taller tranny

[MetroMPG]

I agree with the idea of trying the effective gearing change with oversize wheels/tires first, if you can. Simpler & easier to undo if you don't like the results.

BUT ... I think I've done what a regular driver would describe as a pretty "radical" final drive change. And on a 1.0 3-cyl the possibility for negative effects (moving the engine outside of its usable power range for a given road speed) is even greater.

Yet that hasn't happened (to the point of making FE worse than it was before the swap). The before/after speed vs. mpg chart shows this empirically.

It is true that I now have to sometimes downshift at highway cruising speeds (< 90 km/h / 55 mph) to accelerate and/or climb hills, where before, like almost all other cars, I could have remained in top gear. But I don't have a problem with shifting. It's why I bought a manual.

I'd recommend the same mod to any Suzukiclone hypermiler, with the disclaimer that I live in a relatively flat geography. I'd question its suitability in steep mountainous terrain.

[Silveredwings]

Originally Posted by 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.

I would agree, though the car in question has a fairly torquey 2.5L straight 6. The current first gear is so short (final ratio is 14.64) it's almost useless when I'm not pulling stumps.

There is an AWD version of the same car/engine that was offered with a 3.23:1 final gear. The two reasons I guess they went with a taller grear is mpg and traction.

I frequently start out with second gear (final ratio 8.72) with the stock gearing. Changing the final gear to 2.93 would move first gear to a more useful 12.39. As for the rest of the gear range, I'm not trying to optimize the zero to sixty specs, but how can I tell if I'm really moving out of the sweet spot of the power curve?

[MetroMPG]

Originally Posted by Silveredwings

but how can I tell if I'm really moving out of the sweet spot of the power curve?

Do you have a BSFC map for the engine, or a similar one? That would be the best way to figure it out.

Mainly, I'd think you'd want to ask: at your typical highway cruising speed (where a final drive change should reap the biggest reward), what would be your effective change in RPM? From what to what?

With the BSFC map you could then see how far out of (or towards) your peak efficiency you're heading with the change.

[Silveredwings]

OK, it looks like I'm in need of some more edumacation.

I can easily calculate the rpms vs mph for each gear combo (e.g. 60 mph in 5th goes from 2711 to 2295).

I have no idea where to get a BSFC graph of my engine (model M54B25). If I had one, how would I use it? Isn't it something that shows where the engine produces the most HP per unit of fuel used? The problem with minimizing overall fuel used (maximizing mpg) is that I'm not always trying to get the most HP out of the engine. At 60 mph, I doubt any gear is going to be at peak, and since I need less than 20 hp to go straight and level, the top gear should simply bring the rpms down somewhat. I'm clearly not getting something here.

BTW, I realized another more likely reason that BMW used 3.23 instead of 3.46 in the 325xi AWD version: a 0-60 score that's more competitive in the marketplace. It can reach 60 in 2nd gear whereas mine has to shift into 3rd to get to 60. I'm not sure why this was important for one model vs. another but I'm not a marketeer either.

[MetroMPG]

No, you're right: a 400 RPM difference isn't going to move you far enough on a BFSC map to make your fuel consumption lower at cruise than it is now.

[Silveredwings]

The dotted lines are existing gear ratios, the other lines are proposed. The old 5th and new 4th are about equal. Then there is 1 or 2 more gears (depending on if I do the 6-spd).

If the BSFC roughly follows the torque curve, then it peaks at about 3000 rpm. That would make sense as typical shift points under moderate accelleration. Shifting from 5th to 6th at ~70 mph (a kind of upper range use case), would bring my rpms from ~2800 down to ~2300.



The biggest difference seems to be losing that very short 1st gear for towing a Winnabago up a hill.

(edit: changed mild to moderate)

[omgwtfbyobbq]

Any drop in gear ratio should make it lower, since more fuel is needed to make the same amount of power when the engine is moving slower, but it probably wouldn't be noticeable. Going from 4th to 6th in a 330i only increases mileage by 12% according to C&D, and 400 rpm drop may only net ~4% or a little over a mpg.

[onegammyleg]

Originally Posted by MetroMPG

I'd recommend the same mod to any Suzukiclone hypermiler, .

My Swift must have a different ratio than normal metro's.
I rarely get to use fifth gear around the suburbs , definitely a highway gear for me.

[onegammyleg]

Originally Posted by theclencher

There is a point at which the gearing can get too high..

Yeah , like what I said.
I couldnt imagine any longer legs on my diff.
My Swift (which seems to have many differences to a similar year metro in places) always needs a downshift to 4th on small/medium sized gradients at highway speeds.

I dont know what the ratio is , but it certainly doesnt scream its head off.

I can still be happily driving along in 3rd at 35mph. -- and on the other side , 130kmph in my Swift wasn't over reving the motor in anyway.
In fact it was very happy to sit on this speed.

I wonder how that compares to other metro owners. ???


A diff ratio swap is a great idea , if the car in particular has been fitted with unusually low gears., but for most cars , the cost of this mod is a bit high especially if your not certain that it will bring an improvement.

[Silveredwings]

Originally Posted by onegammyleg

A diff ratio swap is a great idea , if the car in particular has been fitted with unusually low gears., but for most cars , the cost of this mod is a bit high especially if your not certain that it will bring an improvement.

In this case, bimmer forums have batted around $200 as an average cost of a used diff, plus changing a pumpkin with separate half shafts is fairly easy compared to a live-axle setup or worse, a FWD gearbox. Also, BMW does something unusual, they hook the speedo up to the rear axle in the diff so it doesn't introduce an error with the speedo/odo. So, asside from actually getting the diff, it may be easier than trying to get an oversized tire setup to fit.