Mileage - Summer/Winter

[BigBlockMopar]

I'm wondering what everyone's difference in mileage is during the summer and winter times.

Well over a month ago, when summer-temps where still prevalent here (15-20 C / 60/70's F), my Dart got 16 mpg.
Now with temps hovering quite lower (5-12 C / 40-50 F), it's hardly getting more then 13mpg.

I did change one thing when the temps where dropping aswell, and that is installing an overdrive transmission.
But the OD-transmission has a heavier rotating mass then the previous transmission, and it's first gear ratio is a bit longer aswell.The rearaxle gearing with 2.93:1 is not optimal for the motor.
So I don't have a good clue what exactly caused the decline in mpg, but I would also think thicker, colder oils would decrease mpg's aswell in winter time.

[slow67]

Gasoline engines typically get a little less MPG in winter. Air has more oxygen (by weight, aka denser) when its colder. More oxygen in = more fuel in to maintain stoichiometric combustion. But a 3MPG drop is significant. Is your driving more city, more highway, or ...?

[Steptoe]

Winter here is around the 5C to 13C and summer 17C to 26C
I tend to get better milage in winter....If I fill up on a frosty morning when the big holding tanks are cold.
I dont note any significant change in AFRS...just get more out of a tank.
I use 15/30 oil...car is not outside in the open at night so it doesnt get down to the 5C.

Im a bit confused in yoiur explanation in the OD and gearing...
Do I understand motor to gear box to OD So have OD on all gears with a 2.9 diff
What car/ truck? and approx tyre diameter..
Going with around a 25" diam tyre, stand type trans ....And assumin the engine is not built for real low bottom end torque, and is driven around town mostly, interections and stuff....I would imagine that 1st and 2nd would now be way too high....like trying to take off in 2nd and 3rd normal, or worse....yeah that will knock up round town milage real bad.
But hit the open road...rough gess 1500 rpms 60 mph ball park..
That will be real economic... stop start will kill u.

This is why 4 speed autos, 5 speed+ 'boxes have lower 1st and 2nd than 1:1 top/Drive ratio like T350 etc.. and top gear around the 1.6 mark.....then run rear ends like 2.9s 308s, 3.55 so well.

[BigBlockMopar]

'73 Dodge Dart - 60/40 highway/city
Engine: bonestock smogger 2bbl 318ci - 8.6:1cr - using 5w30 synt. oil. - 22vac @ 500rpm idle.
IMPCO convertor&mixer.
A518-OD trans - 2.93 rearaxle gears - Rear Tires 235/60/15.

OD works in 3rd gear only.
Trans gearratios: 2.45 • 1.45 • 1 • OD: 0.69
With the stock 3spd '904'-transmission I had in the summer, the ratios were indeed 'better':
2.74 • 1.54 • 1

The 518-transmission is a 'bigblock' transmission, but with a smallblock-bellhousing. It's built beefier for use in 90's pickups and vans.
Because of the beefy internals, it also takes a little more power to run these, so that would explain some loss of mileage, but I hoped the OD would compensate for that again.
But I guess the less-optimal first and 2nd gears in the trans have a more significant effect on mpg than I thought.

Today I will be replacing the 2.93 axle gears with 3.23 gears. This is primarily to help the engine get into it's powerband slighlty better when doing highway cruising in OD.
Currently with the 2.93 gears and highway speeds (60-65), the car slows down considerably with slight uneven terrain and slopes. More throttle is needed to maintain speed, but vacuum is then in the low 10-15 range, instead of the regular 17-21 range.
When upping the speeds to 70-75, the car starts to run better. The motor gets into it's powerband better and changing speeds/overtaking can be done with less throttle-movements. Vacuum is then just above 15 usually.

3.55 gears would be 'perfect' for my car I feel, but I haven't got those gears laying around.
I do have a 3.9 gearset, but I think this won't help the mpg much again, as it might feel during city driving you're always one gear too low. What do you think?

[Steptoe]

Putting aside the 'weight' of the new trans, if a vechile is working outside its power band, yes has a big effect on performance... both power band and economy.
Dont measure by vaccuum.. every engine has its own unique charactoristic.....on the other hand if the prevuios 17" was efficient, and now dropped to 10" is a sure indicator.
Another effect of changing gear ratios, also changes the load on the engine at a given rpm .. eg cruise..which could very well mean recurving the cruise advance in the dizzy....what could be happening is previously at 17" the VA was all in , now there is no VA advance.....which when a engie is retarded, there is a piont that the power/ efficiecy, economy drops off dramatically...not having the VA active is very likely to retard below that point....plus add with lower rpms, below the piont where the cent is all in, would also retard.

[storm]

This is interesting. I have always found optimum mileage for carby vehicles is obtained when travelling along at the equivalent or just above (+200-300 RPM) engine revs of "fast idle". My car, the new old one that is, has a fast idle of approx 1500 RPM optimum mileage (petrol) is obtained at about 1700-1800 RPM but this makes for a very slow trip cause 100 km/h is about 3000 RPM. At that speed you are out of the idle mixture, which is set rich at the factory, out of the transition and just onto the actual main jet mixture. This is on a Quadrajet, Holleys are just fuel buckets unless tuned properly.

[BigBlockMopar]

working outside its power band,

That's the main issue here indeed.
This 318ci motor, because of it's smogger-low compression, has wimpy torque-figures at low rpms.
Therefore it can not provide enough power with a longer gearing.
I had planned to install 3.23 gears on sunday, which I had to pull from another car first, but that job took longer then I hoped for.
The 3.23's only do a small step of getting the engine into it's powerband better, a 3.55 would be even better. But I'm trying to think ahead for when I'm ready to install the 5.7Hemi in the car. The Hemi will no-doubt have enough low rpm power to work with 3.23 gears, where 3.55's would be slightly too much again I would think.

I do hope to get (way) better MPG-numbers with the Hemi though.

[kikkegek]

pretty funny. I was wondering exactly the same thing recently.

Previous tank got me little over 4km/l (9,4 mpg) and when I filled her up yesterday I only calculated about 3,7 km/l (8,7 mpg).

I didnt change a thing in the mean time, just the weather got colder. We did use the A/C recently because the windows were fogging but cant remember when...that would probably kill my mileage...

will check my mileage again and check if we use the A/C again.

the thing that I donyt get is the TH700R4 gives me a lower first and second gear, but no improvement in our daily driving:
TH400
First Gear - 2.48:1
Second Gear - 1.48:1
Third Gear - 1.00:1

TH700R4
First - 3.059:1
Second - 1.625:1
Third - 1.000:1
Fourth - 0.696:1

I do get better highway mileage. This went from 4 km/l (9,4 mpg) to 5 km/l (11,8 mpg)

any thoughts on that?

[Steptoe]

I have a bottom end torque cam..
With the T350 and 308 diff would get around 13/13 mpg down to 10 around town... about 2600 rpms At 60mph
Swap the diff to the 255 open road 17+mpg round town 14/15 mpg 2000 rpms =60mph
I had to reset the VA and total to suit the new rpm working range.... but the 308 did put the working rpm range more into the engine working range...
Ideally I would have like tpo go the 700r way (finances at the time) with the 308...
Im sure would have picked up a tiny bit more in the around town..the stop start lower 1st 2nd torque) 15/16 range, and it would hve droped the 60 mph down in the 1500/1600 range...even better.

Asuming that mixtures and timing are correct(ed) keep in mind at the end of the day , economy is about the volume (and therefore the volume of fuel mixture) the engine turns over per mile.....which happens to include time spent idling at interestions.

I do believe my biggest killer of around town is idling at lights and intersections, not the gearing ....

[Unhippy]

i get better mpg in the winter....i think mainly because winter weather isn't the best for fast driving and hard acceleration

my biggest MPG killer around town is attached to my right ankle .....i normally get 25 mpg around town....if i'm trying hard and use the throttle like a light switch i can get down to 11 mpg .....but what an 11 mpg

[BigBlockMopar]

Well I went and changed the rear axle gears today.
Took the 2.93 out and replaced it with a 3.23 ratio.
Will still need to change the speedo-gear in the transmission so the speedo reads correct again.
After this I'll fill'r up and do a mileage check again.

I did feel the change in gears on the way home tonight.
It did bring the engine's powerrange down a bit and accelerating felt slightly 'easier'. I still feel a 3.55 gearratio would be best for this motor/setup.

[BigBlockMopar]

I've driven 2 tankfillings now with the 3.23 gears. The 1st tank netted 13mpg and after the 2nd tankfill (with near freezing temps) the car only got a lousy 11mpg.

AFR's are still pretty much the same as during the summer.

I can't grasp how some of you seem to get better mileage in the wintertime. Do you drive longer distances on the highway maybe, where an engine/tranny/rear axle gets warmed up better than on my own usuall 20minute trips?
I do roughly 3 to 4 trips each day, of not more then 20 minutes each tops.

[kikkegek]

I have noticed that my AFR's at cruise and light throttle are about .5 or .7 leaner with the temperatures now around freezing point.

I havent really noticed any change in mileage though...

I think in my case the leaner mixture is compensated by the thicker oil in winter.

also just 15-20min trips a time

[BigBlockMopar]

I've also started using 5w20 oil now. I'm replenishing the current 5w30 oil in the engine with 5w20 at every refill.

What I do feel with the car, is that it's drives 'best'/easiest, when I've driven it and then parked it for an hour or 2 orso, and then drive it again. That's when I feel the engine/car is most responsive. Most likely because all the oils in the drivetrain are warm and thinner IMO.

[Steptoe]

I'm replenishing the current 5w30 oil in the engine with 5w20 at every refill.

I dont understand this...
Even with lots short stop start , cold running, then a long trip, I would be lucky to have to top up my oil a couple 100 mls once between oil changes..if that is what u mean..