I was driving my dads TR6 the other day and playing with the overdrive switch, it got me thinking why did they stop making cars with overdrive ? They bang on about emissions and saving the planet. so why not have a switch that drops the revs down for motorways. They could do this 30 years ago so why not now, it would save your pocket and the planet, seems like a no-brainer to me, or am i missing something ?

 

The older Jaguars, Mk1's and Mk2's etc. used to have an overdrive just like you say.

We used to call it the "moneystick" because it was a significant saving of petrol.

Les

 

5th or 6th is gear on modern cars is overdrive. Overdrive, by definition is any gear ratio less than 1:1....i.e 0.8:1

A TR6 had a 4 speed gearbox; top gear without overdrive was 1:1 ratio (direct drive).

5th gear on my golf is 0.86:1 which is overdrive
6th gear on my golf is 0.72:1 which is even more overdrive

 

To add; The old fashioned solonoid or lever operated overdrive units are technically known as a "gear splitter".

This is found on pretty much any HGV with more than 6 gears (12 speed, 18 speed etc).

These give you a high and low ratio in every gear. My Land Rover had one accompanying the 3 speed autobox and two speed transfer box: 12 forward gear ratios, and 4 reverse ratios

 

In Oz Holden have the AFM system where they shut down every second cylinder when less power is required, i.e. cruising about the city, only available in the V8 autos though so not much call for that system here.

 

Ahhh GM's displacement on demand (they renamed it to AFM).

Very clever yet wonderfully simple: Still uses a 16valve push-rod v8 engine, but has a mechansism on the tappets to disable the valves on that cylinders that aren't needed (sort od like the opposite effect of a VTEC or BMW's Valvetronic ).

 

Thanks for reply. So are you saying its impossible to have some sort of rpm drop system on a modern car?
I realise modern cars like 7 speed mercs etc dont need it as much as old classics but little 5 speed 1.2L cars reving their nut off on the motorway could really do with a drop in rpm. or maybe an extra massive 'final drive cruising gear' would do the trick 'just for high speeds only' if you had the torque for it?

 

The motivation for the old style separate overdrive units was really limitations of old gearbox technology, so they have been superceded by modern gearboxes which now have overdrive built in (in the form of gear ratios that produce a higher rotation speed than the speed of the engine).

There will be a move to more gear ratios as the technologies get better and the improved fuel efficiency justifies the extra weight and complexity - although it may well top out at 7-speed for most cars given the width of the power band from most engines. Electronic shifts will also make it less of a chore for drivers - for example the 7-speed DSG on the latest Golfs.

 

When it gets that easy, driving won't be fun any more.

Les