Safe Speed Forums

80mph speed limit will hit fuel bills
15 November 2011
Proposed new limit for motorways... - ...will increase the cost of your journey - Special What Car? study results

Increasing the motorway limit could hit you in the pocket, according to our research

A new 80mph limit on motorways would hit fuel consumption and increase running costs, a What Car? study has revealed.

These were the results of a What Car? study, after doing steady-speed tests at both 70mph and 80mph with a diesel-powered Ford Focus.

These findings go against the proposal announced by the then Transport Secretary Philip Hammond in September, when he said raising the speed limit would ‘generate economic benefits, worth hundreds of millions of pounds per year from savings of travel time’.

At 70mph, our Focus 1.6 TDCi 115 5dr averaged 51.5mpg with our test equipment and two people on board. However, at 80mph the car could manage only 42.8mpg.

Lower mpg figures mean higher costs for motorists. Assuming the price of diesel is 140.3p a litre, a mile would cost around 12p with our test car, but nearly 15p at 80mph. That’s more than £300 every 12,000 miles.

It would also lead to more frequent fuel stops; the 53-litre tank on our Focus means it can do 600 miles at 70mph, but loses about 100 miles of its range at 80mph.

Drivers would save time going at higher speeds, but only around 10 minutes every 100 miles if the route were clear of traffic.

Formal proposals to increase the speed limit for cars and motorcycles on most stretches of motorway in England and Wales are due to be put forward for public consultation this year, with a view to implement any changes in early 2013.

How the figures compared
Here's how increasing the motorway speed limit from 70mph to 80mph could hit your average economy and CO2 emissions.

Government reasons
Why the Government wants to increase speed limit to 80mph.
• 80mph represents best balance of cost and benefits
• Economic advantages from reduced journey times
• Cars are safer than in 1965, when the 70mph speed limit was implemented
• New UK speed limit would match other EU countries’
• Higher speed limit would mean more ‘legal’ drivers

Our study showed
• Fuel consumption rose by 17% on 1.6-litre diesel Ford Focus
• Focus’s tank range would fall by 100 miles at 80mph
• Running costs up by £300 every 12,000 motorway miles
• CO2 emissions went up by 20% on our test car
• The time saved would be approximately 10 minutes every 100 miles on clear roads

...and we ask ,why would children/ kittens/ cows ( except the old ones in Brake) be present on a UK motorway.

They wanted to get a Costa at the service station?

Increasing the motorway limit could hit you in the pocket, according to our research

Not if you don't go any faster than you do at the moment if won't.

Not if you don't go any faster than you do at the moment if won't.

It doesn't work like that' I'm afraid. Your little oil burner is likely to do even better to the gallon at a constant 30 (and probably better still at a constant 20), I'm afraid!

Basically, the faster you go, the more energy you use. No amount of fiddling with engine output or gearing will change that. Some years ago, when petrol engines were more common, it was true to say that they were generally most economical at a particular speed - 50-ish, because of the dreadful inefficiency of a petrol engine at part throttle. That's not the case with diesels (and is less the case with current petrol engines than it used to be). Diesels are now so efficient that they're remarkably linear. Beyond about 20 or so, the faster you go, the more fuel you use - end of!

It doesn't work like that' I'm afraid. Your little oil burner is likely to do even better to the gallon at a constant 30 (and probably better still at a constant 20), I'm afraid!

Basically, the faster you go, the more energy you use. No amount of fiddling with engine output or gearing will change that. Some years ago, when petrol engines were more common, it was true to say that they were generally most economical at a particular speed - 50-ish, because of the dreadful inefficiency of a petrol engine at part throttle. That's not the case with diesels (and is less the case with current petrol engines than it used to be). Diesels are now so efficient that they're remarkably linear. Beyond about 20 or so, the faster you go, the more fuel you use - end of!

This is really the point. Virtually nobody will do anything different to what they do now. It's just that it wont' be illegal.

It's all about aligning the reasonable behaviour of the majority with the law.

Instantaneous MPG isn't a good way of doing it because the tiniest change in circumstances (e.g. gust of wind, slight incline, aircon pump cutting it, truck passing...) can make quite a big difference at the instant you look at the readout. I find a better way is to put it on AVERAGE and drive for a mile or so at speed "x". The drive the next mile at speed "y" and see whether the average goes up or down. Obviously, it's not so good if the first mile was uphill and the second downhill, but you can knock it down to half a mile - anything just to damp out the wild fluctuations of the instantaneous reading.

I've been talking to these guys recently about some stuff for work. They use good kit - lab grade, and their methodology is robust - same car, same test track, several runs until statistically meaningful lack of variation can be demonstrated:

http://emissionsanalytics.com/the-impac ... emissions/

The graph is for CO2, but that's almost perfectly directly related to MPG.

As for being in the highest gear, forget it! You can use less fuel in 3rd than 6th. The amount of fuel used is almost linearly (see graphs) related to the amount of work required of the engine. At 20 it's not having to shove much air out of the way to push the car along. Seriously, try it, do even 1/4 mile at 20 on average consumption and then do the same 1/4mile at any speed you like (above 20!) and see what the result is. I was pretty amazed!

I don't think the views of someone published in the Guardian and the Independent (and a railway historian - so no conflicts, then) are worth taking seriously.

My views are not based on the opinions of others, just my own.

There is a letter in the Times today from the usual suspects (Brake etc.) crying about the 80mph limit proposal. Apart from the mass slaughter which that think will result, they quote a classic Ministry of Guesswork £1.5Bn cost for this change which, as noted above, will actually make almost no difference.