Lightweighting and aerodynamic optimization of a subcompact car.

[classicalgas]

Wow, only 1.8w, and that worked for you..
How old is your miata? i'm imagining a 1990's-2000's model that didn't have a ton of electronics sapping at the battery all the time.

I see that i can get up to a 20w dashboard solar charger. Possibly this could make a 30ah battery work.

yeah. my vampire loads are minimal, the one I drive most often is a 2000. Before I put a pv trickle charger on it, I could go a week or two without driving it before I noticed starting issues ( though not always a need for the jump box, sometimes it was just slow, or almost wouldn't start....two months, and it might not even turn over. That seemed to be unchanged when the AGM died and I bought the lithium (only about $75 difference in price ) So I decided the pv trickle charger might make the more expensive battery last longer, and get away with something smaller than recommended. So far, so good.

The Shorei weighs just an ounce or two over five pounds , including the battery adapters (from motorcycle to automotive connections ) The special Shorai battery minder wasn't cheap, though you don't need that with the PV trickle.

Some aero tweaks and 150 lbs out of the car bumped my highway mileage on the miata (notoriously geared low, 5th isn't even 1:1) from 26 mpg to 31 ( if I keep it under 75mph)

 

[neptronix]

31mpg, geez, that's still kinda bad.
I think the poor fuel economy in the miatas comes from the final gear.

I test drove a 2023 model and i LOVED The power, but.. hypermiling it, i was only able to achieve 41mpg!
The highway RPM per speed in 6th was the same as my 1.5L car, so i think the gearing is the main culprit, because the car is otherwise very light and aerodynamic.

If i had a Miata, i'd probably upsize the tires as much as possible to lower the RPM.. you'd get improved efficiency and higher comfort as a bonus.

I'd do that on this car but it's gutless already and can't tolerate the gearing change

 

[classicalgas]

31mpg, geez, that's still kinda bad.
I think the poor fuel economy in the miatas comes from the final gear.

I test drove a 2023 model and i LOVED The power, but.. hypermiling it, i was only able to achieve 41mpg!
The highway RPM per speed in 6th was the same as my 1.5L car, so i think the gearing is the main culprit, because the car is otherwise very light and aerodynamic.

If i had a Miata, i'd probably upsize the tires as much as possible to lower the RPM.. you'd get improved efficiency and higher comfort as a bonus.

I'd do that on this car but it's gutless already and can't tolerate the gearing change

I sorta did that, 185 -60 tires on 15" wheels...only dropped 300 rpm at 70 mph, compared to the 50 series tires that would have preserved stock gearing with the bigger wheels. Mostly that was to keep the car's tossability and driftability, lower profile tires make the transition from grip to sliding too sudden for my taste ( and expensive ultralight wheels too prone to damage )

I did put a deeper front spoiler on it, and rigid aluminum belly panels to replace the factory plastic ones, back to the rear of the front tires. Lowered about an inch and a half at the front with a half inch rake, new trunk lid with a spoiler built in, partially blocked and carefully sealed radiator. Hardtop. Found a way to park the wipers lower, in the dead zone at the base of the windshield. From 26 to 31 mpg with no hypermiling techniques is a pretty big percentage jump.

I wanted to do what I could for MPG without hurting handling, daily practicality or acceleration, so weight reduction was my major focus, drag reduction second. The rake and belly pans made a noticeable difference in how planted the car feels at freeway speeds, so I may have added some drag to get downforce ( or less lift )

 

[neptronix]

A 300RPM drop is pretty nice. A big complaint i have with cars with little engine is a high RPM on the highway. I'm fine to drop a gear or two down when climbing on the highway, i just don't want to hear the droning of 3000rpm at around 70mph.

Your car sounds like a hoot. We've used a lot of the same tricks and have the same mentality, but your end result sounds like a lot more fun than mine. Too bad you can't really have it all in a gas car, huh?

Your MPG gains are pretty impressive w/o hypermiling. My lady has been driving my car, who is a leadfoot, and she averages 43.2mpg, where the CAFE rating is 35mpg, so the car modifications really worked out. Of course when i am behind the wheel i am averaging more like 45mpg with >95% city driving and seeing 50's-60's when on highway jaunts doing 65mph in the right lane, cutting into neutral whenever possible down descents, etc.

Your car would benefit from a pulse and glide technique due to the high final drive. The less you have the car in drive, the better, lol. I wonder if you could squeak out 40mpg if you hypermiled it.

I've considered adding some kind of mild hybrid functionality to my car to improve the power and efficiency a bit. With only ~106HP, a 2kw belt drive could help driveability at low RPM and enable me to compensate for the torque loss in 1st gear if i were to upsize the tires to the maximum the car can fit.

But i see a full EV conversion in my future when battery density improves to the point where the end conversion has good range and isn't 1000lbs heavier as a result. I love that nippy handling quality of little Mazdas too much to screw it up with some current era 250whrs/kg car battery. So i will do some kind of hybrid first.