So, I was driving a 2005 Infiniti G35x today and while on the freeway(approx. 73 mph.) I noticed that the rev counter was tickling the 3000 RPM mark. It is a five speed auto but my TL which is also a 5 speed auto is doing barley above 2200 at the same speed. Obviously, this is bad for fuel economy. So I got to thinking that I didn't check this the last time I was in a C280. I normally travel between 75 & 80 mph on the freeway, so all of you who have the M272 engine, what is your RPM at this speed as well as your instantaneous MPG?

Thanks

 

I don't remember the exact RPM, but I am right around 3K RPMS at 75-80 with 2.5L M272 and 7G trans. No instantaneous MPG reading on my car, don't think any of the W203's have that.

But, I don't think using RPM at a particular speed is a good comparison of efficiency at all. So, many variables between engines, aerodynamics, etc.

 

So even if I have an engine with a bigger displacement or more hp/torque, the RPMs would still be as high? I thought having more hp/torque meant that the engine didn't have to work as hard to maintain speed.

 

Don't forget the transmission my car would run about 3300 rpms but would get around 30mpg. if i took the speed down to say 70 i have gotten 32.2mpg.

 

https://mbworld.org/forums/c-class-w...matic-rpm.html

 

Absolutely - depends on how the engine is profiled - hp,torque etc at what RPM & how that is matched by overall gear ratios etc.

You have to take the mpg for the whole package in defined circumstances.

 

and each displacement between the M272 is different.

my 350 barely revs past 2k at 70-75mph in 7th. i've been averaging around 20-21mpg btw with longer trips here and there. with more city or hills in my case I avg around 19.7mpg

http://www.youtube.com/watch?v=qvu7pRA40OQ

 

Keep in mind guys, the C280 4-Matic had the 5-speed transmission and the C230 7-G has a shorter (numerically higher) final drive ratio, leading to a higher RPM in any given gear.

The C350 7G has a 7th gear ratio of 0.728 with a final drive of 2.82. At 2000rpm, it is doing approximately 71.5mph.

The w203 does not show instantaneous mpg. In 7th gear, a C350 SS cruising at about 70-75mph should get right at 30mpg.

 

Wow! I had no idea there were so many factors involved. Now that I think about it, my brother's last 325i would run at about 2800 rpm going 80 mph too and his car was equiped with Vanos, BMW's valve timing system. Thanks for clearing that up. Now, with the engine running that 'high' do you find passing to be pretty effortless since most of the torque is in play? Or do you still have to wait for the transmission to shift down?

Small tangent: does the W203 have those nifty drawers under the front seats?

 

Gearing is the science of horsepower and torque. Essentially, gears multiply the torque produced by the engine and convert it into torque at the wheels that becomes usable. Power is the rating of work. So in essence, an engine with greater power can be geared taller (numerically smaller). An engine with less available power must be geared shorter (numerically higher) to multiply the smaller ammount of torque produced by the engine so that it can move the car's weight. I could go on about this forever.

As for your second question, if you're driving along in the 7G and mash the gas, I'll be the first to tell you, you're not going anywhere for a bout a second and a half which feels like an eternity. If you're in the right gear however, you will have instant acceleration. 7 gears means that you can keep the car in the powerband and upshifts happen pretty quickly under full acceleration. Also, keep in mind you can manually shift the automatic and, there is the 2-second downshift feature. If you hold the shift knob left for 2 seconds, the 7G will downshift to the lowest possible gear for that speed. On the highway, it's not unusual for it to go from 7th to 5th or even 4th or 3rd depending on speed.

 

How do you figure that stuff out? I have never understood what gear ratios and all that stuff means yet you make it sound like there is some formula to it all.

 

There is a formula for it all and you can find all your answers on the WWW. However, IIRC, you're looking at C350s which are indeed pretty efficient. Stay away from the C230 V6 models (2006-2007) with the 7G. as they're not as efficient as the 3.5L and slower than a Pinto.. running on 3 cylinders.. towing a boat.*

* Obviously an exaggeration, but my limited experience with a C230 V6 7G was highly negative. Mild on the gas results in acceleration on par with a Yaris. Hard on the gas results in gas mileage and acceleration on par with a large SUV. Too slow, too many gears - the least desirable facelifted W203 out there.

 

Gearing isn't too hard to understand. Basically, you have an input shaft from the engine, and then an output shaft, and then the final drive ratio. Imagine a bicycle with multiple gears up front where the pedals are, lets say 7, and one gear in the back. The feet provide the power just like an engine would and the cog mated to the rear bike wheel is essentially the rear drive axle coming out of the rear diff.

Lets use our example above. 7th gear @ 2000rpm. On a pedal bike, this would be the largest cog up front, the chain going to the only cog in the rear. Your feet are moving at 2000rpm. 7th gear is .728, so 2000rpm goes in, 2,747rpm comes out. This is how fast your drive shaft is spinning. That goes into the rear diff, which houses the final drive ratio. On the C350, this is 2.82. So 2,747rpm/2.82 = 974rpm.

circumference of a 245/40/17 tire is 77.6".

So, 974rpm * 77.6in * 60mins /12" /5280ft = 71.57mph.

This can be done in any gear.

The reason we have gear boxes is that most engines do not produce enough force (torque) to move a car. Gears multiply that force. Power cannot be multiplied by gearing however, since it is a measure of work. Perfect example is the C350 engine, vs the E46M3 engine. Both engines are rated at 258ft-lbs of tq, but the M3 produces 333hp, vs the C350's 268hp. Why is this? It's because the I6 int he M3 can continue to produce power (work) until 8,000rpm, whereas the C350 stops at 6,400rpm. Why is this important? Lets assume both cars use the same exact gear box. Assuming they both have identical torque curves, weight, etc etc, both cars will accelerate at the same rate! The C350 however, will need to change gears at 6,400rpm, way earlier than the M3 which will continue in 1st gear for another 1,600rpm. As you move up in gears, you lose your torque multiplier.

What is the torque multiplier. As I mentioned, gears multiply force (torque). The C350 engine is rated at 258ft-lbs of tq peak. Gears can multiply this depending on the ratio. So, lets take first gear. 1st gear is 4.377. Final drive again is 2.82. The formula becomes 258ft-lbs * 4.377 * 2.82 = 3,184ft-lbs to the rear axle. There is a final reduction ratio between the axle and the tire size but I don't remember the equation off the top of my head. Now consider second gear: 2.859. So it's 258ft-lbs * 2.859 * 2.82 = 2,080ft-lbs. So essentially, from going from 1st to 2nd, you lose 1/3 of your torque to the wheels. That's why you don't accelerate as fast in 2nd as in 1st. The devil is that the shorter you make gears (numerically higher), the quicker you need to upshift.

In essence, horsepower means much more than torque when it comes to an engines performance rating. Actually the truest metric of an engine's ability is the area under the power curve. The car with the greatest area will win, all else being equal.

There are a LOT more considerations to gearing however, that's why I call it a science. For example, you could gear the 5th gear in that G35x to be taller to bring the rpm's down, but that will space out the gears more. In doing so, you may drop the engine down out of the optimal power band, thus killing performance. Gear it too short however (numerically higher) and you will kill fuel economy. What MB chose to do is give us the best of both worlds by adding 2 more gears. You have 1-5th gear, which is in essence a traditional gearbox, and then 2 extra gears to provide fuel efficiency.

 

Fantastic explanation, Eric..

 

Agreed! I have never had it explained like that before but it simply makes sense. It takes a true genius to take something complex and explain it so even a child could understand it.

 

Thanks Matt!

BTW, the gear ratio's for the 7G are:

1st - 4.377
2nd - 2.859
3rd - 1.921
4th - 1.368
5th - 1
6th - 0.82
7th - 0.728

Final Drive:

C350: 2.82
C230: 3.27

IMO, the 7G was force fed into the C230. It should have a much shorter final drive but then a taller 6th and 7th gear. This would essentially hurt city mpg a bit, but still keep the highway mpg we know today. It would improve the performance of the car at city speeds however. I think the reason this wasn't done is because I believe the 7G gear ratios above are the same for almost all MB's, and only the final drive is different between the different model cars.

 

Safe and happy driving tomarrow fellas.

 

Having driven a W204 C350 7G, I can say the 7G works just fine in the 350. In the 230, it felt like it had shifted to 5th by 20mph under normal acceleration. Drive it harder, they say.. great, now it redlines the first 3 gears. V6 smooth or not, that's annoying.

 

Now it's my turn Excellent!

 

if your concerned about mpg, I don't think rpms is what you should be basing it on. The weight of the car itself has the largest influence on mpg. Yes, driving at a constant rpm saves gas but isn't the biggest factor.

My gsr rev at 4000-4500rpm at 80mph and I still got 35mpg. My stock c32 got 24mpg at 3000rpm at 80mph and 27mpg at around 65mph.

If your concerned about mpg, I would just check the mpg thread and look at the numbers of members that live close to your local area.

 

Remember that once you have that weight rolling it takes very little to maintain it. Then Cd comes heavily into the equation. Even wider tyres have a pronounced effect on mpg & not only because of rolling resistance but because they screw up the Cd.

The W203 Cd of .27 was achieved with 185 or 195/60/15's - don't recall exactly any longer.

 

Thanks for the info, I hadn't considered rolling resistance yet. I'll keep that in mind because I want to put differen OE wheels on when I get one. By the way Glyn, what's it like in Cape Town? I've always wanted to visit South Africa, is it safe?

 

Another for a great explanation post!

Eric, correct ALL 7G's have that gear ratio. You are absolutely correct that using a transmission with the same gearing across the lineup is a compromise. It does work excellent w/ the C350 and other engines and models, but isn't particularly effective in the C230. I would love to see a much shorter final drive and taller 7th (Not 6th), in addition to a taller 1st. IMO, that would do wonders for the overall performance of the C230 with little effect on mileage.

Agreed. It takes a LOT of effort to find the happy medium between those 2 scenarios. Though after 3 years and almost 70K miles, I have trained myself how to achieve it, but not w/o a lot of frustration along the way.

Absolutely. I have found a fairly significant difference in the tires I have used on my C230 and how they affect mileage, acceleration, and the amount of pressure applied to the gas pedal to maintain speed. Tire weight plays a role in that as well, because tires are unsprung weight.

 

You have torque at the axle, in ft*lbs, so to get force at the ground propelling the car foward, you simply divide the torque by the tire radius, in feet.

Tire raidus can be found via the tire size:

255/45/17
255mm * 45% gives you the sidewall height in mm. Divide by 25.4 to get inches, and then by 12 to get feet. (0.376476 feet)

Add half of the 17" wheel diameter, and convert that to feet to get 0.708333 feet. Add that to the tire sidewall to get 1.084809 feet.

Again, now just divide the torque by this figure, and you'll get the foward propulsion force.

This is all theoretical, it doesn't account for sidewall deflection changing the tire radius, tractive capability or the slight amoungs of slip which are required to propel the vehicle, much in the same way a vehicle cannot laterally accelerate without a slip angle.

Generally speaking, a taller tire gives you a taller effective final drive ratio.

 

there it is. thanks for that.