Diesel vs Petrol fuel consumption for the lead-footed
Discussion
Something I've noticed, and thought was sort of interesting, having borrowed my mums 2.0TDI Passat recently.
Diesels seem to eat fuel if they arent driven with great consideration to fuel economy.
I normally drive a 3.0L, 6 cylinder, AWD, petrol Subaru Legacy, and on a long drive, if I sit at 55-60mph, I'll get 31-32mpg. If I sit at "70" in the outside lane, just fast enough for the variable cams to do their thing in 6th gear, I get 29-30mpg. In the Passat, it'll do 60-65mpg if driven at the slow speed, but at the normal, outside lane cruising speed, it gets 34mpg.
So, what I'm wondering is, would a bigger, more powerful diesel engine use less fuel at that sort of speed? Say a BMW 330d? Or maybe a smaller petrol engine, like an ecoboost Mondeo? Or is it just a matter of the power needed to move a car at that speed is going to use fuel at such a rate that mid 30s to the gallon is the best any car relatively boxy estate car can do?
Diesels seem to eat fuel if they arent driven with great consideration to fuel economy.
I normally drive a 3.0L, 6 cylinder, AWD, petrol Subaru Legacy, and on a long drive, if I sit at 55-60mph, I'll get 31-32mpg. If I sit at "70" in the outside lane, just fast enough for the variable cams to do their thing in 6th gear, I get 29-30mpg. In the Passat, it'll do 60-65mpg if driven at the slow speed, but at the normal, outside lane cruising speed, it gets 34mpg.
So, what I'm wondering is, would a bigger, more powerful diesel engine use less fuel at that sort of speed? Say a BMW 330d? Or maybe a smaller petrol engine, like an ecoboost Mondeo? Or is it just a matter of the power needed to move a car at that speed is going to use fuel at such a rate that mid 30s to the gallon is the best any car relatively boxy estate car can do?
Not a LOT to add to this but my R56 Mini Cooper D will go into the low teens on MPG when accelerating hard and having fun.
But when being sensible it will go up to high 60s or 70mpg.
Just all depends upon how heavy the foot is that day i think!
ETA
I'm sure it depends upon the weight of one's foot. You can get surprising mpg out of anything if you are careful.
But is that how you want to drive?
But when being sensible it will go up to high 60s or 70mpg.
Just all depends upon how heavy the foot is that day i think!
ETA
I'm sure it depends upon the weight of one's foot. You can get surprising mpg out of anything if you are careful.
But is that how you want to drive?
Edited by Checkmate on Monday 13th January 03:43
biggbn said:
Something wrong with a passat that only gets 34 mpg at 70ish...
Indeed. The Passat sounds goosed. That’s pretty much the sweet spot for diesel. It should be doing 65 mpg or more at that constant speed. I found it hard to get much less than 50 mpg combined from any of the 2.0 VW diesels I’ve had.
My 3.0 BMW though would struggle to get more than about 40 mpg when driven the same way on the same roads.
I think that’s your answer.
Jim1064 said:
A lot of it has to do with the car's shape and aerodynamics. Flow patterns and drag can vary drastically with small changes in speed.
Yes, our Citroen C2 VTS with the 1.6Hdi engine was really economical for general running around and up to around 60, once above 60 a C5 or 407 with the same engine was more economical, typical motorway speeds could see 35mpg, that boxy shape wasn't cutting through the air.Its probable the bigger cars were geared higher too.
Around town turbodiesel cars aren't any better than petrol. The savings come when utilising high torque at low RPM. So if the RPM has to go up for whatever reason then the diesel is going lose its advantage. And turbocharging requires buckets of fuel so that reduces economy when you have lots of short spurts of acceleration driving round town.
Basically the case for diesel is when you know you're going to be doing a lot of steady state cruising.
Basically the case for diesel is when you know you're going to be doing a lot of steady state cruising.
The last time I was in a big, boxy 4-pot deisel it was an E250 and that did low 40s at "70mph" (which I'm guessing means not 70mph, in my case ~90mph in France.)
Unless you're driving the Passat like the throttle is an on/off button and doing that from stone-cold I can't see how you're doing 30mpg. That or it's got issues that need looking at!
There's data to suggest that the diesel-petrol economy gap opens as constant loads creep upwards (ie. you hold higher speeds) so speed isn't a leveller.
Unless you're driving the Passat like the throttle is an on/off button and doing that from stone-cold I can't see how you're doing 30mpg. That or it's got issues that need looking at!
There's data to suggest that the diesel-petrol economy gap opens as constant loads creep upwards (ie. you hold higher speeds) so speed isn't a leveller.
I will suggest this for some beardy, hyper mileage fun - 1.6 TDI Passat.
https://youtu.be/azQ67YEPCxs?si=goWqKRamEEDFJ1WA
https://youtu.be/azQ67YEPCxs?si=goWqKRamEEDFJ1WA
7 5 7 said:
I will suggest this for some beardy, hyper mileage fun - 1.6 TDI Passat.
https://youtu.be/azQ67YEPCxs?si=goWqKRamEEDFJ1WA
I watched that a while ago. Kept me interested. Started searching for the same car to see how much they cost / how available they are. https://youtu.be/azQ67YEPCxs?si=goWqKRamEEDFJ1WA
Smint said:
Jim1064 said:
A lot of it has to do with the car's shape and aerodynamics. Flow patterns and drag can vary drastically with small changes in speed.
Yes, our Citroen C2 VTS with the 1.6Hdi engine was really economical for general running around and up to around 60, once above 60 a C5 or 407 with the same engine was more economical, typical motorway speeds could see 35mpg, that boxy shape wasn't cutting through the air.Its probable the bigger cars were geared higher too.
Many moons back, I had a succession of big diesel BMW's. Of course they were the staple of PH in being mapped 335d's and therefore the fastest machines on the planet...
I had E9x versions and then F3x versions of them. All were great at mile munching and not too bad in the twisty stuff either. Across all driving, I'd average around 38 mpg. On long runs I'd get into the 40's and might even touch 50 if being particularly parsimonious.
Following on from that I went petrol with an RS3 and S3, then T5 Volvo V60. On the same roads and journeys, my average MPG went down to 28 for the S3 and T5(and lower still for the RS3...). On a run in the S3 and T5, then mid 30's is about as good as you are going to get.
Nothing scientific about this, other than what I've noticed over the last 15 years or so.
I had E9x versions and then F3x versions of them. All were great at mile munching and not too bad in the twisty stuff either. Across all driving, I'd average around 38 mpg. On long runs I'd get into the 40's and might even touch 50 if being particularly parsimonious.
Following on from that I went petrol with an RS3 and S3, then T5 Volvo V60. On the same roads and journeys, my average MPG went down to 28 for the S3 and T5(and lower still for the RS3...). On a run in the S3 and T5, then mid 30's is about as good as you are going to get.
Nothing scientific about this, other than what I've noticed over the last 15 years or so.
With my 335d F30 BMW, no matter how I drive I get an overall 44mpg. So each refill is at 500-520 miles. On one occasion, when there was a tanker strike, I travelled on A and M roads at 56-60 mph, and got that up to 620 miles a tankful. I don’t travel at peak times in congested towns, and there are only so many places that you can accelerate hard and often. I think smaller engines and petrol suffer more than big diesels from varied driving.
AmyRichardson said:
The last time I was in a big, boxy 4-pot deisel it was an E250 and that did low 40s at "70mph" (which I'm guessing means not 70mph, in my case ~90mph in France.)
Unless you're driving the Passat like the throttle is an on/off button and doing that from stone-cold I can't see how you're doing 30mpg. That or it's got issues that need looking at!
There's data to suggest that the diesel-petrol economy gap opens as constant loads creep upwards (ie. you hold higher speeds) so speed isn't a leveller.
Yeah I'm going to suggest an issue somewhere as well, a my big heavy 2010 A6 Avant 2.0tdi 170 would sometimes dip below 40mpg if I was pushing on, but at a steady 70-75 it would have been returning low to mid 40's, and it was the same with my Saab 9-5 3.0tid, which averaged 40mpg quite easily if I recall. The only other diesels I've had were a 2007 Passat estate 2.0tdi 170, and a 2016 Skoda Octavia Scout 2.0tdi 184 auto, and they would both happily sit at 45mpg+ while cruising north of 70mph.Unless you're driving the Passat like the throttle is an on/off button and doing that from stone-cold I can't see how you're doing 30mpg. That or it's got issues that need looking at!
There's data to suggest that the diesel-petrol economy gap opens as constant loads creep upwards (ie. you hold higher speeds) so speed isn't a leveller.
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