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  #1  
Old 12-24-2002, 03:58 PM
ThrillBilly
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Question 96 C-280 high RPM at hwy speed? MPG?

greetings, i just purchased a 95 C-280 to replace my totaled 300E, and have a question about 2.8's RPM's at hwy speeds:

what RPM should this 202 inline-6 auto 4-speed be during at hwy speeds? example 55 MPH = ? RPM. 75 MPH? 80? 85?, etc.

it seems to be noticably higher than my 300E was. i drove the car appox 600 miles home, and got about 24 MPG running FAST without AC at averaging around 85-95 MPH on freeways.

i am sorry that i can not recall the exact RPM's, but i think it was around 3500 at about 80 MPH, and was pushing 4000 at approaching 95 MPH. i will get exact values during travels during x-mas trip this week.

i thought at first that i was maybe "stuck" in 3rd, but some shifting/testing proved that i was using all 4 gears.

does these RPM's seem to be correct for the C-280?

i would also like to hear your milage experiences with the 280.

other: i was hesitant about the "C" car at first (after driving an "E" car for several years); but the drive home found me tired, but with a big grin from ear to ear.

the 202 far exceeded my expectations in every way except for the size. it performs GREAT, and rides like a DREAM on 15" MXV4's.
i would like to UG to 16", but if i thought the ride would degrade from the existing condition, would probably stick with the 15".

thanks in advance for any help. - benton in atlanta
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  #2  
Old 12-24-2002, 05:33 PM
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Your C280 has, I believe the 2.87 final drive. Later five speed cars have the 3.07 ratio.

At 2.87 and the 195/65-15 tires, you would be spinning 840 times per mile. So, at 60mph, your engine is spinning 2400rpm.

At 85mph, you should be at 3400 or so. Keep in mind MB tachs usually read 100-200rpm high. So, at 85 mph, you would probably see an indicated 3500-3600rpm.
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Old 12-24-2002, 06:55 PM
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I believe that gear 4 is a 1:1 ratio on your 722.4 transmission. With a 1:1 ratio and a 2.87 rear end, you should be doing about 100 mph at 4000 rpms. Also, the 722.4 does not have a lockup torque converter, so there may be an extra 100-200 rpms of slip in the torque converter.

All this being equal, I would think that you would be over 4000 rpms at 100 mph and probably be running close to 3000 rpms at 70 mph.

Hopefully, someone else with personal experince on the 722.4 transmission in the C280 can step in and verify if this is the case.
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Old 12-24-2002, 08:00 PM
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Talking C class rpms

My 1994 C180 which has a lot smaller engine than yours has 16 valves and 4 cyls and it goes 2800rpms at 100kph at the moment. This is after I ran a whole bottle of fuel injector cleaner through 3 tanks as it was doing 3000rpms at 100kph before this treatment. The A$10 treatment was very good and successful! I have replaced the entire exhaust system which had internal damage and it now runs like a dream! Costs me a flight to the moon, though,LOL!!!
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Old 12-24-2002, 11:22 PM
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On W202 four speed 722.4 equipped cars, the fourth gear ratio is a direct drive 1:1.

The 195/65-15 tire is 25.0 inches tall. Now to figure out revs/mile, you can do a little simple trig, or look it up at www.tirerack.com. Here's the math:

C = TT (pi) * D

C = 3.14159 *25

C = 78.54

C = (expressed in feet) 6.55

Revs/mile = 5280/6.55 (5280 feet per mile)

Revs/mile = 806 revs.

Now, I checked Tire Racks notes for a typical 195/65-15, and they list 840 revs/mile. Since the tire makers publish this data, I decided to use that number.

At 60 mph, we are traveling one mile per minute. This simplifies things a little. We start at the final drive and work our way forward.

840 revs * 2.87:1 final drive ratio = 2410. This is the number of rpm that your driveshaft is turning at 60 mph. Now, let's go through the transmission.

2410 revs * 1:1 fourth gear ratio = 2410. This is the number of revs your engine is turning. Pretty simple.

Now, suppose we want to figure out 100mph. We could redo the math, but why bother? 100/60 = 1.6666. 2410 * 1.6666 = 4016rpm at 100mph. Okay, not accurate to any extreme degree, but pretty durned close.
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  #6  
Old 12-25-2002, 11:09 PM
ThrillBilly
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Question RPM seems correct, now about your MPG?

thanks all for the help/info on the 202 RPM question.

drive this AM showed RPM's almost exactly as predicted by posters, and as per my further calculations:

60 MPH = ~2400 RPM, 100 MPH = ~4000 RPM as checked on level ground under "normal" loading.

again, on this 200 mile trip, the 280 performed GREAT! this car has plenty of power on the hwy, and rides like a dream on the MXV4's !!!

now, on to the MPG part of the question:

what is your mileage on 202 C-280?
hwy? speed? AC? etc?
city/combined, etc?

i am putting a bottle of Techron in the tank for each leg of the trips over the holidays. it can only help future MPG potential.

your input would be GREATLY appreciated.
thanks, - benton
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  #7  
Old 12-27-2002, 10:34 AM
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I typically average around 26-27 MPG combined city and highway driving. On the highway, I get around 30 MPG at 70-75 MPH. I also have the 722.6 tranny, where revs are a little lower (I pul 2,500 RPM at 70 MPH).
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