nerdy question about trip computers

[snapper65]

I ask only from idle curiosity....

I know from when I got my Cayman S 4m ago that it did about 29-30mpg on the long motorway journey home, and I know that about town (for the last 4 months) ive got a steady 20.9mpg. So on my first long motorway haul in 4 months I decided, just because I was bored, to reset the trip computer. I had exactly half a tank of fuel at the time and I was in 6th idling at about 80 leptons for 60 miles. On resetting the trip its first calculation indicated an average mpg of 29, which i expected. Then over the course of the next 10 miles it fell relentlessly, bottoming out at about 24mpg, which I didnt expect. Then it gradually crept up again to 27mpg, where it stayed (roughly).

So how does it do the maths? and did the fact that the tank was half full when i reset it influence the calculations, and if so, how?

Edited by snapper65 on Tuesday 28th February 16:58

[Rockster]

snapper65 said:

I ask only from idle curiosity....

I know from when I got my Cayman S 4m ago that it did about 29-30mpg on the long motorway journey home, and I know that about town (for the last 4 months) ive got a steady 20.9mpg. So on my first long motorway haul in 4 months I decided, just because I was bored, to reset the trip computer. I had exactly half a tank of fuel at the time and I was in 6th idling at about 80 leptons for 60 miles. On resetting the trip its first calculation indicated an average mpg of 29, which i expected. Then over the course of the next 10 miles it fell relentlessly, bottoming out at about 24mpg, which I didnt expect. Then it gradually crept up again to 27mpg, where it stayed (roughly).

So how does it do the maths? and did the fact that the tank was half full when i reset it influence the calculations, and if so, how?

Edited by snapper65 on Tuesday 28th February 16:58

I suspect the 29mpg reading which appeared right after you reset the trip computer was an artifact.

If you're worried about the change in mpg you just got vs. what the car used to deliver various things can influence this: Gasoline (stale, or fresh, gas blended for a different season); colder or warmer temps; tire inflation pressures; whether you have the A/C on; these can affect gas mileage by a few mpg.

Seldom and maybe never (I can't remember ever doing this) I've never reset the trip computer mid-tank. I always reset it at fill up and I am pretty careful to make sure the fill up level is consistent.

But I have noticed it doesn't take much to affect the mpg reading as I drive. Any idle time, traffic congestion, aggressive passing, climbing a grade (even a fairly gentle grade), even a head wind can cause the mpg to drop 1 or even 2mpg.

Sincerely,

Rockster.