Help working out MPG

[Jawaman]

Ok so not the most PH topic

I've recently acquired a 2001 volvo v70 2.4 petrol (140bhp)

Day I bought it petrol light was on. stuck £20 in and back to red again reckoned 32mpg which I was pleased with.

So I decided to do it properly. Brimmed the tank and started driving.

45 miles later topped it up worked out 21mpg!!!!!

20 miles after that checked it again, 16mpg!!!

I've been using http://www.fuel-economy.co.uk/calc.shtml to calculate MPG

Surely there's something wrong here, or am I topping the tank back up again too soon?

[HoHoHo]

Got an iPhone?

Install 'road trip' and complete when you fill up.

It's accurate if you fill in figures (amount of fuel and mileage) when you fill up.

[Willy Nilly]

Pull up at forecourt

Zero the trip

fill tank to the top

drive until empty

pull up on forecourt

refill the tank

Now, look at the number of litres you have put in the tank and divide this by 4.54, this will give you the number of gallons. The divide the mileage on the trip by the number of gallons.

Easy

[Jawaman]

HoHoHo said:

Got an iPhone?

Install 'road trip' and complete when you fill up.

It's accurate if you fill in figures (amount of fuel and mileage) when you fill up.

Ta seems a handy wee app, have installed it but doesn't change my calculations so far

[Jawaman]

Willy Nilly said:

Pull up at forecourt

Zero the trip

fill tank to the top

drive until empty

pull up on forecourt

refill the tank

Now, look at the number of litres you have put in the tank and divide this by 4.54, this will give you the number of gallons. The divide the mileage on the trip by the number of gallons.

Easy

That's what I'm planning on doing next, maybe I'm being impatient thinking I can work it out after 20 miles!!

[RobM77]

Definitely drive for longer. My daily driver does about 60mpg on a long run, 35 when I'm having fun and 45-50 in general driving. I could easily see any of those figures if I measured the mpg in 20, 30 or even 40 mile spurts. My weekend toy varies from about 3 up to 20 in a similar way!

Brim the tank as much as you can and then zero the trip. Drive it for a decent time, like half a tank's worth at least, and then brim it again, noting the trip mileage and how much you put in. That's the way I do it.

[Jawaman]

I think the last check was a bit disconcerting and that's what's thrown me.

20 miles of empty dual carriageway / motorway averaging around 50mph, nice steady run. And it took 1.2 gallons to top the tank back up, so 16mpg!!!!

[HoHoHo]

Jawaman said:

HoHoHo said:

Got an iPhone?

Install 'road trip' and complete when you fill up.

It's accurate if you fill in figures (amount of fuel and mileage) when you fill up.

Ta seems a handy wee app, have installed it but doesn't change my calculations so far

Whilst it may not change your calcs, it's bloody good at providing a true reflection of costs/mpg etc. Of course, it's more accurate the longer you use it.

For example



|http://thumbsnap.com/0RMhMf1J[/url]

Edited by HoHoHo on Saturday 29th March 21:16

[Dodsy]

Not enough mileage to get an accurate measurement. Did you use exactly the same petrol pump when filling back up ? Cut off can vary quite a bit IME so you fill it to the cutt off using a different pump and get an extra litre in and your consumption looks crap . If you have the same issue across a full tank the % of additional fuel is very low so wont skew the figures much,

The only way to do this properly is fill it until the pump clicks off, then put no more in. Run until empty. Refill again to the click off. Do your calculation.

Also did you do this run from cold ? the first 5 miles or so it will be warming up and running rich using extra fuel, do a longer journey and this has less impact on the overall figures.

[Jawaman]

Dodsy said:

Stuff

I haven't used the same pump twice so far! The last one I used is nearest my house anyway so I'll stick with it.

The 20mile run the car was already up to temp - I had driven nearly 10 miles before I stopped to top up.

Fingers crossed its the pump cut offs then!!!! I do around 180 miles a week so I'll try again this time next week....

[Matt UK]

Dodsy said:

Not enough mileage to get an accurate measurement. Did you use exactly the same petrol pump when filling back up ? Cut off can vary quite a bit IME so you fill it to the cutt off using a different pump and get an extra litre in and your consumption looks crap . If you have the same issue across a full tank the % of additional fuel is very low so wont skew the figures much,

The only way to do this properly is fill it until the pump clicks off, then put no more in. Run until empty. Refill again to the click off. Do your calculation.

Also did you do this run from cold ? the first 5 miles or so it will be warming up and running rich using extra fuel, do a longer journey and this has less impact on the overall figures.

This.

OP, do a longer run or at least half a tank for accurate measures.

Or ease off the throttle a bit.

Or you have a V8 conversion.

Or your car is fooked.

[giblet]

Willy Nilly said:

Pull up at forecourt

Zero the trip

fill tank to the top

drive until empty

pull up on forecourt

refill the tank

Now, look at the number of litres you have put in the tank and divide this by 4.54, this will give you the number of gallons. The divide the mileage on the trip by the number of gallons.

Easy

This, but use a calculator/app to make it easier. Helps to track the cost per mile too

[Jawaman]

Brilliant

If it's had a v8 conversion they've used a very very sick engine!!

I don't think its fooked, not using a drop of oil or water, no smoke, pulls cleanly in every gear.

It had a binding rear caliper when I bought it but sorted that

Tyre pressures are grand too, checked this evening.

I'm just fretting because if it's only managing an average of 20mpg I'll need to get rid!!

[HoHoHo]

giblet said:

Willy Nilly said:

Pull up at forecourt

Zero the trip

fill tank to the top

drive until empty

pull up on forecourt

refill the tank

Now, look at the number of litres you have put in the tank and divide this by 4.54, this will give you the number of gallons. The divide the mileage on the trip by the number of gallons.

Easy

This, but use a calculator/app to make it easier. Helps to track the cost per mile too

It needs to be over a few tanks to get a reliable figure. The first click or second click of a pump probably make insufficient difference to the average figures.

[giblet]

HoHoHo said:

It needs to be over a few tanks to get a reliable figure. The first click or second click of a pump probably make insufficient difference to the average figures.

Yup, I tend to use the same petrol station and usually the same pump. Always fill up to the first click, never any more. My figures seem to match what is on the cars display (if the car has the ability to calculate mpg)