80mph limit on motorways.

[Brigadier]

MPG does not decrease by 20%, i get identical mpg whether i drive at 70 or 80.

Then you are the first person in history to prove physics wrong. Well done.

PP is not necessarily incorrect - if his car is optimised for, say 77mph, it will be less fuel efficient at 70mph and 80 mph. I've deliberately not picked 75mph as the optimal speed as the drag increases with increasing speed.
So while PP is unlikely to have optimal fuel efficiency at both 70mph and 80mph, it is not inconceivable that he can have the same, albeit lower, fuel efficiency at both those speeds.

But I'd be happier to stick with the current limit, and stay on permanent Summer Time

 

[joe-90]

His car can't be optimised for that speed due to wind resistance.

 

[Corgigrouch]

His car can't be optimised for that speed due to wind resistance.

Yes it can, speak to any haulage company or truck manufacturer and ask about the importance of gear and final drive ratios.

When the 90kph speed limiters were first fitted to trucks, the vehicles that were geared for 60mph used more fuel. It's not just about wind resistance.

 

[big-all]

His car can't be optimised for that speed due to wind resistance.

optimised for a specific speed just means to operate the most efficiently at that speed
operating to the most fuel economic speed is a different matter
other criteria can be the economics off additional journeys possible weighed against the additional fuel burned
if you can manage say 4 round trips by traveling 10mph faster rather than 3.5 trips with associated lodging costs then the extra fuel burn will be greatly off set and be much cheaper

 

[Mickymoody]

As I'm colour blind, does that mean that my '95 Celica, have the best MPG?

(As the 96+ version has several modifications to the EGR system, and has less power as a result)...thus the graphics are flawed.

1995 version - 187 bhp. 1996 version 175 ish...due to egr management. Thus proving stats are stats, that prove nothing.

 

[big-all]

As I'm colour blind, does that mean that my '95 Celica, have the best MPG?

(As the 96+ version has several modifications to the EGR system, and has less power as a result)...thus the graphics are flawed.

1995 version - 187 bhp. 1996 version 175 ish...due to egr management. Thus proving stats are stats, that prove nothing.

you need to do the calculations to find out
in general there are several assumptions this includes mpg
productive time at a higher or lower speed connected to productive vehicle and driver usage

 

[joe-90]

His car can't be optimised for that speed due to wind resistance.

Yes it can, speak to any haulage company or truck manufacturer and ask about the importance of gear and final drive ratios.

When the 90kph speed limiters were first fitted to trucks, the vehicles that were geared for 60mph used more fuel. It's not just about wind resistance.

No it can't. Physics proves this.

 

[Corgigrouch]

Agreed, Ya cannae change the laws of physics Cap'n but you can use those same laws to your advantage..

If I was to drive my van at 80 in sixth gear and then drove it at seventy in third, should I expect driving at seventy to return better MPG? Similarly what would give the better economy? Driving at 20 in sixth or driving at 80 in fifth?

By you logic the lower speeds will ALWAYS return the better fuel consumption figures.

Vehicle manufacturers design their cars to return the best fuel figures for the expected speeds, they gear the car and adjust the characteristics of their engines to suit allowing for local emission regulations

Joe, perhaps other drivers use your same flawed logic and this is why they drive 10 to 20mph below the speed limit in a gear that is unsuitable for that speed.... Add to that that Diesel drivers will then be surprised when their variable displacement tubochargers jam up and then they need to have it repaired...

 

[compendiumhead]

Yesterday (Tuesday) I did a little experiment

1200 cc BMW motorcycle

Manchester to central London and back.

Outward: £25 of leadfree. 198 miles at -above 70 mph where possible, maybe 100+ at times to avoid rep=mobiles. (few slow bits thru traffic)
Return £22 of petrol 192 miles (knew where I was going this time), kept under or around 70 mph. No delays.
Saved £3 but one thing I noticed. The number of cars doing well over 80 mph, a few doing over 100 and just about every driver;
Driving to close to the car in front
Hogging the outside lane when there is nothing in lanes 1& 2
Hogging lane 2 when 1 is empty.
NOT using wing/door mirrors
NOT seeing a motorcycle coming up behind despite headlights on
NOT taking a blind bit of notice of 50 mph signs or overhead gantry.

In short, we are all doing 80+ now (except those who can;t)

PS: thanks to the Ford Mondeo driver who thought is great fun to try and waste me on the roundabout just because he thought I shouldn't be able to filter............ hope your door mirror cost a bomb to fix.

 

[Alarm]

PS: thanks to the Ford Mondeo driver who thought is great fun to try and waste me on the roundabout just because he thought I shouldn't be able to filter............ hope your door mirror cost a bomb to fix.

Ball bearings, in your jacket pocket.
Just another method

 

[Gasman28]

PS: thanks to the Ford Mondeo driver who thought is great fun to try and waste me on the roundabout just because he thought I shouldn't be able to filter............ hope your door mirror cost a bomb to fix.

Ball bearings, in your jacket pocket.
Just another method

Steel toe capped boots with the steel exposed - straight down the side of his shiny car - not that I would advocate such a thing of course

 

[Alarm]

That has been known.
One funny f@@ker got the arse because I was filtering and he had been stuck in a jam. Opened door on me, it got closed on his head a few times after, I was being helpful - thas all.

 

[LastMagicBean]

PS: thanks to the Ford Mondeo driver who thought is great fun to try and waste me on the roundabout just because he thought I shouldn't be able to filter.

Yes, sorry about that .
I thought you were a cyclist.

 

[nickso]

That has been known.
One funny f@@ker got the a**e because I was filtering and he had been stuck in a jam. Opened door on me, it got closed on his head a few times after, I was being helpful - thas all.

Works the other way too. One guy filtering past me decided to change lanes just after he passed me but didn't check over his shoulder to see where I was. As I hadn't slowed any he hit me. I don't think he appreciated my cheery wave as he got off his backside.

 

[Alarm]

His fault, hence they call the look back a "lifesaver" .