Does a small V6 give better MPG than a 4 Cyl?

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Elwood Balboa

Elwood Balboa

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I tried asking this on Yahoo answers and just got a load of smart arse ****s telling me how a 4 cylinder gives better mpg but they are unable to explain how, but I cant see how, here's the thing. I used to have a 2.5 V6 that would do 70mph at 1500rpm, my 1.8 mondeo does 70mph at 3000rpm maybe a bit more, which means half the revs for less than twice the engine capacity = more MPG for the V6, am I right to assume that a 3.0 v6 in the same car would do 70 at even lower rpm, and if so doesn't that mean it is more economic, if not why not? I can't get my head round it.
 
I would say there are far to many variables to make any kind of generalisation. LR's new larger capacity TDV8 is more economical than the smaller one it replaced!
 
Depends on the gearing really because a smaller engine will need to rev harder to make similar performance and bigger engines tend to use taller gears. Hence why Saxo boys used to stick a 1.1 gearbox in the VTRs to gain acceleration at the sacrifice of top speed and fuel economy
 
I don't see how there are variables, is it wasted fuel or am I being dense?

car 1 is doing 70mph at 1000rpm with a 3.0 engine

car 2 is doing 70mph at 1500rpm with a 2.5 engine

car 3 is doing 70mph at 3000rpm with a 1.8 engine

which is getting most MPG?
 
But you'll need more fuel and air to fill the cylinders on the 2.5 on each stroke compared to a 1.8 and some engine designs are more efficient than others
 
All else being equal, a 6 cylinder engine is less efficient (and therefore less economical) than a 4 cylinder engine, because it has more moving parts and greater friction etc.

All else being equal, a petrol engine of lower capacity is more efficient than a larger one PRODUCING THE SAME POWER because the larger one will be at part throttle, which creates an inefficiency due to pulling slightly more air (in volume), at a higher vacuum.

However, larger engined cars are typically higher geared (because the engine can pull more torque, so there is no need to be lower geared) and this will result in lower rpm for a given road speed, which (due to friction etc) is often more efficient.

So for the small capacity vs large capacity, there's often not much in it and it can be the 'other way round' than you might logically assume. I have a 2.0 16V Mk1 Golf (170 or so bhp), when driven sensibly it returns 38mpg. Owners of 1.1 and 1.3 Mk1 Golfs (45-50bhp) get about 35mpg in the same conditions.
 
Right, I just think I don't want to believe it, but your answers are all better than those half wits on yahoo, so thanks, but here's the thing, I'm gonna get this 3.0 V6 no matter what, I've already bought it, I've just got to go and get it on Saturday, how am I going to convince her that it's better to have it when we start hemorrhaging money on petrol?
 
I can't work out what car - what Land Rover - you're talking about, my post was just very general facts (based on solid engineering facts though). Have you looked at the official fuel consumption figures. Take them with a pinch of salt, however.
 
paul_c said:
I can't work out what car - what Land Rover - you're talking about, my post was just very general facts (based on solid engineering facts though). Have you looked at the official fuel consumption figures. Take them with a pinch of salt, however.
Click to expand...
Just something to get me between my 88" which I just sold and my 109" that I've yet to find.
 
ricc said:
what car is it that will do 70mph at 1000 rpm... ive never bseen one geared that tall... by some way
Click to expand...

that was just based on the assumption that it would be lower rpm than the 2.5, I should've worked it out proportionally I suppose more like 1250rpm.
 
:doh::doh::doh::doh::doh:

Engine size has got nothing to do with gear ratio, or RPM at any given speed! :doh:

How do people get these ideas into their heads?? :confused:
 
I get that, but more cylinders=less revolutions when coupled with the same gearbox, is that right? I can't even remember what I thought just now.
 
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