This Cannot Be Right - Please Help

[Neverfuel]

I think I am losing the plot....

I appreciate that everyone has their own way of working out their fuel consumption figures. But, last month I did a lot of petrol miles compared to EV and I wanted to try and work out the mpg of the engine alone after grid power was removed from the equation (without the aid of sophisticated monitoring devices). What I found shocked me and I think I need some help to see if my figures are correct, or whether I have misinterpreted them.

I always fill up on the last day of the month and always brim the tank and I would normally work out my mpg per tank of fuel like this:

579 miles divided by 38.78 litres = 14.93 miles per litre, times 4.55 litres to the gallon = 67.933 miles per gallon on that tank. (very good, me thinks)

Okay, I then work out what that has cost per mile, and I add in the cost of electricity. My electricity costs me 10.57 pence per KW so I have set the car charging cost at 10 pence to give me a close approximation of what I use on a daily basis.

The car indicated that I had used £17.65 worth of electricity over the month.
The fuel cost me £1.037 (Morrisons) per litre, therefore cost me £40.21. The total cost this month is £56.86, so divided by the mileage of 579 miles, I have a cost per mile figure of 9.82 pence per mile. I then work out what the electricity would have bought me in terms of petrol, in this case £17.65 would have bought 17.02 litres at £1.037. I add the actual litres used (38.78) to the theoretical litres used (17.02), to give a figure of 55.8 litres equivalent. By dividing this into my mileage of 579 and multiplying by 4.55 litres per gallon I get my mpge of 47.21. (not bad, me thinks)

As I said I wanted to do this because I had done a lot more miles on petrol than electric and wanted to compare with the previous months. However, this prompted me to try and work out what the engine actually did by subtracting some conservative estimates of the EV miles based on my electricity use.

£17.65 of electricity at my rate of 10.57 pence per KW is 166.98 KW.
I then subtracted 10% for losses, so now I have 150KW being used by the car. Dividing that by 8.4 KW (the amount to fully recharge above the 30% SOC level), means I have used 17.89 cycles of battery power. If we then say that each cycle will give 20 miles of EV driving - I had 358 miles of EV driving.

Now, I hope I have my figures right. But even though I have not been entirely precise in the amount of losses, I hope most of you can see my point - if I had done 358 miles on EV in the month, then I had only done 579 - 358 miles = 221 miles on petrol, 38.78 litres of the stuff. So I get the engine to be achieving 25.92 mpg. (shocked, me thinks). Now I know it was a heavy month, but that doesn’t look right to me.

I must be doing something wrong, or the charge cost in the MMCS is telling me porkies, but can someone just let me know what I am missing please, as 25.92 mpg seems to big a gap to my 67.933 figure in the first calculation.
:?

 

[Ozukus]

First question to ask is do you actually believe that you had 18 full charges into the car, as if you don't believe that is the case then your £17.65 is way off.

I have had to run the car with effectively no grid before so therefore at 30% SOC on a semi permanent basis, no charge or save buttons depressed.

When I had this situation the car tended to return around 40MPG, so was quite pleased with that.

 

[Neverfuel]

Hi Ozukus

I think so, as that is about my normal usage. I have started to keep a daily log of estimated EV miles at startup, miles actually run on grid powered EV, number of bars left on the MMCS (if I don't use all the EV range before the next charge) , the daily cost of charge according to the MMMCS and the estimated EV range still to run before the charge begins. I will do this until the end of the month and then post my findings.

 

[jaapv]

Unfortunately estimated miles at startup can only be classified as fiction.

 

[maby]

Unfortunately estimated miles at startup can only be classified as fiction.

It does seem to improve with time. When we took delivery of the car, it generally estimated something over 30 miles and I was lucky to get 20. These days, it estimates 20 miles and I generally get close to 20.

 

[Neverfuel]

Unfortunately estimated miles at startup can only be classified as fiction.

It does seem to improve with time. When we took delivery of the car, it generally estimated something over 30 miles and I was lucky to get 20. These days, it estimates 20 miles and I generally get close to 20.

I only work on 20 as a guideline, but am running a month long test as above to get some real data, I think it would be best to add in outside temp as well, don't you think?

 

[Barnfather]

The numbers you are using to work things out are very sensitive, which I think is your problem

For example - if your calculations on the number of cycles are slightly out - lets say :

Your arbitary 10% for losses, was actually 15% (resulting in 16.7 cycles)
Your EV Range for a cycle is actually 18 rather than 20
The charge per cycle was actually 9Kwh

The above are just examples of slight variations, but the above would result in an eventual result of 35 MPG, which is a lot closer to what you would expect.

Environmental conditions are critical too - temperature, wind etc. You may have also been driving the car sub-optimally if you were doing a lot of petrol miles (motorway?). MPG may also, like any car, be affected by the quality of the fuel used, air pressure of tyres etc.

My point is, unless you have concrete numbers, there will always be a large margin of error when trying to work his out. Perhaps setting up a spreadsheet and logging all this data over a six month period (to cover multiple seasons), may be interesting. Hard work but interesting