davidjohnbamber

Looking at the EPA ratings for these 2 BlueTec models the E250 is 28 city, 32 combined, and 42 highway. The GLK250 is rated at 24 city, 28 combined, and 33 highway. Both these models are within 100 lbs of each other. The GLK has a higher drag coefficient 0.35 versus the E250s is 0.28. There seems to be a huge difference between these 2 models in mileage. Yet, on paper there doesn't seem to be that much of a difference other than drag coefficient between the 2 models.It just seems strange that the E250 gets about 20 to 30% better fuel mileage than the GLK according to the EPA. Just curious I guess. Anybody out there as to why this is?

N_Jay

Wind resistance is the largest portion of energy use at higher speeds.


0.28 : 0.35 is a 25% improvement.

Micah / AF1 Rac

At 9400 feet above MSL I average about 43-45 mpg while generating same peak boost of almost 2 Bar, at 600 feet MSL at same average speed I average 37-39 mpg at same peak boost pressure (turbochargers are awesome huh?) the only difference being air density or density altitude depending on your background. Just like a higher cd(coefficient of drag compared to frontal area) a higher air density makes for lower mpg numbers. If you ever get you pilots license it will be explained ad nauseum, but suffice it to say, the drag is bad and reduces fuel economy, period. Nothing shaped like a brick, only less aerodynamic (looks are very deceiving to the human eye), will ever get truly amazing fuel economy at the same given speed as a sleeker design. Ever wonder why airplanes are shaped the way they are and have been for roughly half a century?

squid23

The drag coefficient does not include the larger frontage. You have to multiply that one out, too.

20swrt

Also, since engineers know about the difference in aerodynamics - they also need to compensate using a different final drive or differential.

Micah / AF1 Rac

You can only compensate so much. I'm deeply involved in the R&D side of the motorcycle business. A Harley Bagger with common height windscreen takes about 21-23 bhp to push through the air at 70mph real speed. A ninja 250cc takes about 15-16 bhp at the same speed. The HD motor may only be spinning 2,500rpm at this speed while the little 250cc twin is screaming along at say 9-10k rpm. Even though the little 250cc motor is at an obvious 4x rpm disadvantage is tends to go about its job using less fuel, this is only due to aero drag. The smaller, slimmer 250cc bike has to move less air to go the same speed.


Don't get me wrong, I'd absolutely love a numerically lower final drive ratio. I could highway just fine at 1600-1800rpm instead of 1900-2300rpm now. We have generous speed limits where I live and pleasure drive.

ImInPA

I would think the fact that the ninja only has a 250 cc engine Vs. the Harley's 1700 cc engine would be a larger influence on mpg. Size does matter when it comes to displacement.

N_Jay

Lower RPM typically produces better efficiency, all else being equal.


But all else is rarely equal.

Micah / AF1 Rac

Quite true, no argument there and the reason I would like to find a lower numerical ratio final drive for my somewhat customized W211 CDI. Oh if only there were an option for a six speed manual swap, AND taller final drive...

N_Jay

Fit an old mechanical overdrive unit.

Eyesncars

I drove my Glk250 this past weekend for 90 miles on the highway and achieved 41.8 mpg at 59 mph. I think the epa underrates the actual mpg on the GLK Bluetec. The truck is truly amazing.

N_Jay

YMMV

Eyesncars

What does YMMV mean?

N_Jay

http://en.wiktionary.org/wiki/your_mileage_may_vary

20swrt

Here are the drive ratios of the GLK250/220 BT. And here are the gear ratios for the E250.

The final drive ratios are different between both vehicles. 2.23 (e-class) vs 3.46 (glk). As I mentioned earlier, engineers need to compensate for added aerodynamic drag that is one of the reasons why the GLK has a different final drive ratio. Another reason is the potential for towing.

Micah / AF1 Rac

Add in tranny ratios to calculate true per gear ratios. Now I'm wondering which is the lowest numerical ratio final drive possible to fit my W211.

C63newdude

You forgot the most important thing: RWD vs AWD. I test driven both. The GLK launches of the line like a shotgun and it seems to have shorter gear ratios. The E250 never impressed me, I found it noisier than the E250.

Another thing to not forget is the set of wheels and tires on each vehicle, this makes a huge difference as wheel.

I would pick the GLK over the E250

C63newdude

I concur!!!It is also fast off the line..