New tyres.
 

Yesterday I had two new tyres fitted to my car and what a detrimental effect it has made to my fuel efficiency. The tyres that were on the car car were just bog standard, run of the mill tyres (not LLR or the like), as are the ones that have replaced them. The old tyres were down below 3mm tread so it was time to replace them.
On the 10 miles home from the garage, via a familiar route, I found that the coasting was being curtailed and on the DFCO downhill sections my speed was a lot lower at the end.
As soon as I got back to my own garage I checked my tyre pressures, and they were at manufacturers recommended figure for the car. This morning I increased the cold tyre pressures to my slightly elevated norm and on the usual commuter run I noticed the same effect.
I know that no two tyres will give exactly the same FE, but I sincerely hope that once all the pips and edges are worn off these tyres I will get back to what I was getting before.
Who would have thought?

Have to take into account the amount of rubber on the new tires compared to the old is pretty drastic. And The fact that you are actually traveling further per rotation of the tire now. May not seem significant but that adds up in a hurry. Just my take on it. I saw a massive drop in mpg when switching to new tires I blames most of it on the non LRR but have to attribute at least some to the additional weight ect.

Not only are there huge differences in RR between tires of the same size - up to 60% - but differences in state of wear also has a major effect - worn tires being much lower in RR than new tires. As you have seen, you have to be very careful in your selection.

My mpg is terrible since fitting the new tyres. I have checked the pressure a number of times (thinking I may have a slow puncture). I have checked for a dragging brake, but all fine. I know the temperature is down a bit now, but not a huge amount (it is only the autumn).
The big problem is these tyres will probably see the car out (the way I drive), and I am not in a position to replace them. Oh woe is me!

Check your distance with a GPS if possible. Your new tires could be a couple % larger now with full tread. Could account for some of it. New tires have always caused my mileage to drop for a while anyway.

Done a bit of research on line, and the tyres my local Honda dealer fitted could hardly be any less fuel efficient. On a scale of A to G, used here in the UK, they are an F. I think a tracked vehicle comes in at G!
I cannot measure GPS distance, but my acceleration feels a bit blunted, so slightly larger diameter/higher gearing may be a factor.

Well, on my beaters I just use whatever proper size the local used tire guy has that he can sling onto the wheel.

Side benefit, I get better service from them. Last time I went to a new guy and he patched a tire he could have replaced. He let me go down the street for change with no collateral and no promise that I wouldn't stuff him, so as I was driving, I turned around and just gave him what I'd have paid for a tire. Worth it. And I'll be going back to that guy.

So maybe my used tire thing has RR benefits? Makes sense but I'd never thought about it.

RR improves as tyres age and wear (traction may also fade).

If you buy a new car your economy peak will coincide with the first set of tyres wearing out, assuming no donuts ;)

The tread difference between a new tyre and a bald on is less than 1% unless it's a all terrain or something.

Not true, and the smaller the tire, the more difference it makes. For example, my car takes a 175/65/14. New tire diameter is 23". My summer tires are 185/65/14 and 23.5" diameter new. If tread wears down 1/4", that has reduced the diameter 1/2" to 23". That's 2.2% and a full tire size down.

My car takes 175/65/14's too. With the difference in tread depth between new tyres (8mm on these ones) to 2mm before changing them, this is a difference of 12 mm in diameter (disregarding tyre distortion).