Welcome! The Lesabre is a nice car, even if yours is a beater, and that generation of Lesabre still looks cool to this day (IMO). The 3800 has a reputation as a robust, long lived, and efficient engine. Can you post a picture of your car?
I would definitely add a temp gauge to the car. They're not expensive. JC Whitney sells a
gauge with sender for $32. I'm sure you could get into one for under $20 if you spend more time looking than I did (and I only spent 90 seconds on it). This would allow you to feel more comfortable with your car's condition, as well as not sweating your butt off in the summer.
There's probably a lot of aftermarket support for the 3800, and you may be able to upgrade the fuel injection, ignition, and maybe even the computer. I don't know how much there is to gain by doing so, but you may be able to do it cheaply with junkyard parts from newer vehicles.
I don't know if it would be difficult (probably would) but if your car revs high while cruising you could change the differential ratio. At least you can do that when the weather is nice.
You probably don't need a scanner to pull codes, but I wouldn't be surprised if you could find one to give you sensor data. However, you can probably get much of your data using DIY methods and contraptions, using gauges, meters, and LEDs that tap the sensor's wire.
In my sig, there's a link for a fuel rate meter; in the linked thread, there are links to similar/better ideas. You may be able to pull a DIC (Drivers Information Center, the factory-installed MPG gauge) from a similar car in a junkyard.
Most real improvements that people get are from driving techniques, not car modifications. Unfortunately, it sounds like you're already doing almost everything you can, and can't do most of the useful modifications. The usual things that would help are ruled out by various conditions you posted...
- No coasting rules out Pulse & Glide and downhill coasting. My 1987 FWD Cadillac's transmission took over 200,000 miles of awful abuse without ever complaining but that's no assurance that yours will survive thoughtful and careful usage of neutral. However, in my experience, the fear of the 'N' is mostly superstition; some vehicles won't cool the transmission sufficiently in 'N' but other than that it's no problem as long as you rev-match when putting it back in 'D'.
- No drafting -- well, would it bother you to draft tractor trailers at a lengthy 3 second following distance? Most people follow much closer than 3 seconds, but results posted by members here show that 1.5 to 3 seconds is the most effective drafting distance. It's not hard to find tractor trailers driving at your speed.
- You already conserve your momentum and limit your top speed.
- You can't do any grille blocking or install a warm-air intake because of your fear of overheating.
- You can'd do aerodynamic modifications because you need to look respectable when you arrive (and your car doesn't leave much room for improvement without making it look bad). You might be able to experiment with an extended air dam. You might be able to remove the hood ornament (that would be a shame) and replace the likely wire-spoke hubcaps with interesting smoothies but they'd probably look too out of place on your car.
Increased tire pressure may be a worthwhile experiment. You sound like you're decided on your tire pressures without having experimented and actually tested to see how it affects traction. Your car is probably pretty light with relatively large tires, so you may not be able to safely run maximum pressure in the summer, but it's almost certainly necessary to run the car's recommended pressure in the winter. Running less than that is just begging for a blowout on the highway, even at 55 to 60mph.
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