Talk about an arrogant set of posts. Duke's tone is, unfortunately, one of, "I understand this, how could you readers be so stupid as to not understand this!". When I read his first post, I wondered, not what revs per mile meant, but rather where did he get this number? I'm sure others had the same question.

If your goal is to communicate your thoughts clearly to a widely varied audience, then you ought make sure you explain all the assumptions clearly and with as much simplicity as possible.

As I was reading the posts, I kept thinking of the computer techo geeks that I have to deal with occasionally in course of my management consulting business. I often have said to them, "Slow down. Remember that the people you are talking to do not know what your abbreviations stand for and they mostly can't keep up because you are talking too fast. So if possible, slow down explain it in layman's terms." I see this over and over again. They are generally introverts who are in their own little world, and have difficulty dealing with people. Not necessarily placing Duke in this category, just making an observation.

Apparently, you are some sort of automotive engineer. That's fine. My experience is that engineers often have a holier than though attitude because they think they are smarter than everyone else (they often are, in their area of expertise), and think in terms of black and white, true or false. Psychologists will tell you this is true. Makes perfect sense. Gray areas could be dangerous in engineering.

Anyway, no offense meant, simply stating my opinion. I'm sure this is a useful formula to someone. Personally to me, it's a simple as this. My 300E is a high rev small six. The factory optimized the gearing to cause it to run at around 3,000 rpm at 70 mph. Works fine. I don't need to know the details.

Steve

Engineers, including computer types, do tend to have a narrow viewpoint -- that their description or solution is the only possible one. They also love to use acronymic terms, partially because they are used to them and sometimes to confuse general management people. In my consulting days, I always told management clients to deal with engineers and computer people the same way they do all others; ask them if they can do a specific thing and how much it will cost in time and money. Actually, anything you can do with paper and pencil can be done by the computer -- but sometimes you may not want to spend that extra high dollar amount to achieve a minor result.

All four of my cars, with minor variation, run 2700 to 2800 rpms at 70 MPH --
on a fairly level interstate highway ... two of them are sixes (the 230S and the ML320) and two are V8's (420SEL and Corvette). I do wonder why a 190E runs at such high rpms at that speed, but am not familiar with the car and engine.

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Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)

The 190E has a very underpowered engine compared to of course todays cars. It has high gearing to compensate for low power. Also starts off in second gear and has a very short and almost useless first gear. The Automatic is a 4 speed so without that nice 5th gear that is in the cars you mention it needs to rev that high at highway speed to be able to still move when needed. Plus that rev is right in the middle of the power band at 70MPH. Get a 5 speed stick and you get lower revs and of course better gas mileage. Sorry laymans terms as I am an IT/Engineer.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

I have a similar observation. It seems that management and marketing types think that if engineers would just slow down and expand on all terms, that they would fully understand both the problem and the solution.

In effect, it's thinking that all those years in college are completely unnecessary, if only people would talk slower. Sorry, but this is a misconception. Going to first principles only gets in the way, and doesn't solve the problem, nor even explain it.

This is absolutely normal.

If you've changed tire size from specification, you'll continue to know how fast your engine is running but not your road speed.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles

management and marketing types should just think a little faster

If you didn't understand where 849 revs/mile came from why didn't you ask?

It's no mystery. If you look at any tire at The Tire Rack and click on "specs" the revs/mile and a lot of other data is displayed. The tire manufacturer's web sites also list similar data for every model and size tire they offer.

Don't most guys look at these sites and check out the specs when they are looking to buy tires? I have to assume some basic level of knowledge on the part of other members of this forum.

I assume that most guys can look up specs as easily as I can, and since tires are often discussed it seems that most would know about basic specs such as revs/mile.

If you don't, just ask, and I can explain as I have several times over the years, already - or maybe I'll refer you to the archives.

I find it a little irritating that a guy drags up a six month old post where the question was answered and then gives a bum answer. This happens (bum answers) way too often.

The above applies to "tire calculator" links that are offered up. They use "rigid body" dimensions to calculate revs/mi like you were driving on solid steel "tires" like on a railroad wheel. These calculations will not give accurate results if you are trying to compute revs at a given speed in any gear.

And correct information is only a few clicks away. If your make and model tire is out of production and specs are no longer listed, just look at the specs for a couple of same size tires. The revs/mi and other dimensional data for each size usually vary no more than one percent from high to low even across manufacturers. For each size group there are industry standard dimensions and other characteristics that are promulgated by The Tire and Rim Association, so the basic dimension and revs/mi data is nearly the same for all tires of a given size.

The 849 revs/mile data I quoted is the Michelin spec for the 185/65VR-15 MXV tire that were OE on my car, and other makes/models/speed rating tires that were OE are essentially the same as are replacement tires of the same size.

Another tidbit you may want to file away is that revs per mile increases about 2 percent by the time the tread depth gets down to the wear indicators. If you don't know what those are, The Tire Rack has some good tire tutorial pages that explains tire basics.

Duke