One of the advantages of Ford cutting costs and building the new Ka on the same platform as the Fiat 500 is that it gets to share Fiat's engine technology. Which means that a common rail diesel is available for the first time in the Ka, so suddenly it becomes more useable. The diesel gives it the extra punch to make higher speed journeys easier and it's no longer consigned to the supermarket run.
Not that the diesel gives any monetary advantage. The petrol Ka emits 119g/km of CO2, the diesel manages with 112g/km, so neither gains any tax advantage because neither comes in under the magic 100g/km. If you factor in the 12mpg difference (the 1.3-litre 74bhp TDCi has an average fuel figure of 67.3), you've got to be doing nearly 20,000 miles a year to make an actual running cost saving with the diesel. No wonder Ford reckons the TDCi will only account for four per cent of sales.
So buying a diesel Ka as opposed to a petrol one is not a financial decision. This is even more true given the diesel is £700 more than the petrol (and the diesel is only available in top-spec Zetec trim).
But from a driving point of view, the extra cost is worth it. The Ka diesel seems happier in itself and you've got lower expectations of how the Ka should handle. Somehow, in the TDCi, you're not as aware of how fantastically awesome the last gen was to drive. You accept it for what it is - with this engine, the Ka simply becomes a tool to get from A to B, nothing more. It's not entertaining and it's not trying to be. The petrol is too hamstrung by its immediate ancestry.
The headline acceleration figure isn't impressive at 13.1 seconds from 0-62mph, but around town and out on the motorway it never feels that slow. You can certainly sit in the fast lane without feeling like a mobile chicane. The extra torque over the petrol helps of course (107lb ft as opposed to 74lb ft), but so does the low turbo entry point and the fact it's only got five gears. It means the engine rarely drops below that turbo entry figure of 1,800rpm.
The lack of a sixth gear means there's a little bit more diesel rattle than would be ideal, which never completely disappears even on the motorway. But there's less NVH than there is in the petrol at high speeds. Which is, after all, what's important in this car and what makes it a better all-round tool. It should be just as comfortable sitting at 70mph as it is tootling around town. It's not interesting or entertaining, but that's where diesel city cars need to work. This car, sorry Ka, does.