Hi, I asked this question last week and got one or two responses. I'm looking for more. My 91 190e runs at 3500rpm while traveling at 70mph. Is this normal? It seems high. Thanks

849 revs per mile tires times 3.27 axle ratio and add a couple of percent for converter slippage.

You do the math.

Duke

Hi thanks for your reply. However, I'm confused. Would you please explain this a little differently? Thanks again.

revs per mile x 3.27 = about 3000 + at 60 mph.

so i suppose duke means 849 revolutions of the tire per mile, then after getting that number divide by 6 and multiply by 7 to get rpm at 70 then add 2 to 3% to that figure for slippage.

clear as mud?

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Yeah, that's the deal.

Tire revs/mile times axle ratio equals revs at 60 MPH in direct drive. Revs at any other speed can be ascertained by linear proportioning, plus add a fudge factor if the car has an auto trans with a non-locking torque converter.

In the case of my five speed, since fifth is indirect, I have to multiply by the gear ratio:

(849 x 3.27)0.80 = 2221 revs at 60 or (2221) 7/6 = 2591 revs at 70

Duke

Hi. you'll need to know the Circumference measurement of your tyres.

Then divide that number into 5,280 (#of feet in mile) to get the number of times or revs. the wheels turn in a mile x the axle ratio to achieve rpms.

Duh!!!

Use the tire revs per mile specified by the tire manufacturer. Most tires of a given size are usually within one percent, so you can use nearly any manufacturer's data for a given size.

Computing the revs per mile from static dimensions will not yield accurate results because tires are not rigid bodies.

Actual loaded revs per mile figures are usually about 2-3 percent greater than the rigid body calculation using inflated OD.

Why use the inflated OD to calculate a revs per mile figure that will be inaccurate when manufacturers specify actual loaded revs per mile based on testing?

Did I say Duhhhhhhhhh!!!

Pneumatic car tires do not behave the same as steel railroad wheels.

And just in case you didn't notice, this thread is six months old.

How about another Du...!

Duke

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

seems a bit revy....my honda does 4,000 at 70 and thats from low gears...but I never drove a 190E...just surprised a benz is geared that low.

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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

my 84 190e is about 3500 rpm at 70 mph in 4th gear.........yes this is normal.
joe