gearing and revs?

[Lawnmowerboy]

Hi all. Was talking to one of the chaps at work today and he reasons that most mpg us gained at peak torque in top gear? We were trying to work out how many revs my 205 will be doing at 60ish?
Gearing is as follows and tyre size is 165 60 r13. Also does anyone know what sort of mpg driving sensibly between 40 to 60mph?

 

[Lawnmowerboy]

Gearing in top and final drive is as follows.

 

[711jrp]

2529 give or take............worked out at 100kmph

 

[Lawnmowerboy]

Okay cheers. Would you mind posting the calculation. I had a go and it was doing about 115k revs at 60

 

[AndyStobbs]

I posted it totally wrong. Too hard to do in my head. Try again when I'm not on the phone if someone else hasn't beaten me to it.

 

[Lawnmowerboy]

Cheers chaps. Bit embarrassing I can't do it really. Doing my advanced maths and science this year to get it out the way before my engineering hnc starting in sept.

 

[Lawnmowerboy]

No the circumference calc was right I got that bit but it was putting in the final drive and 5th gear ratios where I facked up

 

[AndyStobbs]

I fluffed it initially on the metres per minute. It's 1609 metres per min. Divide one into the other to get wheel rpm and work that backwards through your ratios.

 

[711jrp]

Okay cheers. Would you mind posting the calculation. I had a go and it was doing about 115k revs at 60

The tire firms give a revs per Km stat so I just looked one up and it was 576.44 x the final drive x top gear gives you engine revs per Km, x100 for engine revs in an hour at 100kph then divide by 60 for Rpm at 100kph.

Pete

 

[hotrodder]

Peak torque is generally where the engine is at it's most efficient... volumetric efficiency and brake specific fuel consumption. Not got much in common with mpg though.

Aerodynamic drag goes up at the square of speed i.e. double the speed 4 times the drag, triple the speed and the drag goes up nine fold. That suggests that slower is always more fuel efficient but there's various other frictional losses to overcome (which are a bigger deal at slow speeds where the air doesn't put up much resistance) and a running engine needs fuel whether it's moving a vehicle or not (power for leccy stuff etc). Think of the last point in gallons/hour and (for that portion of the total amount of fuel used) the faster a journey is made the better. Calculating speed for max economy quickly gets into major headache territory but it envariably ends up somewhere slightly faster than the min speed the car will comfortably pull in top gear without labouring the engine. For most cars that's (probably?) about 40 - 50mph, maybe a bit faster for something slippery, especially if it's got the torque to pull long gears at low rpm

 

[Lawnmowerboy]

Cheers hotrodder and other gents. Sorted that out ; )

 

[hotrodder]

Here's the maths although i strongly only masochists read further ... http://www.ingveh.ulg.ac.be/uploads/education/meca-0478/notes/SPEHPerf4_Conso_2012.pdf and http://www.transportenvironment.org/sites/te/files/publications/Road load determination which kinda carrires on where the first finished, road/vehicle loads and simulating 'em on a rolling road

 

[Shox Dr]

That reminds me. A mate worked out many years ago his newly purchased car was it's most economical at 50something MPH. So he would drive at that speed regardless of the Posted Speed limit, conditions or traffic, all in top gear.

It was comical to go anywhere with him. He wouldn't open windows nor the sunroof for fear of increasing drag. He drove from Yorkshire to Hastings on holiday one year and made his lass sit in the back. As his MTB would only fit in the front seat, and he didn't want to fit the bike rack to the back, and use petrol.

 

[Lawnmowerboy]

How did I know Yorkshire was gonna com in tut somehweh lad

 

[normspanners]

if you want to save fuel costs run it on 50 % used veg oil costs about 40-50p ltr

 

[Psychosaffa]

if you want to save fuel costs run it on 50 % used veg oil costs about 40-50p ltr

Where on earth do you purchase veg oil for 50p a litre? Best I've found was 90p a litre, and my Hilux runs on it without any modifications.

 

[Lawnmowerboy]

Unfortunately mine has the cav pump so no straight veg for me. What about a mix?

 

[PhillipM]

Yep, whilst peak torque might be the most efficient spot, it's only with the engine in isolation and the throttle wide open, as soon as you add vehicle speed in and air drag it all goes to pot, as to run at peak torque and WOT you'd be doing a fair old speed unless you're engine only makes about 50bhp.

 

[Lawnmowerboy]

Well, 60

 

[normspanners]

The cav will work on it, it is the low pressure pump on the back that suffers, you just need to fit an external lift pump to create the pressure but as said 50 /50 is fine.
My mate has 205 that I sold him it has never seen real diesel !! and it still gives him 45-50 mpg.

Where on earth do you purchase veg oil for 50p a litre? Best I've found was 90p a litre, and my Hilux runs on it without any modifications.

I buy 20lts of new for £16 which is 80p at macro so I am not going to pay anywhere near 90p for used.