Peugeot 205 5th gear swap (8.2% improvement at constant 50 mph)

[renault_megane_dci]

What's the "after" test ?

[HypermilerAX]

"B" test, rather. The test I made in the second post but with the new gearing.

[HypermilerAX]

I corrected the volume of the first test. I took 1,00 L for the first bottle I put but the station volume is not as accurate (only 10 mL) as weighing.

Recap :

Before :
Temperature : 25,7°C
Pressure : 1021,1 hPa
Humidity : 43%
Air density : 1,197
Wind speed : 13 km/h NNW
Sunny
Engine at operating temp
Tire pressure raised to 2,8 bar

Fuel consumed : 840 g = 1015,7 mL (827 g/L at 25°C)
Distance driven without EOC : 28,56 km
Consumption : 3,556 l/100 km
Recorded average speed : 80,26 km/h

Corrected consumption (80 km/h and air density 1,2) : 3,547 l/100 km

After :
Temperature : 20°C
Pressure : 1020 hPa
Humidity : 77%
Air density : 1,220
Wind speed : 12 km/h NNW
Clouds but dry road
Engine at operating temp
Tire pressure raised to 2,8 bar

Fuel consumed : 748 g = 901,2 mL (830 g/L at 20°C)
Distance driven without EOC : 27,30 km
Consumption : 3,301 l/100 km
Recorded average speed : 80,23 km/h

Corrected consumption (80 km/h and air density 1,2) : 3,255 l/100 km

Before : 66.16 mpg
After : 72.28 mpg

This is exactly what I expected. It's an 8.2% improvement for a 15.1% higher ratio (MPG gain = 55% of ratio change).

[monah78]

I have also a Citroen AX (Gasoline MY88 AX 954 cm3) and I want to increase gear Ratio.

I see you change your front Tires from 145/70R13 to 155/80R13. I want to do the same with Toyo NanoEnergy 3 155/80 R13 but I was wondering why only in front ? Do you experience some problems when you turn (Friction of the tire against shape ?
Other point I want to change my current 4 speeds Manual transmission with a 5 Speeds one with longer ratio (I'am looking for a MA5/O from a Peugeot 208 1,4l HDI Blue Lion).
According my caculations with those 2 modifications (tires & gearbox) 1000rpm speed (km/h) will be :
I : 8.62 (12x41*17x61) instead of 7.38
II : 16.28 (21x38*17x61) instead of 13.94
III : 25.13 (29x34*17x61) instead of 22.33
IV : 34.51 (41x35*17x61) instead of 30.98
V : 43.27 (47x32*17x61)

Do you experience some driveability issues with such ratio ?

Thanks by advance for sharing your experience.

[renault_megane_dci]

I wouldn't want to be a pain but are you sure the gearbox would work in your application ?
It's been a lot of time since the AX era and a gearbox mated to today's 1,4 HDi is probably a stronger unit than the one you had in your car.

That being said I once fitted the diff of a diesel variant in my 1981 Mk1 Golf GTi to good effect.

[HypermilerAX]

I have the same doubts. Even if the 1.4 HDI's gearbox is still a "MA" type like the AX, I believe that there have been some modifications since. I have only seen the MA/5O gearbox on pics and from what I could see the body is not exactly the same so I'm not sure if the holes for the screws are exactly in the same position. The best is to see the gearbox before you buy it and check.

Secondly, that gearing will be way too high for that low torque engine (75 Nm at 3700 rpm). I have 44,7 km/h/1000 rpm but with an engine that has 25% more torque than yours (97 Nm at 2250 rpm) and at lower rpm which means that it has around 40% more power at 2000 rpm. And it is already quite borderline, it's fine for highway cruising but I have to mash (80%) the throttle on 3% inclines to keep 90 km/h. I increased ratio by 19% and top speed dropped to 140 km/h. What you want to do is +40%, I think the 5th gear will be undriveable with top speed no more than 110/120 km/h on flat and without wind.
Try the diesel gearbox (after 92), it has 20% higher ratio (43x33 and 17x61), that should be enough.

As for the tires, I mounted them only on front because :
- it's a front wheel drive so you only need them on the front to have a bigger gearing, I have no driving issues with having different sizes on front and rear, it's just 5,7%.
- they don't fit at the rear anyway (I have only 2,5 cm free with stock tires and the 155/80 would probably rub on the body)

You don't need to put bigger tires. You have enough room for improvement on the gearbox ratio to keep the original size. It's always better to keep the lowest tire size, less rotational weight, and change the gearbox. I have some good experience with the Yokohama Bluearth which exists in 145/70.

[monah78]

Thanks for your advices.

I've made some investigations on the web this week in order to assess mechanical feasability and driveability replacing my current AX 4 speeds gearbox (MA4) with a long ratio one from low consumption version of a Peugeot 208 1,4 HDi BlueLion (MA5O). Let's show the results. From a high level mechanical point of view it seems to be feasable replacing only gearbox itself (tag 1 in red on the picture) and adapting the clutch case from my current MA4 (bottom view) to the MA5O (upper)
Sketch of MA5O vs MA4 : (See in my ecomodder album picture6256-ma-gear-box-change]
Those basic assumptions has to be transformed in reality but hope is remaining !
I will visit next week a "car breaker" who have a gearbox from a crashed 208 1,2 VTi (normally same interfaces) to have a look and confirm if it's really feasible.

The fuel economy potential of such a gearbox switch is amazing. To illustate on a graph that I have plotted in a BSFC=f(Torque, RPM) graph the iso-speed lines corresponding to my AX10E and the effect of the regearing switching from MA4 to MA5/O (See in my ecomodder album picture6255-ma4toma5o)
Estimated fuel economy can rise up to 45% @ 70 km/h (43,5 mph) and 18% at 90 km/h (56 mph) corresponding to an average depending of the use of 20 to 30%.

As you mention the main drawback will be driveability. According my simulations the time to go to 120 km/h starting from 80 km/h on the last gear will be increase from 17s to around 29s !!!. An option will be to decrease gear to 3rd to recover similar acceleration.

Max speed (originally 140 km/h) will not be longer reachable on 5th gear but it seems to be achievable in 4th gear (even 150 km/h) according the graph. This point has to be checked but personally I never overpass 110 km/h => so for me it's not an issue.

For curiosity, I have made the same graph with your diesel gearbox 92 (MA5/L according internet websites). See the graph (in my ecomodder album 'picture6258-ma5l'. The potential fuel economy benefits will be divided by around 2. I will keep this option as a "less critical" and less costly path if my initial idea doesn't finally works.

Finallly let's share the fuel economy target I have with my White Citroen AX 10E "K-Way"
(picture6257-co2-water-fall-whiteax in my ecomodder album)
I will be very interested with your feed back on the other items.

Waiting your interesting comments

Nota : I'm not able to share the link to pictures in my posts due to limitation of ecomodder website (too young and not enough posts since registration to be able to share links)

[renault_megane_dci]

I Have kind of the same experience :

I fitted a diesel diff in my Mk1 Golf GT1.
Top speed took a hit since the car was barely able to pull peak torque in top gear but this was 4500 rpm @ 180 kilometers an hour.

[MetroMPG]

Originally Posted by monah78

Nota : I'm not able to share the link to pictures in my posts due to limitation of ecomodder website (too young and not enough posts since registration to be able to share links)

I've fixed your account so you can post photos now.

Bienvenue a EcoModder! Welcome to the forum.

[monah78]

Thanks MetroMPG allowing me to illustrates my projects.
You will find below the sketch to jump from MA4 -> Last release MA5O.

The potential effect in fuel economy of such a gear ratio change on a Citroen AX :

Same fuel economy graph potential with AX 1,5 D MA5L Gear ratio instead of AX 1.0E MA4.

And finally my project road map overview to reach the 3l/100 km target on my Citroën AX starting from a current 4,9 L/100 with lowering idle speed to 650 rpm.


Thanks to Ecomodder, I earn that with an EOC kill Switch we can decrease fuel consumption. I'am currenty adapting such a switch on my car (very simple on an cabureted engine like mine). I will give you a feed back soon.