by yesplease » Tue 12 Jun 2007, 17:34:11
$this->bbcode_second_pass_quote('Terrapin', 'I') suspect the JD’s Miata already has a 5 speed and unless it is revving over 3k at 60mph there isn’t much if anything to be gained by another gear. It simply moves the cruising RPM out of the efficient power range. The wind resistance alone at 60mph (particularly with the top down) takes some power.
Lemme see, *scribbles*, a Miata needs ~10kW at ~60mph, and maybe ~12kW with the top down at 60mph. It has a 1.8L engine that makes ~80-90wkW@~6k rpm iirc, depending on model, and just about any automobille engine built within the past few decades has relatively poor efficiency at anything less than about half load. So, if we're at half load at 60mph, we should be geared to make ~20-24wkW peak, i.e. the engine speed should be a third of peak, or ~2k rpm. From
here, most Miatas seem to be spinning at ~3krpm@60mph, so, the cars are at about a quarter load and BSFC is correspondingly poor, I'm guessing in the ~400-550g/kWh region, when it could be at ~250-350g/kwh at half load or greater. Otoh, by gearing it so tall, the top speed will be cut to ~90mph, so even though we are now able to get 50+mpg at a ~60mph cruise we also won't be able to go 100+mph in 5th any more. We'll have to settle for pulling pretty quickly up to 90mph in 4th. So, it depends on what the driver wants more, being able to go ~100mph+, or getting ~50+mpg@60mph in 5th. Since the shorter fifth hurts mileage, as well as increases the odds of me getting clocked going ~100+mph, I'd probably go w/o it, and leave the optimal ratios for acceleration at the track. But, most people like having the shorter gearing so they can blatantly break speed limits.

Btw, the last generation of vehicles to have fairly efficient engine/gearing ratios died in the mid 80s.
What you said about the Insight and nutty "Hypermilers" is generally true, however, I'd much rather nab an older car and mod it to the point where it can pull ~50-80mpg@60mph for a grand or two, and cruise with semi drivers on the highway, than pay the exorbitant prices hybrids and other high mileage vehicles seem to be getting. A/C isn't so bad since it only takes ~1-2kWh at highways speeds, and generally isn't on all the time (too cold). It'll cost take a couple mpgs, but like I said before, I'd rather have an old compact with some sound deadener, mp3 player of some sort, belly pan, wheel skirts/deflectors, A/C, appropriate gearing for whatever speed I want to cruise, and several thousand dollars in my pocket.

$this->bbcode_second_pass_quote('Professor Membrane', ' ')Not now son, I'm making ... TOAST!