Manuel conversion and MPG??

chris1811 · Post by **chris1811** » Fri Jun 19, 2015 3:40 pm

Pffffftt my 2.0L diesel insignia only just did 380-400 on an £80 tank so the Lude doesn't seem all that bad tbh

Post by **Vtecmec** » Fri Jun 19, 2015 3:41 pm

bb1boy wrote:drokking hell, I must have a hole in my tank then.. £30 gets me less than 100 miles!

I did Essex and back last weekend (208 miles) and had more fuel at the end than when I filled up with £30.

Post by **lewd lude lover** » Fri Jun 19, 2015 3:59 pm

There is a reason trucks are restricted to 57 mph init. 55-57 is the 'sweet spot' apparently. Anything more and you are burning fuel moving air out the way and only that. Air acting more like toffee the faster you go I can believe there is a point where you enter the diminishing returns curve. Thats all remembered internet teaching so I am happy to be taught a better lesson if one is available.

Put it this way. Hooning at 80mph for 25miles to meet gayno used more fuel than half way to gatwick did on a gentle cruise. 55 is a horrid speed to go at on the motorway though so I normally just clip along at lorry overtaking speed, about 60-63.

Thats easy in the truck now though. Not so easy in a car with some 'pep'

Post by **Vtecmec** » Fri Jun 19, 2015 4:00 pm

The magical 56mph is always the speed used by car manufacturers when they quoted MPG figures............

bb1boy · Post by **bb1boy** » Fri Jun 19, 2015 6:37 pm

I hear what you're saying, but a Prelude is surely more aerodynamic than your truck so the air displacement/speed curve would be different, no?

Post by **Vtecmec** » Fri Jun 19, 2015 6:40 pm

It does vary apparently, not just only with the vehicle, but also wind direction, ambient temps, humidity, etc.

'92 Si VTEC · Post by **jjmartin349571** » Fri Jun 19, 2015 7:46 pm

I read that the most fuel efficient speed is actually more like 20mph, anything over that and drag is causing big problems.

I'm not sure that these figures take into account anything exciting like pumping losses though, there's so many variables that affect fuel efficiency.

I just put fuel in when the light comes on. Which is far too often

YouTube · Post by **Sailor** » Fri Jun 19, 2015 11:39 pm

Surely fuel efficiency isn't actually related to speed. If so, all you'd have to do is to plot power against consumption.

"Whole car" is what counts. For example, if you can't do 20mph in top gear, the system is less likely to be efficient than at a higher speed. Then there's drag, and rolling resistance, and. And.

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Sat Jun 20, 2015 9:52 am

lewd lude lover wrote:There is a reason trucks are restricted to 57 mph init. 55-57 is the 'sweet spot' apparently

Or it could be that 56mph is 90kph (give or take rounding), which is the European speed limit for HGV's and was imposed purely for safety reasons, nothing else...

That's not to dispute the general argument about lower speeds giving better fuel economy (in general) etc. But there is no 'one speed' that is best for fuel economy on all cars. It's dependent on a massive range of things (as many others on here have said) so is very different for every car.
As an example, in CX petrol Turbo's, if you sit on the motorway at anywhere between around 55-70mph, it sits 'on boost', guzzling fuel (you'd be looking at something between 18-25mpg). Cruise at <something quite a bit more than 70> and it sits nicely off boost and you can get over 35mpg.

Post by **Donald** » Sat Jun 20, 2015 10:32 am

Sailor wrote:Surely fuel efficiency isn't actually related to speed. If so, all you'd have to do is to plot power against consumption.

"Whole car" is what counts. For example, if you can't do 20mph in top gear, the system is less likely to be efficient than at a higher speed. Then there's drag, and rolling resistance, and. And.

Sailor is awake. Q=mc•ΔT is the simple equation if anyone wants to measure their own fuel efficiency with a cup of water