I was getting 30 mpg on trips at 70mph steady state on the highway. 32 at 65 mph.

for many reasons we had not driven long highway stints since early 2019.

just made a highway trip. Steady 70 mph. 26 mpg. What the heck?

just hit 32,000 on the odometer.

the dealer replaced the exhaust system because we had a check engine light on almost constantly for 23 months.

that is the only change we had.

any ideas or similar experience?

 

Maybe the fuel isn't as good as a year ago?

 

Which engine? My Si6 (petrol) doesn't get close to that...

Same fuel from same source?
How are you measuring consumption - based on what the vehicle reports or what you actually put in the tank?

 

It’s tricky to compare on anecdotal evidence.

Are you comparing the exact same trip?
Same length, duration and speed?
Same driving style eg acceleration and traffic?
Same temperature both for the engine and your AC use?
Same tyre pressures?
Windows or sunroof open vs closed?
Is the weight the same, empty truck for example?
Same tyre size and brand?
Lifted or not?
Same fuel grade?

Seriously, I’m not saying there isn’t something wrong but comparing fuel economy with all the variances there are is very tricky. You’re talking a 12% ish drop which is not insignificant but the fuel economy my wife gets in the same vehicle as me making the same trips is about 20% lower even though she doesn’t speed or anything, she just isn’t as smooth in her driving.

 

TD6 diesel

 

I perceived a slight drop in my TD6 mileage after about a year or so. However, it was well within the variance of 1) initial enthusiasm for the mileage I was getting vs. reality 2) Differing conditions while driving the same route, ie: wind direction/speed, temp, vehicle speed 3) my own driving habits evolving.

It's funny though, because lately I feel like the mileage has increased back again at 45k miles.

 

Indeed pretty much what he said. I was using some cheap diesel for saving on money for like a year due to financial problems and switch to Shell V-Power like 2 months ago. The difference in performance and mpg seems unreal.
Every little thing affects mpg, even a good cleaning of the engine(at least for mine).