Engine oil grade v fuel consumption

Not an area I've bothered to look into much before but in theory thinner oils might reduce internal friction enough to affect power and therefore possibly fuel consumption. I've never considered it to be likely that the effects are going to be big enough to be noticeable between say a 10/40 oil and a 5/30 though.

However I was away on business in Aberdeen for a month recently which involves a 550 mile drive up from the London area. I go up there a fair bit and have done for many years so I know exactly what mpg each car tends to give on that route. The current beast, 2001 Focus 2.0 ESP always give 33 mpg. Doesn't seem to matter if I drive a bit faster or slower whereas previous cars have been more speed sensitive in their mpg.

On the way up last time sure enough the two tankfuls were 32.7 and 33.3 mpg averaging exactly what I was expecting. What oil was in the engine when I bought it 12k miles ago I obviously have no idea but it's been topped up with some old 10/40 semi synthetic I had knocking about and yes I know that's A) not the recommended oil and B) I shouldn't have been such a lazy git and should have changed the oil long ago.

Anyway, while I was up there and had the use of a car hoist where I was working I finally got round to changing the oil and filter. A friend found me some fully synthetic AC Delco 5/30 for only a tenner a gallon which seemed like an absolute bargain. Absolutely nothing else was touched, tyre pressures included. On the way back I noticed that the fuel gauge was going down more slowly than expected. I know exactly how many miles it takes to reach each mark on the gauge at a given mpg and something was clearly behaving differently. The first half of the trip was quite normal with little traffic and I filled up in Widnes near Liverpool after 367 miles to find I'd got 35.7 mpg. I drove at my usual speeds and it also took exactly the same 6 hours door to door to get from Aberdeen to Widnes that it taken to get from Widnes to Aberdeen on the way up. I stopped at friends there briefly to collect and deliver stuff on both legs of the trip. The average speed was therefore identical and no obvious reason for the mpg to have altered.

The rest of the trip wasn't so good. Leaving Widnes at 4pm on a Friday afternoon I crawled out of the town in heavy rush hour traffic, got a brief clear run on the M6 and then sat in a bloody accident induced tailback with the aircon on for 45 minutes. After 2 hours I'd only covered 80 miles. Finally I got past Birmingham, the roads cleared and I gave it some serious beans down the M40 because there are no speed cameras there, everyone else drives like lunatics anyway and I'd been on the road for 9 hours already and wanted to get home. So basically the second tank was a mixture of heavy traffic, a solid traffic jam and a 90 plus mph blat for the last 100 miles. None of these conditions ought to have been good for fuel consumption but the gauge was going down even more slowly than the first tank.

I finally got home and before polluting the rest of that tank's mileage with any cold starts and general pottering about I filled up to satisfy my curiosity to find it was a somewhat astonishing 38.0 mpg. The only time I've managed 38 mpg before was an experiment shortly after I bought the car when I drove at 50 mph for 90 miles to see what I could squeeze out of it after noticing that the consumption was generally s**te compared to the Mondeo that preceded it. I've never got even close to that at normal motorway speeds though.

I've had the car for 3 years, done several Aberdeen trips in it at the same mpg and there's nothing else I can attribute the extra mpg to than the oil change. To go from 33 mpg to 36 mpg would be astonishing enough at 10% extra economy but 38 mpg on the same roads is beyond belief. In fact I've almost recovered the cost of the new oil in the petrol savings on one journey. I've worked out that if I'd put in that oil when I bought the car I'd have saved £170 in fuel costs already.

The acid test will be in a couple of weeks time when I go back up there. If the extra economy repeats on the route north then I'll consider it proven. I'll also put the car on the dyno there to see if the power has altered. Last time it was run on the old oil it gave 142.5 bhp so I'm expecting that must surely have gone up if the economy has too. If both factors have changed then I have to conclude that the oil has made a material difference.

To those using cheap, i.e. thick non synthetic oil from your local car parts shop or supermarket it might just turn out to be a very false economy. You could be losing a lot more in fuel costs than the extra cost of a modern thin synthetic oil.

-- Dave Baker Puma Race Engines

Reply to
Dave Baker
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The message from "Dave Baker" contains these words:

Drag, not friction, Shirley?

Reply to
Guy King

I wouldn't argue with your findings with your 2.0 Focus and oils, but I've found those sort of mpg variations with different brands of petrol in my 2.0 Primera. Good ones for my car being Shell, Esso, Total (best). Poor ones being supermarket, BP. Did you fill up with different petrol perhaps? Another factor with mpg on fast runs could be tail winds, if the wind changed round so it was a tail wind both ways then who knows what mpg you would get.

I've tried synthetic and semi of different grades and never noticed any mpg or power differences of much note, which is not to say there aren't any. I usually get over 40mpg on a long fast trip by the way. Just a thought, isn't the 2.0 Ford engine prone to sticking valves with non-Ford oil?

Reply to
Steve B

I've been keeping records of my fuel use for about 4 years (first it was curiosity, then a strange compulsion, latterly a requirement for business expenses).

Brand of petrol doesn't really seem to correlate with mpg in my experience. Every now and again I get a slightly anomalous figure but nearly always there is a counterbalancing tankful afterwards, so I put these down to a problem brimming the tank.

However, I have recently had the car (Civic 1.6) serviced, and the last 4-5 tankfuls have seen a step-change upwards in mpg of about 2-4 mpg, consistently. There is no similar increase on the mpg graph in the past, nor can I really discern any steady drop in mpg from any one service to the next. Indeed, the only noticeable trend on the graph is a simple seasonal cycle, winter driving being about 2-3mpg poorer than summer.

So, I wonder if the last service (just an oil-change according to the book and the dealer) has made some significant difference ? Oil grade is 10W/40 semi-synth, and they don't use anything expensive (about =A39 on the bill last time, iirc).

I thought 33mpg was pretty poor for a 2.0 Focus, too ;-) My long-term average is about 41, with a best of 46 and a worst of 35 (part tank of short journeys around town in winter).

-- "Do NOT look into laser with remaining eyeball!"

Reply to
John Laird

I ran a 1.8 Focus for 65k miles.

I too found that it was heavier on fuel when running on 10w-40 oil -which i put in for the summer, as I was caught short wihthout any 5w-30 and figured it would be ok.

Mpg dropped as you have experienced by about 1.5mpg.

Putting 5w-30 back in it for the winter (despite the colder weather) restored the economy.

Tim.

Reply to
Tim..

My car has a mpg computer on-board. It computes and updates a readout on the dashboard every 5 seconds. With that I have been trying to work out the most economical driving speed for *my* car. So far a definitive figure has not emerged, but throughout these tests (still on going), what surprised me was the *enormous* difference a car's road speed made on mpg.

For your experimental result to be meaningful then, the road speeds need to be the same and constant on both trips, up and down the motorway. Start the experiment (filling up to the brim) at one filling station at one end of the motorway and finish at another filling station at the other end. Change the oil and repeat the same identical motorway run in the opposite direction.

A full tank is heavy. The 45 L full tank in my car weighs about 100 lb. You have to be careful to make both trips carry roughly the same weight. The procedure above should eliminate this variable if the tank is filled right up at both ends.

For a sub-compact, the most economical speed is said to be around 55 mpg. Whether a less viscous oil makes any *material* difference I have yet to see any published evidence. Hence, I'll be waiting eagerly for the report of your experiment. Lucky you, you have a dyno at your disposal. :)

Reply to
Lin Chung

I've kept fuel records for every car I've owned for over 20 years and I don't find the slightest correlation between fuel brands and mpg nor do I actually think it possible to measure one because mpg varies so much between tanks anyway as the mix of driving and the ambient temperature changes. Only on a long trip at a steady speed or as an average over several tanks can you determine trends.

FWIW I filled up when I got to Aberdeen at the Asda supermarket and used that whole tank in general pottering about during the month and got 30 mpg which is exactly what I'd have expected. I changed the oil and filled the tank again at the same place the night before setting off so the type of fuel had nothing to do with my increased motorway mpg. What brand I bought at Widnes I have no idea but it's always the bog std unleaded.

The early hydraulic Zetec engines suffered from lifter pump-up if the oil was too thick which held the valves off their seats. It was actually nothing to do with valves sticking as was commonly thought. The Focus engines don't have hydraulic tappets anyway.

Reply to
Dave Baker

The Focus is quite a big heavy car and my version also has the 205 low profile tyres with corresponding increased drag. It's not the world's best vehicle for economy. However over 20 years of fuel records there isn't a lot of difference in mpg at motorway speeds between most of the cars I've owned other than the really small economy ones such as a 1.1 Polo I ran briefly.

Mid 30s mpg seems to be about it at 80 mph indicated whatever the vehicle. It's possible to do much better at lower speeds though, or at least with some cars. I once got 51 mpg out of a quite heavily modified injection Fiesta by doing 50 mph when that car's normal mpg at 80 mph was about the same as the Focus. However as I said previously the Focus doesn't really respond mpg wise to low speeds like a smaller lighter car would. It has too much rolling resistance due to its weight and tyres.

Reply to
Dave Baker

Interesting, but I would rather use the oil that the manufacturer designed the engine to run with. I would put it down to you driving differently knowing in advance you wanted to prove a point. It's similar to people who think that certain brands of premium petrol make a car faster.

I take it you don't really know where shop or supermarket oil comes from then! Similar to large companies selling products to supermarkets to sell under their own brand name for a lot less.

I hope you studied hat type of oil filter you used as they ARE very different and some are useless.

Reply to
Neil

Since I had no idea it *would* make a difference and hadn't given the matter a moment's thought prior to finding out how the mpg had altered that clearly isn't an issue. As I took pains to point out nor was my average speed or journey time at variance. Whether the oil is indeed the whole issue is another matter. Longer term testing will reveal the answer.

Similar to large companies selling products to supermarkets to sell

... and I take it you haven't spotted the significance of my sig, or no doubt my website

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One thing I do know a reasonable amount about is engines and the bits that comprise them.

Reply to
Dave Baker

Ummm, I thought that's what I just did.

About 74 lb actually. Petrol has a specific gravity of about 0.75.

Reply to
Dave Baker

=================================== I think that there are too many variables to confirm that your improved fuel consumption is related solely to the change of oil. For example, others have mentioned wind direction and of course there are many other things. However your theory appears to have quite a lot of support in knowledgeable circles. This quote lends support to your theory:

"Less well known, however, is the fact that the fuel consumption can be significantly improved just by changing the lubricants. For example, it is possible to decrease the amount of fuel consumed by modern cars by up to

5% simply by switching from a typical multigrade oil to a "friction-modified" lubricant with a lower viscosity."

The full article here:

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Cic.

Reply to
Cicero

I agree and it will take a few more tanks and another long journey before I'm certain there's a lasting improvement and of what sort of magnitude. It was definitely a much warmer day coming back down than when I went up and that on its own would have helped economy somewhat. I don't think there was much wind either day but again I may have had some help from it on the way back and a headwind on the way up. Looking back over my records of previous trips for 10 years or so there does seem on a couple of occasions to be slightly better mpg driving back south than going up but then the economy of previous cars was much more affected by average speed than the Focus which seems to be relatively immune to it.

However your theory appears to have quite a lot of support in

5% is much more like the maximum I would have thought oil visocity could possibly affect mpg but certainly that second tankful of mine in quite adverse driving conditions was a big surprise at 15% better than the norm. I have no idea how that tankful in heavy traffic and tailbacks and then a fast final stretch could have been better than the first at steady speeds on open roads. It's almost as if the oil change was steadily improving something inside the engine. Flushing out crap maybe. Maybe the old oil filter was blocked or faulty too and soaking up power from the oil pump. I have no idea how long it had been on there prior to me buying the car of course.

-- Dave Baker Puma Race Engines

Reply to
Dave Baker

Odd, I could get over 40mpg on the motorway out of a 2.0l A6. Which obviously has 205s & a bit more weight than a focus.

Reply to
Duncan Wood

I used to be able to return 38/39 mpg on a long 80mph run in my old 2.0 Mondeo which has the same engine as Dave's Focus but which is a little heavier. That was also with 205 tyres.

Reply to
Paul Giverin

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Dave Baker" saying something like:

I can feel the difference in my Tranny - the recommended is a semi-synth

5/30 but when I use a 10/40 there's a slight but noticeable loss of power. Changing back to the 5/30 restores it.
Reply to
Grimly Curmudgeon

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Dave Baker" saying something like:

Downhill, innit?

Reply to
Grimly Curmudgeon

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