't have a penalty to the value of the car).

erwins
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  #23  
Old 09 December 2010, 01:41 PM
dewey dewey is offline
 
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Originally Posted by erwins View Post
With a 2-year-old car, I would take the one with fewer miles, all else being equal. If there is a price difference, it would depend on what the amount was. I don't think 2 years of city driving--with just an average number of miles per year--would be so detrimental to the car that it would offset the fact that there are twice as many miles on the other car.

erwins
As you must have guessed there are actual cars involved. The price difference would be around $2000. $24,000 vs $26,000. The cars are Volvo S60s with AWD.

dewey
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  #24  
Old 09 December 2010, 01:51 PM
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How do you know that the miles were highway miles?
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  #25  
Old 09 December 2010, 02:20 PM
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NorthernLite NorthernLite is offline
 
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Originally Posted by Chloe View Post
How do you know that the miles were highway miles?
IF the vehicle in question has an information centre then you can check for average speed. For instance, I recently sold a three year old Chevy Cobalt with 18000 km adn an average speed of about 25 km/h. When asked I told the customer that my best guess was that the vehicle was used for in town travel but because I didn't know the original lessor I couldn't say for sure.

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Originally Posted by CannonFodder View Post
The salesperson is always going to claim that high mileage cars have mostly 'highway miles'.
Why would you say that? Unless I know the exact history, I would make no such claim.
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  #26  
Old 09 December 2010, 03:18 PM
dewey dewey is offline
 
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Originally Posted by Chloe View Post
How do you know that the miles were highway miles?
I don't. It is only a best guess on my part. The owner might have been using the car in the city to do deliveries. But that is so many miles over such a short time that I am assuming that it is mostly highway miles.

dewey
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  #27  
Old 09 December 2010, 03:35 PM
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Originally Posted by NorthernLite View Post
Why would you say that [the salesperson would make a possibly untruthful claim in order to further a sale]?
Because all car sales people are without scruples and would do anything to make a sale. Just like all lawyers are evil sharks who prey on the weak and defensless, and all bureacrats are lazy-shiftless workers who would be useless in the real world.
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  #28  
Old 09 December 2010, 03:57 PM
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Ah, yes. Quite right, quite right.
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  #29  
Old 10 December 2010, 07:36 AM
Troberg Troberg is offline
 
 
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Quote:
Originally Posted by jimmy101_again View Post
Most of the ones that come on new cars are considerably more complex than that. I believe they use the fuel injector on time and the cars speed to calculate the MPG. There's a crap-load of compute power on a modern car and about a zillion engine and vehicle operating parameters measured every second so there's more than enough data to get a reasonable value for the instantaneous MPG. No need to depend just on something as qualitative (and inaccurate) as the intake manifold pressure.
Are you sure about that? I've not picked apart a really recent car, so I might be wrong, but the car industry isn't likely to rethink stuff that works, and the people who cares about that gauge are likely to not be technically minded anyway, and could probably have been fooled by a randomly swaying needle, so there is no incentive for them to abandon the cheapest method.

Someone with insight on the matter?
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  #30  
Old 10 December 2010, 06:17 PM
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"Highway Miles" to me sounds like a means for car salesmen to turn a high mileage car into a good thing. Pfaff! Magic!
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  #31  
Old 10 December 2010, 08:51 PM
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Originally Posted by redspider View Post
"Highway Miles" to me sounds like a means for car salesmen to turn a high mileage car into a good thing. Pfaff! Magic!
It's not comfined solely to car salesmen. Customers often use the term to explain while their high mileage vehicles should be worth more than fair market value when being traded in.
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  #32  
Old 10 December 2010, 08:59 PM
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GenYus234 GenYus234 is offline
 
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Technically, they are "selling" their car to the dealership in return for a reduced price on a new car.
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  #33  
Old 10 December 2010, 09:51 PM
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Originally Posted by GenYus234 View Post
Technically, they are "selling" their car to the dealership in return for a reduced price on a new car.
Great Googly Moogly, man!! Do you realize that you are only one of probably four people who I've encountered in the last four years who understand that concept.

People come in and ask me what their trade in is worth. I ask them how much they want for it and they get all coy and shifty eyed then tell me it's my job to tell them how much. And when I check and see what I can buy a similar vehicle for they get all upset because I'm trying to cheat them.
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  #34  
Old 11 December 2010, 12:49 AM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by Troberg View Post
Are you sure about that? I've not picked apart a really recent car, so I might be wrong, but the car industry isn't likely to rethink stuff that works, and the people who cares about that gauge are likely to not be technically minded anyway, and could probably have been fooled by a randomly swaying needle, so there is no incentive for them to abandon the cheapest method.
But it didn't work before. The old style "efficiency meters" were just a vacuum gauge with a relabeled face. Instead of a scale range of a couple inches water (vacuum) they were labeled as "gas-mileage-sucks" to "gas-mileage-OK".

The newer meters (after 1996) tie directly into the car's computer system so they have access to pretty much all of the engine's data. (Which probably includes the intake manifold pressure). But the fuel injector dwell time is also available and is a much better measure of how much fuel the engine is using at a particular moment. Rescale the injector dwell time and use the wheel speed to get the car's speed and ta-da you've got a reasonably accurate measurement of the fuel consumption rate. Total cost in a new car? Basically $0 in parts since all the sensors are already there for other reasons. Just the cost of the bit'o code in the car's computer that does the number crunching.

Half hour of googling didn't turn up anything definitive except one or two references to after market meters that connect to either a (post-1996) car's data port or are connected to the (generally single point) fuel inject wire. The JCWhitney web site no longer lists the old-style vacuum based gauges.
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  #35  
Old 04 November 2011, 05:09 AM
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Just chiming in here with a little real world experience. (Also trying to inject a little life into this catagory. It's been a bit quiet lately.)

Last May, I bought the first brand new car I've ever owned. (Kinda cool driving a car with only 17 miles on it! )

The car I drove to work before that, a 2001 Buick Regal with a 3.8L V6 had 230,000 miles on it when I bought the new car. I'd done some maintenance on it, including plugs and wires, replacing all of the fluids, a brake job including rotors and calipers, new front hub bearings (FWD) and a new CV joint on the left front axle, and of course, an alignment. Truth be told, I thought it drove pretty nicely. Turn the key for about a second, let it go, it was running. Transmission felt firm and solid. The AC chills down till it's nice and crisp on a hot summer day.

I drive 62 miles one way to work each day. I've been at this job for over 3 years new, so that meant I ran the miles up on it pretty quickly in that time span. But it always drove fine.

I thought, perhaps it was just because I was used to it. But, during the winter, sometime for a week, I would drive my 4WD crossover instead due to weather conditions. (We had a pretty stout snowfall average last winter.) But, whenever I got back into the sedan, it felt just fine to me.

So, that meant that most of the miles were indeed highway miles.

Since last May of course, I've been driving the new car to work. (Just went over 17,000 miles. But hey!! They're almost ALL highway miles!!!!) I've let my son drive the Regal since then while he's been working on college degree. All of the miles are now city miles. And the roads around here have been in rougher shape then before as both cities, counties, and the state have been deferring road maintenance.

Last Saturday, I drove the car for the first time since last June, and I have to tell you, it seems rougher to me than it did when I handed it over to him. No, he doesn't abuse it. In fact, he's a good cautious driver, who will get on my case if I go over 5mph over the limit. I know him and I know he's not abusing it. The city street environment is, I'm convinced of that.

How many miles has HE put on it? Not sure, the odometer LED's quit. (Along with the transmission gear indicator and a few LEDs on the radio panel buttons.) The driver's rear view mirror, which had a heating type defrost system failed. It fogged over so bad, I was forced to replace it. It's also gained some squeaks and rattles it didn't have before. All this due to it's new "city life?" Hard to say. Just age and coincidence? I don't think so. I really do believe city drivng is harder on a car.

Just my observations and $0.02 worth.
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  #36  
Old 04 November 2011, 05:36 PM
dewey dewey is offline
 
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Quote:
Originally Posted by DrRocket View Post
Just chiming in here with a little real world experience.
Thanks for chiming in. I had forgotten about this.

As it was I did not buy either of the Volvos that I was looking at, the high mileage one or the low mileage one. Instead I bought a 2007 Mercedes C280 with AWD and 14000 miles. I have had it a year and have had no trouble with it yet. I only put about 10000 miles a year on it so it will be a pretty good test of city miles.

dewey
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  #37  
Old 06 November 2011, 01:52 AM
JFC
 
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In private aircraft, for reasons that should be obvious, "miles" are not used as a measure of wear. The preferred measure, as far as I know, is "engine hours", or the number of hours an engine has been in use. After so many engine hours, the engine ought to be replaced.

Let's just take two vehicles that are mostly used for suburb travel, but assume one is primarily used for delivery (as in a mail carrier). I would say the mail carrier's car is probably going to have a lot more engine hours per mile (among other things) than one that's just in constant use to get from place to place, just from all the idling in driving from mailbox to consecutive mailbox. Now, just how damaging is all that idling? I don't know.

I'd love to know what the average lifetime mileage expectancy is for a mail carrier vehicle (or it's engine, at least).
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  #38  
Old 09 November 2011, 08:27 PM
kanazawa kanazawa is offline
 
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Quote:
Originally Posted by JFC View Post
In private aircraft, for reasons that should be obvious, "miles" are not used as a measure of wear. The preferred measure, as far as I know, is "engine hours", or the number of hours an engine has been in use. After so many engine hours, the engine ought to be replaced.
So...is the "obvious" reason that the tach/Hobbs meter records engine run time in hours, and that there is no odometer on the airspeed indicator?

I'm guessing as a practical consideration on multiengine aircraft, some engines will have differing times in service, and the actual mileage of the aircraft won't be relevant in all cases.
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  #39  
Old 09 November 2011, 08:44 PM
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Quote:
Originally Posted by NorthernLite View Post
Great Googly Moogly, man!! Do you realize that you are only one of probably four people who I've encountered in the last four years who understand that concept.
It helped that when I traded in my last vehicle, the dealer told me that they could only give me the wholesale value because it was too old and battered for them to resell. Given that it had two major dents, a pinkish paint job, and was leaking oil from the transmission, I figured that they had a fair assessment.

Quote:
Originally Posted by kanazawa View Post
So...is the "obvious" reason that the tach/Hobbs meter records engine run time in hours, and that there is no odometer on the airspeed indicator?
Yup. It might also be that some aircraft weren't required to have a mechanical airspeed indicator that could be used for that purpose.

Quote:
I'm guessing as a practical consideration on multiengine aircraft, some engines will have differing times in service, and the actual mileage of the aircraft won't be relevant in all cases.
That is also correct. Maintenance is divided between engine and airframe (everything not engine). So a twin engined aircraft could have three different schedules for maintenance.
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  #40  
Old 13 November 2011, 05:38 PM
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Quote:
Originally Posted by NorthernLite View Post
Great Googly Moogly, man!! Do you realize that you are only one of probably four people who I've encountered in the last four years who understand that concept.

People come in and ask me what their trade in is worth. I ask them how much they want for it and they get all coy and shifty eyed then tell me it's my job to tell them how much. And when I check and see what I can buy a similar vehicle for they get all upset because I'm trying to cheat them.
See, I checked both Blue Book values before I went in so I had some idea, and then (he wanted to resell it) I knocked some $$ off of the relevant one. He gave me a better deal than most would, but he was the owner and we had bought 4 new cars from him, and I was trading it in to buy a 5th. He did squirm a little though.

He took it for a drive and tried all the seatbelts and everything first of course.

And then he ended up giving me three hundred more because I had to have a safety to get it in my name (my mother had given it to me) to trade it in and it had a few problems, but he covered that because I did it there and he would have had to have it done to sell it anyway. If I'd been thinking clearly I would have just put my mum's name on the ownership of the new car too. She's on the insurance so I can get a family rate. (It's legit.)
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