taller tranny

[MetroMPG]

Depends on how you look at it. I think 8.8 is fantastic if you've otherwise plateau'd your FE!

[SVOboy]

Haha, I only get like 26-27%,

I wish I could plateau, but not enough time and it seems my numbers just keep improving.

[MetroMPG]

the FE improvement from the transmisison is likely closer to 5% (5.6%, based on the single 70 km/h point comparison) than 8.8% I claimed before.

[MetroMPG]

Collected a bit more info:

[ZugyNA]

Originally Posted by MetroMPG

All of this is a long-winded way of saying that the FE improvement from the transmisison is likely closer to 5% (5.6%, based on the single 70 km/h point comparison) than 8.8% I claimed before.

I'm wondering how this 5.6% mpg gain compares to the % change in overall gear ing? Not sure that you mentioned this.

[MetroMPG]

The taller transmission lowered RPM by 19.8%. It was lowered a further 5.8% because the OEM tires on the Blackfly are taller than the tires originally spec'd on the taller transmission car.

[Compaq888]

Originally Posted by Gary Palmer

Compaq888: Did you get a replacement vehicle already?

Yes I did. 99 civic dx 5 speed manual.

[MetroMPG]

Some rough temperature calculations:

So roughly 2.5% of the increase could be attributed to the temperature difference, which leaves about 5.2% of the FE improvement attributable to the transmission change.

[Silveredwings]

Wow, there's some great info here. Now I'm considering doing something like this.

The bimmer is geared extremely short (IMHO): final drive is 3.46:1

5-Speed ratios:
1 - 4.23
2 - 2.52
3 - 1.66
4 - 1.22
5 - 1

With 3.46, the final ratios are:
1 - 14.64
2 - 8.72
3 - 5.74
4 - 4.22
5 - 3.46

Being RWD, I can change the final drive to 2.93:1 (also avail 2.79, 3.07, 3.15, 3.23, 3.38). That would give me:
1 - 12.39
2 - 7.38
3 - 4.86
4 - 3.57
5 - 2.93

I don't think this is too radical. In comparison, the Firefly went from (correct me if I'm wrong):
1 - 15.00
2 - 8.31
3 - 5.62
4 - 4.01
5 - 3.32

to:
1 - 12.03
2 - 6.67
3 - 4.51
4 - 3.22
5 - 2.67

...which is still taller.

BTW, phase 2 would entail dropping in a 6-speed tranny:
1 - 4.35
2 - 2.5
3 - 1.66
4 - 1.23
5 - 1
6 - 0.85

With 2.93, I'd get:
1 - 12.75
2 - 7.33
3 - 4.86
4 - 3.60
5 - 2.93
6 - 2.49

Now all I need is to find the hardware.

[onegammyleg]

I haven't read the other postings on this thread , so perhaps I am repeating others words.

When doing a final drive ratio change , especially in a 4 cylinder , you may drop the car out of its usable power range at normal speeds.
This may force much higher cruise speeds to keep the engine happy , but at the sacrifice of aero drag losses.
Also it may force staying in lower gears longer - the total of the two may result in worse FE.

I wouldnt go nuts on a diff swap , you could test it with oversize tyres and see if a similar ratio works OK then go with the diff swap if your see an improvement.