Last post on Jan 04, 2010 at 11:53 AM
You are in the Subaru Legacy & Outback
What is this discussion about?
Subaru Legacy, Subaru Outback, Sedan, Wagon
#70 of 75 Re: 2010 Subaru Legacy 3.6 R [saedave]
Jan 01, 2010 (7:29 pm)
The economy is better, though I am not sure why. Is there data on engine rpm at 60? Have overall gear ratios changed? Multiple possibilities come to mind. In my sales manual, the original economy data was updated with a later insert.
#71 of 75 Re: 2010 Subaru Legacy 3.6 R [marvinlee1]
Jan 02, 2010 (7:44 am)
The economy is better, though I am not sure why
One of the changes from the 3.0R to the 3.6R, I believe, is a new automatic transmission including both gearbox and torque converter. Even with the same overall gear reductions, efficiency could be different AND the 3.6 has better low end torque which if utilized optimally by the trans would improve efficiency. Another factor is different tires which alone sometimes adds (or reduces) mpg by as much as 1 or 2 mpg. Perhaps someone has compared the Bridgestones with the Continentals on TireRack's ratings?
#72 of 75 Engine speed and economy
Jan 04, 2010 (10:18 am)
Automatic: 1: 3.540; 2: 2.264; 3: 1.471; 4: 1.00; 5: 0.834; final: 3.083 Overall, top gear, (0.834 X 3.083) = 2.571222 Tire size: 225/50R17 Tire revs per mile: 804.6 Engine revs at 60MPH = 2068.8, or about 2100 RPM. Overall gear ratios are 1: 10.91382; 2: 6.979912; 3: 4.53093 4: 3.083 5: 2.57122
A (non-standard) 225/55R17 tire would give (777/806.4) 0.96569724 of the 2068.8 engine speed at 60MPH, or about 1997.83 revs per mile at 60 MPH.
Contrast the above with 2010 Toyota Camry XLE, which has P215/60/16 tires and has the following gear ratios:
1: 3.300; 2: 1.900; 3: 1.420; 4: 1.00; 5: 0.713; 6: 0.608
Camry XLE V6 Final Drive Ratio is 3.685. Overall XLE ratios are 1: 12.1605; 2: 7.0015 3: 5.2327; 4: 3.685; 5: 2.627405; 6: 2.25048
At 795 tire revs per mile, the overall Camry V6 XLE engine RPM at 60 MPH, per gear, is: 1: 9667.5975; 2: 5566.19; 3: 4160.00; 4: 2929.75; 5: 2088.787; 6: 1789.13
Comparing the revs per mile for the Subaru Legacy 3.6r to the Camry V6 XLE 3.5 shows this, Theoretical first, then road speed at 3000 RPM:
Subaru Camry at 3000 RPM
1st 8781.26 9667.60 20.5 vs. 18.6
2nd 5616.04 5566.19 32 vs. 32
3rd 3645.59 4160.00 49 vs. 43
4th 2480.58 2929.75 73 vs. 61
5th 2068.80 2088.79 87 vs. 86
6th 1789.13 N/A & 101
Interpretation: First gear in the Camry is lower than in the Subaru, probably helping the Camry zero to sixty times with a more rapid launch. The Subaru engine runs about nine percent slower in first gear than does the Camry. Second gears are quite close. The Subaru engine runs slower in third gear. The difference of the wider first-to-second ratios of the Camry and Subaru is large. The Subaru second gear engine RPM is 63 percent of the first gear RPM. The Camry second gear engine RPM is 58 percent of its first gear RPM. Thus, the Camry engine not only runs slight slower in second gear than the Subaru (5566 versus 5616), it runs even slower as a percentage of the Camry’s engine RPM in first gear. The driver perception will be that of a greater drop in engine RPM for the Camry when going from first to second gear.
The Camry and Subaru fifth gears are similar, where the Camry engine RPM is just under one percent higher. The greatest difference is in the Camry sixth gear, which acts as a true overdrive by lowering engine RPM about 13.4 percent, relative to the Subaru. Therein likely lies much of the Camry’s better highway fuel economy rating with the greater drag of the Subaru’s all wheel drive likely accounting for most of the remaining difference. The Subaru automatic transmission lacks a sixth gear and the Legacy 3.6R would benefit from having it. The Camry engine speed is 14 percent lower at 60MPH and the car uses 11 percent less fuel. Considering the fuel economy negatives of all wheel drive and higher top-gear engine RPM, the Subaru engine would appear to have high fuel efficiency.
#73 of 75 Data coments, preceding post
Jan 04, 2010 (10:21 am)
The data table arrangement did not survive the posting process. In addition, it is possible, and even likely, that arithmetic and rounding errors have found their way into the numbers. Corrections and other improvements are welcomed, as are all comments, critical or otherwise.
#74 of 75 Re: Data coments, preceding post [marvinlee1]
Jan 04, 2010 (11:50 am)
I can interpret it just fine, thanks for taking the time to do that analysis.
#75 of 75 Re: Engine speed and economy [marvinlee1]
Jan 04, 2010 (11:53 am)
Nice analysis. I keep a spreadsheet of all Subaru gearing combos that interest me so I compared with your calculations. I have slightly different numbers for the 2010 Outback with the 5spd auto.
It looks like the difference is in the tire size. The standard tire is a 225/60-17, aspect ratio 60 not 50. This gives revs per mile of 730. RPMs 60 = 1878, much more comfortable.