Waht can cause a sudden drop in miles per gallon?

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Hello, I did a 50 mile run today, about half way I noticed it said average MPG of about 49. Being a hot day I would have expected maybe 60-65. By the time I got back it was showing 43 average mpg. Its an old car but diesel and always got good milage. At one point at about 50 miles an hour it felt a bit bumpy (dont think I imagined it) But I think that improved.
Any ideas what it could be? I checked the tyre pressure and put a basic obd2 on which didnt find anything.
Cheers.
 
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Engines air less efficient on hot days. Turbo diesels usually have an intercooler (radiator to cool the air) as much as possible after the turbo, simply because cool air provides a better, charge into into the cylinders.
 
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my petrol car , has an efficiency display and shows what using up the petrol, AIRCON is a huge proportion on that , especially yesterday driving 3hours home, I usually get 44ish MPG , i got 50MPG driving last sunday to Devon in the rain,
Yesterday driving home , first part of journey was 35MPG with the Aircon on max blowing (it a convertable and the roof was open, but we still needed to blow cold air into the cockpit) overall the journey was 39MPG home , same trip.
 
Check out for black smoke in the rear view mirror, especially if you think the motor may be running less than smoothly.
John :)
 
my petrol car , has an efficiency display and shows what using up the petrol, AIRCON is a huge proportion on that , especially yesterday driving 3hours home, I usually get 44ish MPG , i got 50MPG driving last sunday to Devon in the rain,
Yesterday driving home , first part of journey was 35MPG with the Aircon on max blowing (it a convertable and the roof was open, but we still needed to blow cold air into the cockpit) overall the journey was 39MPG home , same trip.

A myth buster test, tested that - cars run more efficiently with all windows closed and a/c on, than with a/c off and windows open. Cool and wet, provides best engine efficiency.
 
cars run more efficiently with all windows closed and a/c on
Exactly , hence my example of very LOW MPG , which I expected and was happy with, but just to demonstrate how bad it could get, especially on a convertible, when not bothered by efficiency.
 
How many miles on the diesel?

Could be due to hot weather,

But could also be partially blocked intake manifold, diesels like to coke up the insides, seen many a PSA/ford 1.6 & 1.4 hdi do this.

Other makes don't also though
 
average MPG of about 49. Being a hot day I would have expected maybe 60-65. By the time I got back it was showing 43 average mpg.
I usually get 44ish MPG , i got 50MPG driving last sunday to Devon in the rain,
Yesterday driving home , first part of journey was 35MPG with the Aircon on max blowing (it a convertable and the roof was open, but we still needed to blow cold air into the cockpit) overall the journey was 39MPG home , same trip.
Just enjoy all those numbers - some of us dont get anywhere near them.
 
It happened to me not so long ago.
I soon realised the engine wasn't reaching operating temperature because the thermostat was stuck open.
£15 and half hour work sorted it.
 
Just enjoy all those numbers - some of us dont get anywhere near them.

Plus 1
Some car owners are obsessed with fuel consumption figures. Sure AC means lower mpg, but you can't have your cake & eat it too.
My only income is a pension, but I still enjoy the comfort of older petrol driven cars that were executive transport in their day.
The intake of damp cool air is always best for power (ask a racing driver) hot dry air & the use of AC will have an effect, sometimes dramatic ... get over it :whistle:
 
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