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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
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