135i Mileage Ratings

Just a quick question - why is the 7 speed DCT rated at 18/25, while the manual is 20/28? This seems like a very large gap compared to most cars. The new Audi S4's S-tronic transmission is more efficient than the manual..

The quoted gear ratio specs for the 135i are very strange as BMWUSA shows a DCT (automatic) 3.46 final drive ratio with a 1 to 1 direct drive 7th gear.

http://www.bmwusa.com/Standard/Conte...fications.aspx

That would indicate something like 3,000 rpm @60 mph in top gear, but owners have reported it's closer to 2,100 - similar to the MT.

If the ratios are correct, the car is really geared low and would be hard pressed to achieve even 25 mpg.

Tom

The overall gearing in the DCT is lower (numerically higher), this improves acceleration at the expense of fuel economy. BMW's DCT has no overdrive, 7th is 1:1, while on Audi's 7 speed S-Tronic, 5th is overdrive, 6th is double overdrive, and 7th is triple overdrive, making RPM's at highway speeds substantially lower. Here are Audi's gear ratios (anything smaller than 1:1 is an over driven ratio):

1st: 3.69
2nd: 2.24
3rd: 1.56
4th: 1.18
5th: 0.92
6th: 0.75
7th: 0.62

And BMW's DCT:

1st: 4.78
2nd: 3.06
3rd: 2.15
4th: 1.68
5th: 1.39
6th: 1.20
7th: 1.00

Mr ONE that was an amazing reply.

FWIW,

I was asking about the final drive ratio in this thread:
http://www.bimmerfest.com/forums/sho...al+drive+ratio

Answer I got was its an error on the web site. That number was the final drive ratio when the Steptronic was the AT option for the 135, but with the DCT its apparently 2.56:1.

I'm still curious about the gas mileage rating for the 135 with the DCT. Its notably lower than for the MT, and it a good bit lower than the new F30 3 series 335 with the 8-speed AT.

Thanks
Bruce

While it appears to be true that the 135i's differential ratio is taller (numerically lower) than the web site suggests, I stand by my statement that 3 overdrives will markedly reduce engine RPM at highway speeds. The 135i's DCT is geared to accelerate as it's primary task.

Originally Posted by one35i

While it appears to be true that the 135i's differential ratio is taller (numerically lower) than the web site suggests, I stand by my statement that 3 overdrives will markedly reduce engine RPM at highway speeds. The 135i's DCT is geared to accelerate as it's primary task.

OD or not is irrelevant. What counts is the overall ratio - gear x final drive. Having an OD (or 3 or 8) only reduces engine RPM when compared to the SAME FD ratio - otherwise it's apples & oranges. So without specifying the differential ratios, your figures are meaningless.

Incidentally, according to their US website, the current Audi A4 has three transmissions - one 6 speed manual and two 8 speed autos (Multitronic CVT and Tiptronic) but no S-Tronic. The Audi's overall ratios in 8th are higher (lower numerically) than BMW's DCT in 7th, but this is a function of total gearing, not simply having an OD or direct drive top gear.

http://models.audiusa.com/a4-sedan/d...specifications

Tom

My original statement was an oversimplification of the gearing for the sake of brevity. You are correct that if the Audi has 9 inch diameter tires or an NHRA like 6:54 rear gear ratio, then 3 overdrives are "meaningless." However, I doubt either one is true.

Originally Posted by one35i

My original statement was an oversimplification of the gearing for the sake of brevity. You are correct that if the Audi has 9 inch diameter tires or an NHRA like 6:54 rear gear ratio, then 3 overdrives are "meaningless." However, I doubt either one is true.

OK, next time provide the fd ratio (and tire size, if appliccable) and we'll do the math.

Of course, neither of us has explained why the MT 135i is rated at 28 highway versus 25 for the DCT - given that both turn about 2,100 RPM per mile in top gear.

Tom

Originally Posted by johnrose

Mr ONE that was an amazing reply.

Originally Posted by johnrose

Mr ONE that was an amazing reply.

But very BORING. On this forum facts are the last thing we want. Opinions are MUCH more interesting, even if they are absurd.

ProRail, I like your style!