[nicktyler]

Hi Guys

I tried the search and came up with some vague answers but thought I'd see if it can be cleared up. Basically I am going on a very (to me) long motorway journey of 330 miles each way and I was wondering what the optimum cruising speed is for a manual 335i.

I drove for 110 miles in mild traffic and rain a while ago and had the cruise control on at between 80-85 (autobhan) and returned nearly 37MPG which I thought was pretty good.

I have a dyno that shows my max torque is at 2800rpm so am I right in thinking that if I sat in 6th at 2800rpm and diddn't touch the brakes that would be the sweet spot? I prob wouldnt do that as it is 88mph if I remember correctly.

I know I can't control the traffic but lets assume there is none.

What do you guys think?

Cheers

Nick

[creepy coupe]

Best I ever saw was 38 MPG sitting at 80 mph over 380 miles. But there are many variables, wind direction, AC on or off. I'd be very happy with 37 MPG at 80-85 mph

[HighlandPete]

If you are wanting the highest mpg, then you need to slow right down.

It isn't only engine efficiency, but the the spread of the BSFC (Brake Specific Fuel Consumption) map across the power output, which indicates the best mpg. Wind resistance is still the biggest enemy when cruising at speed. Typically 20 - 25% more fuel used to cruise at 85mph, compared to 70mph.

Tests under the control of guys like Peter De Nayer have consistently proven cars still give the best mpg return at the lowest speed a car will run in top gear. What's that.... about 55mph?

HighlandPete

[creepy coupe]

Originally Posted by HighlandPete

If consistently proven cars still give the best mpg return at the lowest speed a car will run in top gear. What's that.... about 55mph?

HighlandPete

Which is why average MPG was quoted at 56 MPH

[nicktyler]

Oh so I am way off with the max torque thing. just below 37mpg is proper good for 80 then (I was on V-power as usual). There was even a bit of catterpillaring with the traffic and slowing to 50 sometimes.

I suppose there is a balance between time and MPG to think about as well. For my specific situation I think wind speed etc is a bit too scientific but good to know.

I am also going from London to the Lakedistrict so on average over the whole journey it will be uphill

I'll stick to 80 if traffic allows so as to not go mental. 55 is dangerously slow really. If I get near 37mpg again I will be happy.

[creepy coupe]

Originally Posted by nicktyler

Oh so I am way off with the max torque thing. just below 37mpg is proper good for 80 then (I was on V-power as usual). There was even a bit of catterpillaring with the traffic and slowing to 50 sometimes.

I suppose there is a balance between time and MPG to think about as well. For my specific situation I think wind speed etc is a bit too scientific but good to know.

I am also going from London to the Lakedistrict so on average over the whole journey it will be uphill

I'll stick to 80 if traffic allows so as to not go mental. 55 is dangerously slow really. If I get near 37mpg again I will be happy.

You need to stop worrying about MPG and enjoy your 335i.

[nicktyler]

Don't worry I do. I normally struggle to get over 20mpg. I just thought As I was going to be a captive audience, just sitting on the motorway for so long I would see how much MPG I could get and save some money on the way.

[HighlandPete]

Originally Posted by nicktyler

Don't worry I do. I normally struggle to get over 20mpg. I just thought As I was going to be a captive audience, just sitting on the motorway for so long I would see how much MPG I could get and save some money on the way.

Just remember the more you push that loud pedal, the less money you will save.

HighlandPete

[HighlandPete]

Originally Posted by creepy coupe

Which is why average MPG was quoted at 56 MPH

A recent test using several cars across the latest production models, showed that we can still go down to about 40mph in most manual cars, and return the best cruising mpg. With increased consumption as the speed rises.

With cars tuned for the EU test regime, that makes sense, as it is low speeds and low loads, where the test turns in the latest headline grabbing figures. So 40mph is the new 56mph.

HighlandPete

[takumichinoku]

Exactly what creepy coupe said. Either that or your sat in the wrong car.

[Subzero2003]

i got 37mpg with AC on down to Gatwick airport in the pouring rain the other day, which forced me to drive at about 60mph the whole way, whilst constantly going through deep puddles (which must have created more resistance, hence forced the car to use more fuel).

My normal average for hacking it to work and back everyday whilst not hanging around is about 24-26mpg.

[phil200tdi]

65mph is good in mine.. Not holding up the trucks and still making relaxed progress - I have had 450 miles out of a tank on a few occasions (multiple trips not one long journey). Once my valves have been cleaned and the HPFP replaced I'll go for a real economy run - hoping for 500 miles!

In the UK the more important thing with all the traffic is to not touch the brakes at all. Slow down in gear and speed up again gently, carry momentum. Its quite funny watching morons fly up behind someone, slam the brakes on and then speed up hard again to the bumper of the next car 3 metres further on.

[xenon]

You're not way off with the torque insofar as the engine is at its most efficient when it's at the rev-range that produces the most torque. However, as has been said, the wind-resistance is the biggest factor and it's clear that it takes more effort to be at 2,800rpm in 6th gear going at 88mph then it would to be at 2,800rpm in first, going much slower. Therefore the most economic is probably 2,800rpm in first but hardly practical.

[doughboy]

Best speed for MPG (335i)

zero mph, parked on the the drive

Probably about 56mph in sixth.

[rsmallwo]

Originally Posted by xenon

You're not way off with the torque insofar as the engine is at its most efficient when it's at the rev-range that produces the most torque. However, as has been said, the wind-resistance is the biggest factor and it's clear that it takes more effort to be at 2,800rpm in 6th gear going at 88mph then it would to be at 2,800rpm in first, going much slower. Therefore the most economic is probably 2,800rpm in first but hardly practical.

Sorry to burst bubble but....nope!

Gearing is a massive factor with combustion engines. As already stated, the lowest speed in Max gear is the most efficient mpg wise. The 56mph is the official best mpg all round

[xenon]

You've not burst my bubble but I disagree. Anyway, what's the "lowest speed in max gear"? How fast is that? I can go lower than 56mph in 6th so what's the lowest speed? The speed at which it's about to stall? That's the most efficient running? Sure.

Of course gearing is a "massive factor", how could it not be? But the engine is at its most efficient when it's producing the most torque. That's irrefutable.

[nicktyler]

I can't imagine goign at 65mph or lower for 300+ miles though. It would be pretty boring. I am going on holiday and would like to have some time there so will pay the extra and balance out the speed with economy. Once I get to the Lake District I plan on Destroying the MPG figure on the mountain roads!

I got the thing of the torque as I heard one of the guys on Top gear say it (or at least strongly imply it in not so many words). I think it was when they did the Long range challenge and JC drove the Jag.

I'm pretty sure they implied it was max torque in top gear for mpg as the gear amplifies the torque and the speed allows you to travel further than in first. IIRC it was May in some tiny polo densil so speed would have been about 55mph at max torque in top gear?

Just that ours happen to do 88 as there not designed the same way and that will bring the issue of wind resistance into it on an exponential scale?

I might have just made all of that up though

[phil200tdi]

You are more likely to travel back in time at 88mph than you are to get the best possible fuel economy!

this thread seems confused.... either you want maximum efficiency or you dont. If you want to be reasonably economical then drive at 70-80mph and dont worry about the 10% extra fuel you could be getting by driving slower.

[jimpom23]

Not sure what speed is best but I was very surprised to have the fuel light come on (60 miles left) and get as far as 360 miles! Whole journey barely dropped below 80 (kph of course)

[HighlandPete]

Engine efficiency and the most economical speed are not directly related, even with gearing in the equation. We are not using anything like the power output where engine efficiency is optimum.

What HP are we needing to cruise anyway? At the speeds where mpg will be at best point, the speed has to be slow to get away from high wind resistance so we are only into sub 50HP demands anyway. No performance engine is really efficient at that output. BUT, mpg will be at peak.

It is of benefit if the engine is running near optimum torque rpm, as BSFC is typically lower in that range, even at part throttle and low HP demand. But even that is not necessarily the best mpg point, as a higher gear with less losses and road load can still mean consumption is less at lower rpm. That has been proven many times under test, so clearly understood.

We will always come back to the slower we drive the more economcal a car will be. Which is completely different to an engine driving a continuous load, like a water pump, a tractor at plough, or a marine engine driving a prop, where efficiency, rpm and load are matched to BSFC and resulting Litres per Hour (l/hr) is optimised.

HighlandPete

[78WJM]

56mph is used in the stats because it's equivalent to 90kph, nothing to do with that being the most efficient speed.

[pjs]

It will do about 44mpg at a steady 60. I know cos I get bored and try stuff out from time to time. Doubt you'll get much better than that unless you are on 17" tyres. I used to average 26mpg when I used it, now the wife has it on the school run she averages 20 mpg.