Speed effect on MPG

[chris]

I'd heard all the figures about how driving affects your MPG, etc in specific relation to driving at 65MPH.

Sooo, I've done a little experiment.. Despite not being in work over xmas and new year today was my final driving day of my weekly experiments.

Quite shocking really!

I drive my wife's car, as my car has the child seats in it - it's a lot more economical too, being a 1.4 petrol mini, but anyway:

65MPH = 59.9 MPG
70MPH = 49.8 MPG

(whilst I obviously don't want to admit I've broken the speed limit, I wanted to see what affect it'd have one my fuel consumption - driving slightly above the limit (by 5mph, I saw a consumption of 42.3MPH for a trip to work and back home. I've no idea what that would average out to over a week, but that was enough to put me off!)

This is driving ~40miles a day on a motorway, then a couple of miles urban roads, using the same petrol station for fuel and driving at the same time pretty much each time - each period for a week.

Soo, I've decided that I'm going to try and drop my speed by 5MPH, I drive in quite early, and leave before the traffic builds up on the motorway so I hope I shouldn't effect other folk on the road, but almost 10 miles extra per gallon, that's surely got to have a reasonable impact on my fuel bills!

Once we've finished the house, I'm going to save up then for a new car (well, not "new" new, but ya know!) - more than likely a diesel now that I'm doing this amount or driving, which is likely to increase in the next month or two as well.

[SarzWix]

When I drove to Slough for my sister's wedding, in my V reg 1.3 Jimny, I drove pretty much the whole way at 55mph; apart from the fact that the car wasn't built for speed, any time I went above that I could actually see the fuel gauge dropping... You get used to it quite quickly, and find that all the hair-raising lane swapping and other carry-on mainly happens in the outer 2 lanes, the slow-lane is quite a chilled out place to be

[chris]

I can see the fuel gauge drop in "my" car - which is a CR-V. It's really not built for long m'way driving, and I feel like such a tool for getting a petrol version.

I still had to over take a few folk, but the annoying thing was people hovering to my rear-right roughly at the same speed not letting me pull out. Then they'd slow down again after I did, then speed up and overtake me, slowing down again. Nowt as strange as folk!

[SarzWix]

At 55 I found I was doing about the same speed as most of the trucks. Had to overtake them every now and again because following the same one for longer than about 20 mins was too soporific But other cars mostly passed me and I didn't have to worry about them at all The only bad bit about it (apart from it taking about 7 hours to get there) was when there was a bit of a hill and I didn't get out from behind the truck quick enough and then I'd have a truck behind, one in front, and another trying to overtake us all at about 2mph faster; felt a bit like being in a truck sandwich and that wasn't pleasant!

Only took ¾ of a tank of petrol to get there though

[bearded bloke]

Mine goes nearly two tonne & has the aerodynamics of a brick,but if I'm really steady & on flat land it can do just over 42 mpg (that was the NE France leg of a road trip to Turin a few years back),over the complete journey (just over 2500 miles) the average consumption was 38.6 mpg.

Before rushing out to buy a diesel model have a play with a calculator & workout the costs not in mpg but in mp£,the difference in pump prices almost negates the potential savings in most cases,the other thing to check is the road fund license price for same model of each fuel type.

[veggiechicken]

I make the same journey as you Chris about twice a week - about 55 miles M4 and 20 miles urban/rural. Plus round town stuff during the week. My car's a 1.4 diesel and I "try" to stick to the speed limit The car is 4 years old and average fuel consumption from new is 56mpg.
If I slow my speed and avoid the hilly route I can average 60+ on that journey. But, as BB says, the petrol/diesel price difference has widened over the years and diesel cars may cost more? My road tax is £30 (I think?).

[chris]

Indeed, I have done, they seem to tip the scales past ~20k miles p.a. The Tax band will go up by 1, but we're still saving from my previous car's tax band (THAT hurt!)

Edit: I should also add, it was a bad buy.. a mini - with two young kids?

[scarey55]

We were driving back from Paris on New Year's eve and we were hoping to make it into the house in 2012 (missed it by 2 mins ) We usually drive the Autoroute at 110 kph (68) but, as we were hurrying, we drove at the speed limit 130 kph (81) and the effect on the fuel gauge was staggering, usually do it easily in half a tank but, as you said, we could almost see the gauge dropping as we watched it.

[WillyNelson]

I might be playing devil's advocate here a bit, but...

I think a lot of it is to do with small underpowered engines just aren;t designed for m-way cruising. They're fine for pootling about town but when you have to get up to 60+ they have to rev so high that they're drinking fuel.

We had a hirecar before xmas as ours was in the garage and the difference in a 1.4 petrol from a 1.8 diesel was staggering. 4500rpm as opposed to 2500k, 66 more little explosions every second, and I could see the fuel guage dropping. A motorway cruiser needs a bigger, more powerful (ie torquey) engine to enable it run taller ratios in 5th & 6th gear, which will in turn lead to fuel economy in the high 40s & 50s. And a small diesel is useless as an inner city runaround because of the added cost never being outweighed by the mileage.

So it's not always a case of changing how you drive, but also of suiting the car you buy to the driving you do.

[Kristen]

I read about hypermiling a year or two back and have been doing that since. On rural journeys which I know like that back of hand (school run for example), and thus where I can anticipate the road ahead, I get 30% better MPG compared to how I used to drive. Infuriates my wife when I drive 10 seconds behind a vehicle, but when there is no chance of overtaking following closer just means I have to brake when they brake ...

In essence try to never use the brakes - obviously not possible when you come to a T-junction at the bottom of a hill!

And coast in neutral (yeah I was taught that I wasn't in control if I wasn't in gear, but no one has ever been able to tell me why)

SatNav is handy because it gives me earlier warning of roundabouts than the road signs (mine alerts at 800 yards). On a dual carriageway I select neutral from the 800 yard point and usually only drop from 70-ish to 60-ish, and if there is any traffic in front of me I will have caught it up by the time I reach the roundabout.

When I am not [gently] accelerating then I coast in neutral, rather than maintaining a steady throttle. (so called "Pulse and Glide" style of driving). I think that has little gain, in most vehicles, on a motorway, but even then I use it on downwards slopes which have less/no friction effect. On roads I know well I can time the Pulse to then give me a long Glide on more downhill sections. Its pretty flat in East Anglia, but every downhill slope helps.

If I am going to have to brake (following at 10 seconds and clearly car(s) in front are slowing/stopping, or approaching a roundabout with stationary traffic) then I first slow down in gear. Apparently modern engines don't use any few costing in gear and dashboard of last few cars I've driven have all said 200 mpg when coasting in neutral and "---" (blank) when coasting in gear. Usually get several hundred yards for "free".

I maintain constant-throttle up hills (speed drops), rather than constant speed. Surprising how much extra throttle, and thus fuel, it takes to climb a hill

These days I'm disappointed if I can't get 10mpg more than my wife (same car, same route). She drives "Normally" and is not a tearaway, as such, but it shows what can be achieved without too much trouble. Keeps me concentrating too, trying to keep the MPG dahsboard readout at a figure that I want

[bearded bloke]

Come on who else has still got their petrol ration book,remember back in '73 when there was the oil crisis & the government looked to reduce the nations fuel consumption & imposed a national maximum of 50mph.

[veggiechicken]

On a more general point, if the powers that be were to make the inside lane on a 3 lane motorway - a "slow" lane with a max speed of 60, then it would become acceptable to drive at that speed without having to put up with go-faster freaks driving in your boot, nipping in and out around slower cars and lane changing ad nauseum.

[janeyo]

Drives me bonkers being behind people in perfectly good driving conditions who do not drive at the speed limits. It's dangerous as can make people too impatient to get past them.

I have a mondeo estate diesel. I get 50mpg average out of it (mostly to town and back and school runs), more in cold weather than I do in hot weather. When mr janeyo drives it, as he has been ferrying me about for 6 weeks as not able to drive after c-section, he has only got 46mpg! He drives with a much heavier foot than me, and I don't hang about! Pretty sure if my dad drove mine he would have it up at the 60mpg mark lol

[veggiechicken]

Come on who else has still got their petrol ration book,remember back in '73 when there was the oil crisis & the government looked to reduce the nations fuel consumption & imposed a national maximum of 50mph.

I have somewhere!!

[scarey55]

I might be playing devil's advocate here a bit, but...

I think a lot of it is to do with small underpowered engines just aren't designed for m-way cruising. They're fine for pootling about town but when you have to get up to 60+ they have to rev so high that they're drinking fuel....

....So it's not always a case of changing how you drive, but also of suiting the car you buy to the driving you do.

I think you have a good point there Willy. The car I mentioned is a Renault Twingo 1.4 LEV? and I expect that it isn't designed for long motorway journeys which probably added to the bad fuel consumption.