Diesel or Gasoline car rental

Are you sure you can be certain which you'd get? Generally hire car deals say "or equivalent" so you expect a Golf diesel, but get a Focus petrol.

Unless you are taking a very long trip, with lots of driving, the difference in fuel costs is likely to be insignificant compared to other holiday costs.

Presently diesel is cheaper per litre in Portugal/Spain and, generally, diesel cars are more economical in terms of km/l, so diesel would save a few euros for a longish trip.

I have hired a car more than 20 times in Portugal and I have never been offered a diesel. Nor an automatic. They cost more to purchase.

Diesels are more cost effective for longer motorway runs but many modern unleaded cars are pretty economical too. I have a larger diesel automatic that does 50 mpg on a very good day on the motorway and my small unleaded manual runaround gets over 70 on the same journey.

Unleaded is slightly more expensive to buy in Portugal, but it’s not sufficiently different to impact on your choice of car.

We rented a car in Coimbra (in October) and weren't aware there was a choice. Our mini cooper was diesel - unexpected but luckily we figured it out before refueling the first time!