You're downshifting to decelerate. What you want to do for economy is downshift merely to keep the engine revs up. As long as the revs are up, the ECU shuts off the fuel injectors and you use no fuel. You really don't want to get into "compression braking" to decelerate. The whole point of the exercise is to COAST as much as possible and use the DFCO (deceleration fuel cut off) feature of your ECU to use NO fuel while you're doing that.

SOoooo... it's actually better to stay in a higher gear so that the load of the engine slows you down LESS and you can coast "free" for a longer period.

It makes more sense when you can watch a real-time MPG display and see what's going on. It goes something like this: (typical modern small 4-cylinder)
Gentle acceleration from a stop = 14-18 mpg
REALLY gentle accel from a cruise = 25-30 mpg
Cruise = 30-40 or more depending on the car and the speed
Lift throttle coasting in gear above 1200 rshared_pm = infinite mpg (zero fuel)
Idling at a stop = 0 mpg!

Obviously, it is to your greatest benefit to extend the amount of time you spend coasting in gear vs doing anything else. Every fraction of a mile you spend coasting in gear is "free".

Watching the Scangauge in my car, if I'm on a typical city trip (under 25 miles total), every time I lift off of the throttle for 2-3 seconds, I gain about a tenth of an MPG for that trip. I lose between .5 and 1 mpg every time I stop and wait at a light, and about the same if I have to make a sudden stop (no "free" coast down to build up some mpg) and accelerated from it.

So, if I make a 20-minute trip and figure that the EPA rating on my car is correct... I'm starting at 29 mpg. If I can do a 3-second coast every 15 seconds or so, I've gained 80 tenths or 8 mpg just from that. Puts me up to 37 mpg. Of course gentle acceleration is good for an easy 10% in my car, too, so there's another 3 mpg. This is how I can easily get 40 mpg in the city in my car.

You're doing good by trying to not use the brakes, but if you're downshifting to slow down, you're STILL wasting momentum. Try to conserve and use as much momentum as you can.

Just got back from lunch, about 18 miles each way. Even with shitty holiday traffic, I managed to trip 49 mpg on the way there and 51.2 on the way home. Tank average is over 49 right now.

 

Um ...so I ran it like a banshee and the tanks came out as follows
Filled it up in Ft.smith
Ran it down I-40 for 248.6m/5.3g=46.9mpg; some of that ended up on the fender, I just didn't believe it was full.
Took that tank down 78 to Prattville, Al. Might as well have been Interstate 70 mph limit for 318.7g/7.2g=44.2mpg.
Next fill was 129.0m/2.7g=47.8mpg don't remember where the fuck I was.
Got here and ran the last tank out running all over the place 496.1m/11.5g=43.1 mpg.

1192m/26.7g= 44.6mpg and I was in a hurry. Might take it a little slower on the way back up...but I doubt it.

 

Gotta love a car that can return that kind of MPG even without trying!

 

Well it's an auto as well, I tried to see if it had the dfco as yours does, after 3 sec or so the engine pitch does change a little but it's not that noticeable at 70, no scan gauge so I dunno for sure. Halfway across the country on 70 bucks is pretty sweet though. Just got back so when I feel like freezing my ass off I'll get the new stuff up...hell of a headwind on 40 though so I think those are going to be the best numbers, car was alot heavier for the return as well, not much worse but not better.

 

Drove to Orlando yesterday and proved once again that my car is capable of much better mileage just running around town than it is at Interstate speeds.

Drove from Safety Harbor to I-275 on Hillsborough. Cruising speeds from 45-60, a few lights. 48.1 mpg average by the time I got on the Interstate. (and was still rising if I'd stayed in that environment)

Cruising through Tampa at about 60-65, mileage steadily dropped to about 46. Cruising a steady 68-70 through Lakeland, it dropped to under 45. Stabilized at about 44.5, but I got tired of driving 70 and kicked it up to 75. Then it steadily dropped again all the way down to 41.8.

On the way home, traffic was dense, so I was able to cruise 55-60 through much of Orlando and got the daily average back over 43. When traffic broke, I went with the fast flow at around 80 mpg most of the way to Tampa.

I'm convinced that Tampa > Orlando is slightly uphill, because even travelling faster, I managed to keep the MPG over 42 for the highway portion. Adding in another off-highway stretch on Hillsborough got the final day's average to 44 mpg.

I need to buy a TDI. I would if they were smaller. (Toyota! Where's my diesel Yaris???)

 

Tdi golf is back, or you could put a pd150 in the Spit I think those are rated at 150hp/ 270ft/lbs. They also mount longitudinally so it would go in relatively easy. There are a few Miatas running with them.

Do it for the children.

 

I gave serious consideration to putting a diesel engine into the Spitfire when planning my engine swap. Ultimately, opted not to. The Yaris 1.4D would have been ideal, but I didn't want to have an oddball not-available-in-the-US engine in my already oddball car. Not much else that's available in the US seemed like a good fit.

The TDI Golf is still too big. I like a car that tips the scale well under 2500 pounds... hard to find these days. (even the Yaris is nearly 2400)

 

Jetta > 3400
Plus 2 kids and a sled full of toys
Still pulled 47.2 up I-75/I-20 from the fl state line to Birmingham.
I-40 headwind slapped me back to 44ish.

 

I alway decelerate so I can stay moving until the light turns green again to go. I mean most times your going to have to stop. But a good bit I can keep rolling and by the time the other cars accelerate I'll be right at there pace to accelerate up to the speed limit without having to idle or go from a stop.

 

When you downshift gently to keep your revs up and gain the benefits of DCFO, you're not hurting anything. The wear on the clutch is almost nothing because the engagement is smooth with no torque applied (either from the engine to the drivetrain, or from the drivetrain to teh engine), the wear on the engine is negligible, and there is no risk of doing any engine damage from over-revving because you're taking it easy. But, that's not really "compression braking" if you do it right. Your goal should normally be to have the car in a gear that keeps the revs fairly low to provide LESS resistance to your coast and allow you to coast further while maintaining the DFCO condition. (note that if you're coasting downhill at speed for a good long ways, it's actually more conservative to shift to neutral and take the fuel hit for letting the engine idle and extend your coast longer, but that doesn't happen much in FL)

If you're downshifting with the intent of compression-braking, you're going to put a lot more load on the clutch and the transmission. A hard downshift is just like a hard launch or a hard upshift, it's hard on the equipment. You also risk over-revving if you get careless, and that can trash an engine in a hurry.

The main thing is simply that brakes are a lot cheaper and easier to replace than clutches! But, there's a time and a place for everything. If you're driving through the mountains, going downhill, and just trying to maintain a speed without too much gravity-induced acceleration, it is quite appropriate to put the trans in a lower gear and let the engine braking regulate your speed. But that's quite a different thing from trying to use the engine to SLOW the car in normal driving.

Back to "efficiency" driving: Aggressive downshifting to slow the car is no more beneficial than using the brakes. In either case, you're scrubbing off momentum that could carry you further. You're turning momentum into heat either in the brakes or in the engine. The true economy would come from getting off of the gas sooner (when possible, of course) and coasting further rather than braking (either by engine braking or by using the middle pedal). Every extra fraction of a second you can spend in a DFCO coast rather than idling is going to improve your MPG a little bit.