No gas guzzler tax for the M-DCT cars???

[skierman64]

Looks like the final drive ratio on the M-DCT is 18% taller than 6MT. That's the difference between 2500rpm 6MT and 2050rpm DCT at the same speed. I'm not an engineer just a business major, so if my calculations are wrong speak up.

As a reference, I have a friend who put the 6MT into an E36. The first 5 gears are all the same ratio as the stock 5 speed. The 6th gear dropped the RPM at highway speeds about 300-350 rpm and increased the mileage about 4mpg during interstate cruising.

I have a feeling a 450rpm drop at highway speeds due to the DCT, will yield substantial mpg savings. Not to Prius levels, however it might do as well or better than my 5 speed E36.

I don't know how the EU calculates it's mileage seen in swamp's post above, but I have a feeling we'll see larger than a 5% increase on the US Highway rating over the 6MT.

Time will tell.

----------------------
Edit based on Enigma's comment below

Looks like the difference in final gear ratio is only 6% or a drop of 2200rpm to about 2070 at the same speed. Not as substantial as I thought. As I said, I was a business major.

There should be some saving in the 7th DCT gear being a 1.0 ratio. Less parasitic loss in the transmission.

[enigma]

Originally Posted by skierman64

Looks like the final drive ratio on the M-DCT is 18% taller than 6MT. That's the difference between 2500rpm 6MT and 2050rpm DCT at the same speed. I'm not an engineer just a business major, so if my calculations are wrong speak up.

Um, you also have to look at the transmission gear ratios which are also different. The total change is far less than your quote implies.

[GregW / Oregon]

Originally Posted by enigma

Um, you also have to look at the transmission gear ratios which are also different. The total change is far less than your quote implies.

True. T Bone did extensive analysis of this in his computer simulations. I tried to find it but there are too many posts.

[swamp2]

Originally Posted by enigma

Um, you also have to look at the transmission gear ratios which are also different. The total change is far less than your quote implies.

Correct.

Gear ratio x final drive
6MT, 6th:.... 0.94 x 3.846 = 3.615
M=DCT, 7th:. 1.0 x 3.154 = 3.154

Difference is 13%. That is pretty significant.

I have no idea if CarTest (for performance simulation) accurately predicts mpg but the option is there. It predicts:

city/highway
6MT:....13.5/19.8
M-DCT:.12.9/21.5

These do not correlate well with the EU specs, not even the right prediction of which is better for urban driving and a much more dramatic benefit in the highway figure.

As a point of reference CarTest predicts this as well:

city/highway
E46 M3:..........15.1/24.9
E60 M5 (SMG):.10.1/15.8

Seems pretty decent but have not really checked those closely (T-Bone/Greg how do these look comared to real life and or vehicle specs?). Further I have all my simulations with MT at .25 s shifts. This is a really fast drag/track shift and absolutely would not be the norm for regular driving. I should do a sensitivity study on that....

[skierman64]

A 1.7mpg increase over the 6MT on the highway sounds good to me.

[nusevad]

Nice analysis Skierman and Swamp. Lets home these figures are true and potentially lower the GGT!

[enigma]

Originally Posted by swamp2

I have no idea if CarTest (for performance simulation) accurately predicts mpg but the option is there. It predicts:

city/highway
6MT:....13.5/19.8
M-DCT:.12.9/21.5

These do not correlate well with the EU specs, not even the right prediction of which is better for urban driving and a much more dramatic benefit in the highway figure.

Does it have an option to predict differently for auto vs manual gearboxes. For purposes of economy the DCT is unique in that it behaves half like an auto (no lift on shift) and half like a manual (reduced losses). That combo when combined with the auto like tendancy to shift at lower RPM should result in better urban economy.

[man02195]

I put this together using the European data. The Euro cycles aren't the same as EPA cycles, but these numbers show the relative change by having 7 speeds vs. 6 speeds.

[darkalley]

I spoke to someone in the know at BMW and the DCT will NOT avoid the GG Tax. Sorry......

[skierman64]

How about a reduction in GG tax over the 6MT since it's supposed to get better mileage?

[GregW / Oregon]

Originally Posted by skierman64

How about a reduction in GG tax over the 6MT since it's supposed to get better mileage?

Yes, may go down to $1,000.

[skierman64]

The $300 saving will pay for a few tanks of gas. That's ok with me.

[swamp2]

Originally Posted by enigma

Does it have an option to predict differently for auto vs manual gearboxes. For purposes of economy the DCT is unique in that it behaves half like an auto (no lift on shift) and half like a manual (reduced losses). That combo when combined with the auto like tendancy to shift at lower RPM should result in better urban economy.

I don't think so. Unfortunately the mpg prediction is a bit black box. I did test for sensitivity to shift times and it does not take that into account. A big and incorrect limitation when simulating a dual clutch system for sure. So when comparing two identical cars like M3 with and without M-DCT I think the only variable in the calculation is the gear ratios. This means the difference IRL should be a bit more than predicted due to the conservation of power to the ground.

[swamp2]

Originally Posted by man02195

I put this together using the European data. The Euro cycles aren't the same as EPA cycles, but these numbers show the relative change by having 7 speeds vs. 6 speeds.

It seems your numbers are purely based on analytical/numerical units conversion, right? This is an OK approximation but the way the cars are tested is critical. As we know heavy foot or city = poor mpg and cruise at low speeds on the highway = great. The EU and US test procedures are quite different and neither reflects real world driving all that well. Lastly the numbers do not really reflect the difference in the number of gears. The effects visible in the numbers are from the different gear ratios and essentially no deceleration from wind and rolling loads during clutching with the MT.

[M3_2010]

Originally Posted by swamp2

We already quite a bit of info and info on M-DCT and we can at least bracket the likely difference between 6MT and M-DCT.

M3 Coupe:
MT (EU): 17.9/9.2/12.4 (l/100km urban/extra-urban/composite)
M-DCT (EU): 17/9/11.9

So

M-DCT gains: 5%/2%/4%

MT (US): 14/20 (mpg city/highway)

Estimates M-DCT US EPA mpg rating: 14.7/20.4 (big assumption which is probably not very accurate: % gains on EU and US test cycles are the same). Also if the least aggressive AT modes give the best mpg can the best AT mode be chosen for the test?

CAFE std is used to figure GGT and is approximately: combined mpg = (1/(0.495/city + 0.351/HW)) + 0.15

M3 MT "combined" = 19.1, GGT = $1700.

But we know this is not quite right, the M3 MT GGT is in actuality only $1300.

Using the 14.7/20.4 figures (not sure if they have to be rounded or not, anyone?) we have M-DCT "combined" = 19.8 mpg. Perhaps we will see as good as 15/21 (or with lucky rounding) which yields a "combined" = 20.3.

So the best/worst case calculated combined gains are 1.2/.7. Lets use those gains to do some bracketing (see table below)

20.5 +.7 = 21.2 -> No change
20.5 + 1.2 = 21.7 -> $1000
21.4 + .7 = 22.1 -> $1000
21.4 + 1.2 = 22.6 -> No tax

I guess I'll place my bet that the GGT goes down to $1000. I suspect GregW might have some comments/suggestions/corrections on this work. Greg?



For reference the tax schedule is as follows:

Combined fuel economy of: Amount
at least 22.5 mpg No tax
at least 21.5, but less than 22.5 mpg $1000
at least 20.5, but less than 21.5 mpg $1300
at least 19.5, but less than 20.5 mpg $1700
at least 18.5, but less than 19.5 mpg $2100
at least 17.5, but less than 18.5 mpg $2600
at least 16.5, but less than 17.5 mpg $3000
at least 15.5, but less than 16.5 mpg $3700
at least 14.5, but less than 15.5 mpg $4500
at least 13.5, but less than 14.5 mpg $5400
at least 12.5, but less than 13.5 mpg $6400
less than 12.5 mpg $7700

thanks for the info.... CAFE has new MPG figures which are lower? This would make it easier for the tax collectors.....

[///MML]

any final word on GG tax for M-DCT cars? Is it $0, $1000, or $1300?

[skierman64]

We'll know in about 2 weeks when the first guys start taking delivery in the states.