Need to rent a car in Rome. Looking at gas vs. diesel. Need to know the price of each one to see which we one we should rent. Also, is the price by the gallon or liter?

They use litres in Europe. Gallons (USG) are a North American thing. Gallons (UK) are a British thing.

Diesel costs more per litre, but the cost differential is compensated by the better fuel economy of diesel fuelled engines.

Diesel. Costs more, but the car gets way better gas millage, unless you rent a Nissan Micra or other super small car, then gas is better. The Micra I has gets 53mpg on gas.

According to this site: http://www.hidden-italy.com/car/drive06.html diesel is cheaper than gasoline in Italy.

Yesterday's average price of diesel in Italy was €1.50/litre. Petrol was €1.63/litre.(http://www.prezzibenzina.it/)

This is not universally the case in Europe, and the two prices fluctuate differently, so the difference between petrol and diesel can change.

But, with the better mileage of a diesel engine on a like for like car, even in the UK (where diesel's usually costlier than petrol) hiring a diesel car will always create a lower fuel bill than hiring the same car with a petrol engine.

Diesel cars offer better mileage. It can also be cheaper, depending upon the country (as noted, it is cheaper in Italy and is cheaper in Denmark, too). Diesel performance has gotten better, but I still find it a bit rougher running than gas. Of course, you aren't racing and the savings are worth it. Pricing is in liters.

You may not have much of a choice anyway. You are kind of beholden to what the agency has in stock.

Diesel/gazole is much cheaper in France than gas/petrol, as much as 8-10 cents per litre less.

But in reality will you get a choice? When booking any car it is always subject to availability.
Just book the size of car you need and see what turns up.

Agree, you won't get a choice. You'll get whatever car they have in the size you booked, or sometimes a size bigger.

You can always ask for one or the other if you have a preference. My dad used to always say, "You'll never know unless you ask."

In all likelihood, if you rent a really small car it will take gas. If you rent a medium-size one, it will be diesel. You can ask, but don't expect your wishes will be granted. They'll give you what they have available, and unless a very small car, it will most likely be diesel.

<i>In all likelihood, if you rent a really small car it will take gas. If you rent a medium-size one, it will be diesel.</i>

The past two small cars (Citroen C3 & Ford Fiesta) I've rented have been diesel.

Diesel tends to be less expensive in Europe than petrol. The opposite is currently true in the US. Given the 13 Eurocent difference quoted by Flanner (which means about 70 cents per gallon in US dollars), get a diesel if possible.

And do you REALLY need a car in Rome? They drive like Italians. That's not a compliment.