Altering the back axle ratio for economy

[bmw114]

When i was young my dad had a Ford Consul 375 with a 3 speed gear box, then we had various cars with 4 gears, then the norm became 5 gears, now you can get cars with 6 or 7 gears where is it all going to end.

My 520i does 1400 rpm at 30mph in 5th, 1800rpm in 4th and 2200 in 3rd but will quite easily poddle along in top at 25mph.

So if i could alter the back axle ratio from what ever it is now, lets say 3.8 to maybe 4.3 I could just do 30mph in 4th at 1400rpm and then in theory do maybe 70mph in top more economically.

I know a ton comes up at 4800rpm so do you think i could get enough BHP or torque at about 4200rpm to move the car along at 100mph.

I think it is geared for four people and a bit of luggage but normally its just me and the wife so surely the gearing could be altered a litte bit.

Has anybody out there tried it?.

>> Edited by bmw114 on Monday 14th November 15:20

[RobM77]

The 6th gear is pretty high on modern BMWs, so I would suggest that you find a car in the current range with a a 6th gear and a similar Cd and power and find out what final drive the 6th gear produces.

I recently had a 6 speed 1 series 1.8 diesel, and that would just about hold 50mph in 6th, but anything less than 50mph became a struggle for it. Great for the motorway though, as the revs dropped right down.

[Andrew Noakes]

For a start you'd have to change it in the right direction - going from 3.8:1 to 4.3:1 would lower the overall gearing.

I'm sure it could be done, but I doubt the gains would be large enough to offset the cost of the conversion.

[bmw114]

Sorry about the back axle ratio but you know what i meant.

I have a Mitsubishi Pistachio and it was made for economy, it weighs 700kg, developes 74bhp and the back axle is 3.778 and the 5th gear is 0.697.

Unfortunatly it has`nt got a rev counter but you don`t us top gear below 55kph, On a trip over the Troudos mountains we covered 388k and i put in 16.6 ltrs of petrol so thats 65mpg which is`nt bad.

The M5 from 1992 onwards had a back axle change from 3.91;1 to 3.71;1 for better fuel economy, but i still don`t know what my axle ratio is.

Any help?

>> Edited by bmw114 on Monday 14th November 17:40

[r988]

The easiest way is to find out what other cars uses that same diff centre, I think you drive a 1 series so look up what other 1 series diff ratios are, it probably uses the same diff as a 3 series so check those out as well, they would be far and away the easiest/cheapest way of a final drive change. A lot of American cars like the Corvette/Viper etc have a really tall 6th gear that is solely for economy. So you could always try a different gearbox, most modern cars are geared taller for economy anyway though.

120i has a final drive ratio of 3.38:1 and the 120d a final rive ratio of 2.47:1 (taller=better economy but you need the torque to drive it)

120i gearbox is
1-4.35
2-2.50
3-1.67
4-1.23
5-1.00
6-0.85

120d gearbox
1-5.14
2-2.83
3-1.80
4-1.26
5-1.00
6-0.83

130i gearbox
1-4.350
2-2.495
3-1.665
4-1.230
5-1.000
6-0.851

final drive: 3.23:1

Those can all vary due to what market it is being sold in though, a bit of research should soon find out what you have. In all honesty I doubt whether it's worth the hassle and expense of the change though, compared to the benefit received.

[rsvmilly]

Cheapest way to modify the gearing is to put larger circumference wheels on it. You could do this with larger profile tyres - assuming they'll still fit in the arches.

Makes the speedo over-read, though.

[Pigeon]

No, it makes the speedo UNDER-read... for the same road speed, the wheels, and therefore the speedo drive, will be turning more slowly, so the reading is lower.

However, since most speedos over-read, you could decide to change the gearing by just the right amount to make the speedo read correctly...

[rsvmilly]

Pigeon said:

No, it makes the speedo UNDER-read... for the same road speed, the wheels, and therefore the speedo drive, will be turning more slowly, so the reading is lower. However, since most speedos over-read, you could decide to change the gearing by just the right amount to make the speedo read correctly...

You're quite right. I even meant that as I was typing. (Hits self)

I can't even blame alcohol!