Hwy versus city driving is really subjective unless you keep extraordinarily accurate records of some sort (I don't) - and really of limited value. What might be more useful and objective is to replace that % value with the car's reported Avg Speed. Most cars can report this these days - and it seems likely this would actually give a clearer picture of mileage along a spectrum of city/hwy driving combinations than the gross % estimate. Thoughts?
You are correct, this is a totally subjective criteria, but not useless if you can be honest with yourself when making entries. And I agree that including average speed is a good idea, although my car doesn't track this.
Average speed is only slightly less speculative than city/highway, and that only if the vehicle tracks your average speed for you. In fact I'd say I could approximate my "city/highway" percentage better than I could my average speed, if I had to do the estimating myself.
However, just like city/highway, average speed leaves out extremely pertinent details -- such as how many times you stopped the car and from what speeds. Someone who gets up to 60mph and then stops at a red light every mile for their entire commute might have the same "average speed" as someone who drives on a 40mph country road without stopping. That would make for quite the difference in mileage.
City/highway is no less useless -- it is entirely subjective and that's why I don't use it. But anybody can guess at their city/highway percentage pretty well, whereas it is more difficult to guess at average speed if your car doesn't measure it.
Hm. City/Hwy is a total guess in my case. Avg speed would be a more uncertain guess, as none of my vehicles track it. To be honest, I don't have the faintest idea.
Other than that, there are other useful figures, like avg temperature, which is again something I can only guess. But it definitely impacts fuel consumption. So does the average length of trips (through the distance driven with cold engine). Wind - way too complicated (and at least 2-dimensional) to even make a guess. OK, the last one might be a bad idea
True enough as far as average speed not being a whole lot more useful than City/Highway percentage. I have an instrument that tracks my average speed and therefore it's not a guess in my case. Still, it's not even close to a perfect indicator as BDC's expample is spot-on. In my case, I get better (higher MPG) at lower average speeds as I use a lot of hypermiling techniques which really come into play in city driving. Hiway driving is mostly a function of your car's efficiency, the speed you drive and environmental factors.
I'd, personally, rather track average speed because it's not a WAG for me!