Anyone swapped diff ratios for better mileage???

[Robert K]

I have a 1991 535i that typically pulls 23-24 on the highway. Since I'm running an EAT chip, I've got to put premium in it, which was $2.75 the other day here in western Kentucky. Just was wondering if anyone here had actually swapped in a taller diff ratio in their E34 for the purpose of getting better mileage. If so, which model do you have, what ratios did you swap, and what were your results?

Grace and peace,

Robert K
1991 535i

[bahnstormer]

imo its not worth it unless u're trying to get these gains at 5th gear cruising situations
in which case if u're at 2000rpm at 80mph in 5th with a 2.64 diff or something
then u'll be dropping down to 4th ro 3rd gear for passing and using even more gas

changing diff for mpg isnt gonna work as well as u'd expect

more effective to change tires to thinner ones and remove as much
weight from car as possible

[Paul in NZ]

the mileage from the e24s?(635s) and the e 28 535s were better than the e 34 for TWO reasons...they are lighter and at 60 mph they rev at about 1800 rpm instead of (mine anyway) 2250.A freind has a 635 and he always gets better mileage than me,and i think i am a more economical driver than him

[Kalevera]

Man...I wish I got 23 or 24 on the highway.

Granted, I like to push it on the freeway, but -- as with today, I get ~ 20 or 21mpg.

Robert, your car should have a 4.27 if it's an automatic. You could install a 3.91 and it would bring the rpms down...probably less than 100 revs...but still.

best, whit

[Jay 535i]

If you did as you suggest, you'd probably just use the accelerator more, negating the advantage.

If highway mileage is what's concerning you, you'd probably get more of a result from lower resistance tires, or less use of A/C or driving with windows/sunroof open. Drag increases with the cube of speed, so anything you can do to reduce drag will offer benefits at highway speeds.

Of course, you could always just try to drive less. Save the planet, man.

[pundit]

As people have suggested the effect would be less than you expect.
Fitting a taller diff, while it would lower the revs for the same road speed, has other implications. It increases engine load. Even assuming cruising along a flat road one of the major influences on fuel consumption at higher speeds is wind resistance. You could liken the energy required to overcome wind resistance to that of driving up an incline. The greater the drag, or the greater the incline both increase engine load. Being in too high a gear for a given load can actually negate the theorectical benefit of lower revs provided by taller gearing as more fuel is required per cycle to counter the increased engine load than would be in a lower gear at higher revs. Generally manufacturers work out the optimum gearing based on vehicle weight, engine power, average loads, aerodynamics and type of driving conditions. As with most things though, this is a compromise.

[SRR2]

Unless you're spending your entire driving life in stop and go traffic, overall drive ratio will play a direct role in fuel usage. It's really not all that complicated. It takes X horsepower to move the car down the road at Y MPH. The overall drive ratio determines at what speed the engine runs at a given forward speed. The economy question boils down to "At what operating point is the engine most efficient at delivering X hp?" And generally, these engines are more efficient at, say, 1800 RPM and 10% throttle than they are at 2200 RPM and 8% throttle. Unfortunately, without having a complete set of engine characteristics in your hands, it's something of a guessing game to determine the optimum overall ratio. Suffice it to say that the drive train engineers at BMW struck a compromise in balancing performance and economy, within the limits of the number of gears in the transmission. Of course if you want to maintain possession of your baked confection with the contemporaneous ability to consume it, you'll figure out how to install a two-speed rear drive, just like in large trucks. Or, find a compatible six-speed transmission with a tall top gear.

[Paul in NZ]

Or, find a compatible six-speed transmission with a tall top gear.

and thats one reason why the 540 is as good as or better than the 535 in the economy stakes

[Bill R.]

in this car to begin with. My average highway mileage on a flat highway running with the cruise on at 75 to 80 is typically right around 26 to 26.5. This is stock chip, stock wheels and tires no wider than normal. Here's the specs below on the changes they made in 91 for yours to increase acceleration from a stoplight and around town and yet not give up much on the highway if any. The epa mileage for my car is 15/21 and i get better than that on both. The epa for your car is 16/21 , so i don't know if stepping up to a taller ratio will help or hinder. As you can see below they made numerous changes.

DRIVETRAIN

The following 1991 models have a new EGS automatic transmission shift program along with a

recalibrated transmission and a changed rear axle ratio.

Model

Final Drive Ratio

Old New

535iA 3.91 4.27

735iA/iL 3.91 4.27

750iL 3.15 3.64

850iA - 3.64

The lower final drive ratio provides improved acceleration off the line while enabling the use of a new

"EGS Map" which is biased towards early upshifts at light throttle, yielding increased performance

without sacrificing fuel efficiency. During light throttle duty cycles, 4th gear can be reached as early

as 40 mph with the converter clutch engaged.

More important than the actual acceleration time is the heightened feeling of "power on demand". As

the system detects the driver's desire to accelerate (by comparing throttle opening to road speed

and gear position), it is programmed to downshift sooner than previous models to make maximum

use of the engine's power. The cars are still equipped with a kick-down switch; however, in most

driving situations, the transmission will have shifted to the lowest gear permissible for a given road

speed before the switch is activated.

The major changes to the transmission include the torque converter and converter clutch solenoid.

Due to the lower rear axle ratios, the torque converter stall speeds have been decreased

to

between 1954 and 2135 rpms with the M30 engine, and between 1815 and 1980 rpms with the M70

engine. These figures will be slightly lower (50-100 rpms) if the engine is not yet broken in. The lower

stall speed converters are not interchangeable with previous models. The frequency of torque

converter clutch engagement/disengagement has increased on these models. In order to ensure the

reliability of the converter clutch solenoid, the construction of the solenoid has been changed from

aluminum to steel.

These drivetrain components are certified by the EPA together as a complete system. These parts

are not compatible with past models and fault codes will be set if parts are interchanged between

systems.

I have a 1991 535i that typically pulls 23-24 on the highway. Since I'm running an EAT chip, I've got to put premium in it, which was $2.75 the other day here in western Kentucky. Just was wondering if anyone here had actually swapped in a taller diff ratio in their E34 for the purpose of getting better mileage. If so, which model do you have, what ratios did you swap, and what were your results?

Grace and peace,

Robert K
1991 535i

[pyro]

i got a dumb m50 which requires 3$ gallon gas...