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Engine guides and mods

Gearbox Fluid Change

Although smart say this is a non servicable item, it would be foolish to use the same oil year in, year out. I recommended you change it every 3rd service. People have found the gearchange is much improved after doing this.

Modification Details

This page now covers the following:

fortwo (450)
Roadster (452)

fortwo 451)


Although the drain plugs are hard to get to, do not jack your smart up to access it.
 This will raise the fill hole and increase the capacity. If you really must jack it up,
 level the car by jacking the front up level with the back.

Drive the car until it is at full operational temperature, this allows the fluid to
 warm up and therefore flow better from the casing.

Undo the top drain plug first, there is 2 reasons for this:

It allows the fluid to flow out easily from the lower drain

If you take out the lower plug first and can't undo the top one because it is
seized or the head becomes damaged, you have no way to refill the transmission.

If you do the top plug first and you cannot open it, you have no problems,
you can sort it out in your own time.

Official Service Schedule For Gearbox Fluid

Smart's stance on this has always been the same. In their view, the transmission
fluid is a non service item and does not need changing for the entire life of the car.

It's not an oversight, this is what they have said in all of their cars since 1998.
They clearly don't have a clue and they should be ignored.

Evil's Recommended Service Schedule

Anything between 40 and 60 thousand miles will be fine.

ATF Vs Gear Oil

You hear it a lot online where people say that you can use either 75w90 gear oil or ATF in a smart gearbox.

Just because you can, doesn't mean you should. Getrag who makes the gearboxes obviously know what they are talking about as they make some impressive transmissions for some amazing cars. They design the gear box to use a specific type of gear box fluid. There are no benefits to using the wrong type of fluid. Neither are rare or expensive.


ATF is a thinner fluid designed to lubricate by increased flow movement.
Gear oil is more viscous and therefore clings to the gears.
The thicker gear oil can introduce viscous drag which reduces efficiency.

ATF expands when it gets hot.
Gear oil burns off when it gets hot.
If you use gear oil, keep an eye on the levels.

ATF acts like a coolant inside the gearbox.
Gear oil is far less efficient at cooling.
Above spec temperatures aren't a massive issue but could cause issues with seals over long durations.

ATF is not a lubricant. It's a specialised fluid that also has lubricating properties.
It contains many specialised additives such as viscosity index improvers, kinematic index improvers,
rubber seal rejuvenating agents, sheer transfer additives, gasket conditioners, corrosion inhibitors, detergents etc.

Gear oil is (normally) a synthetic lubricant containing extreme pressure additives and anti wear additives.
It contains these in order to be able to run correctly in hypoid gears.

ATF is designed to run in gearboxes using straight cut, spiral bevel, helical tooth and planetary style arrangements.
Smart gearboxes do have helical tooth cogs but they don't have hypoid gears.

ATF has a longer life than gear oil which would need a more regular changing frequency.
Since smart (wrongly) decided that the gearbox fluid wasn't something that needed changing,
they went for a fluid with a longer life span.


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