Does a car run more efficiently if it is used more often?

No, but you are more efficient at using the car, because now you are familiar with its quirks and behaviors and it is second nature to you as you drive your daily commute. There may have been a bit of a break in period as you used it more, but that’s more for new cars.

Do you know what the previous owner’s mileage was? And are you calculating this yourself or are you relying on a display from the car?

It has had two owners before me and is 7 years old. It had done 37,000 miles with a full service history (ie it was taken into the Renault garage every year for a service and the mileage noted in the service book and stamped with the dealer’s stamp. That sort of thing is hard to fake in the UK I think). I did wonder if I was just getting used to it. I do know that you need to take your car out for a long run occasionally if you just use it to pop to the shops so I was wondering if the long runs i do everyday have helped improve efficiency

If I just drive around town and only for a short time, the car never warms up to top efficiency, and the stop and go traffic eats into my mileage. If I only get onto the freeway and drive an hour or so each time, my mileage goes way up. So I’d ascribe it to better driving habits rather than anything new the car is doing.

Maybe. It’s just hard to imagine that my driving is that much different. If anything I would say I drove more cautiously when I first got the car than I do now as I was used to driving an older car that billowed out smoke if I tried to accelerate hard or drive about about 60mph!

IMO the way the vehicle would get better would be like when you sit around still for a long time then try to move again, your joints are stiff. When a car sits around for a long time most of the oil drips down into the pan and leave all the small cracks etc. it was in. As you start to drive it regularly the oil gets pumped throughout the engine and don’t have the opportunity to drop all the way back into the pan, so on subsequent trips, there is more oil available than when it say for months. Not only that, once you start driving the car depending on the gas, gunk that was in the injectors gets blasted out, on top of that, the rings will start to reseat better.

No. It will be more efficient if it is allowed to fully warm up and is driven many miles while warm. That constant starting up, driving a short distance, then shutting it off and letting it get cold isn’t efficient.
Your MPG would/will vary if you are hitting a head wind or are blessed with a tail wind. How warm or cold the ambient air makes a difference, too. Drive it like you’re mad at it!
Want to make that diesel last a long time and always run strong and smoke less? NEVER idle the engine! (Idling is the WORST thing you can do to a diesel!!!) If you have to run it to warm it up, run the engine at a minimum of 1000 RPM. Keeping the engine hot reduces varnishing of the cylinder walls and sticking of piston rings, therefore keeping compression at a maximum and blow-by at a minimum. This is good for efficiency!
The quality of fuel could be responsible for a variation of MPG, too. Winter fuel has (should have) anti-gel added, summer fuel does not.

@kritiper – who are you saying no to? i understand that it is less efficient to stop and start and do ‘urban’ driving. My question was to do with the length of time it took for the mpg to settle down. I drive for 40 minutes on a motorway doing 65–70mph every week day. This is not a variable. It was at least a month of this kind of driving before my mpg settled at 51/52 mpg. Until then it was lower. I have just remembered that it was actually 46mpg that first two weeks. I drive the same route to and from work so any head/tail winds would cancel each other out. It’s been winter the past few months with not much variance in temperature.

I have been wondering about the fuel quality. In my old car the mpg was fair consistent but occasionally I would have to buy diesel from a different, local garage. Usually I buy from the supermarket as the price is very competitive. The local garage is not very much more, which is unusual for a rural petrol station. My mpg would suffer badly. I supposed it was cheaper quality fuel would make sense of the lowish price of the fuel.

I think I have too much time on my commute to contemplate these things!

@Stinley I drive my car very consistently every week and the MPG varies. The amount or type of anti-gel could be the deciding factor in your MPG. Different fuel makers could have a blend of fuels and additives that could alter your results. How full your tank is or how much extra weight you have in your car are factors as well. For the fuel MPG to settle down might have to do with how much fuel was in the car when you bought it, what type/blend it was, where it was bought and how long it sat in the tank before you bought the car. Fuel will degrade over time and could lose it’s optimal potential efficiency (BTU’s).

If a vehicle is left to sit for long periods of time the seals will dry out. This can lead to engine inefficiency…