What's the absolute maximum range of your Duster?
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I guess I don't know what your combo is, and why 14.7. What gears, what converter, what engine, how many carbs. 440 w/ 3.91's ? slant six with 2.76's? The only classic (and some at the time weren't really classics but now they are..LOL) cars we traveled in that got that kind of mpg was just recent. Our '77 Ramcharger Full Time 4x4 360 4bbl with 3.55's got 13 mpg on just about a 4,000 mile trip, and last year our '77 Dodge Van tradesman 318 2bbl got 14 mpg on about a 1200 mile trip. I think I'll get the Van better for this year after I pull the heads and re-work them. Again, you must have a wild child to be at 14.7 mpg in a A-body. Or, leaving marks at every stop ??? LOL
See your /6 Duster has a 7 1/4 rear end.
Lots of those came with 2.76 ratio. Had a 74 /6 Dart Swinger that came 2.76, automatic.
A little taller rear tire will give you actual more miles per gallon.
Kind of doubt you have a 3.23 ratio, but you can check. 2.76 rear gears will be best for mileage if that is your main concern.
Would think you could be closer to 20 mpg if set up right with that /6.
Had a '69 Satellite with stock 318, 4bbl upgrade, 727, and 2.94 rear gears. That one got 20 mpg. Everything was set up right working well together.
9.2:1 compression ratio in '69 helped too, more power out of the same fuel.
☆☆☆
Lots of those came with 2.76 ratio. Had a 74 /6 Dart Swinger that came 2.76, automatic.
A little taller rear tire will give you actual more miles per gallon.
Kind of doubt you have a 3.23 ratio, but you can check. 2.76 rear gears will be best for mileage if that is your main concern.
Would think you could be closer to 20 mpg if set up right with that /6.
Had a '69 Satellite with stock 318, 4bbl upgrade, 727, and 2.94 rear gears. That one got 20 mpg. Everything was set up right working well together.
9.2:1 compression ratio in '69 helped too, more power out of the same fuel.
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Jumpin' Jehoshaphat ! If it's a stock slant 6 and at 14.7, I'd be look'n into that before I even thought of leaving. Ouch to that!! Even at 70 mph, I'd think 20 + would be more than reasonable to get. We got 17 mpg with a '70 dodge truck with a 340 and mild cam at 65 mph on a 2000 mile trip back in the late 90's. Thermoquad and 2.76's.
A stock 225, 833 o/d. 4.10 in a 8 3/4 and 32" tall tires. The guy gets 26+ on the highway.
Ooh, love that old cable servo cruise control. I remember those!!!
Princess Valiant
A.K.A. Rainy Day Auto
The best I ever get with my daily driven slant is 16.5 mpg. I think that's pretty normal with a slant.
I never understood that 36 mpg claim from the feather duster ..... unless you're driving in idle the whole way.
Every 318 I have ever driven gets better mpg than the slant. The 318 doesn't have to work as hard to keep a dart propelled.
I never understood that 36 mpg claim from the feather duster ..... unless you're driving in idle the whole way.
Every 318 I have ever driven gets better mpg than the slant. The 318 doesn't have to work as hard to keep a dart propelled.
Some truth here. Some slants are hogs it seems. But my Dad's '68 valiant 225 got 28 mpg doing 70 on I-25 going back and forth 100 miles each way to work. I think if they are "right", mid to higher 20's can be common. I've had 318 2bbls get better over 20 easily. I think my brothers Aspen R/T with the 4 speed OD got 26 mpg on the road. It went on a lot of trips. Clean burn, valves that seal, timing spot on makes a big difference. Huge in fact.
It depends on "which slant" you're talking about. The 170 is a gas miser.
Absolutely, and revs nice also. Mine would get close to 30. But that was highway cruising, and of course it was not stock. For the OP get a vacuum gage and learn to use it as a mpg tool.
adriver
Blazing Apostle
Absolute is relative as we know.
But I can get and honest 20 to 23 on the highway with the 16 gallon tank.
I run a stock 318, lock up 86 trans, Q jet and a 2.76 8 1/4 rear 215 70R15.
So it's around at least 300 miles for me.
My advice, as someone that has driven cross country, is don't run 70 on a long trip.
That will make a big difference.
Slow it down and stop for gas every 200 miles.
Be cautious.
You rear will probably need it anyway.
But I can get and honest 20 to 23 on the highway with the 16 gallon tank.
I run a stock 318, lock up 86 trans, Q jet and a 2.76 8 1/4 rear 215 70R15.
So it's around at least 300 miles for me.
My advice, as someone that has driven cross country, is don't run 70 on a long trip.
That will make a big difference.
Slow it down and stop for gas every 200 miles.
Be cautious.
You rear will probably need it anyway.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I once drove my Barracuda all day on a tankful, (67 liters, or 11.8 gallons US) at speeds up to 85mph.
On the way back, I followed a late-model Corvette who was caroosing, so didn't get quite the mileage. She was geared 85=2100
The factory long-stroke slanty can do very well cruising at low rpm, from point to point. But as soon as it gets into stop and go driving, it all falls apart. With hiway gears she just can't get the ignition timing she needs.
If it was me, I'd be installing an adjustable, stand-alone, dash-mounted, dial-back timing box; and tune it as you drive. Not for vacuum, but for highest speed at smallest throttle opening. There is NO OTHER Way to get the ignition timing right.
Let's say;
With 2.76s and 27.7" tires, 65=2250 at zero-slip. She could be wanting over 50 degrees. But your factory D will be lucky to make 20 plus your initial. Factory timing is around TDC so 20 it is. If you advance the unmodded D to 12*, you might then get 32. But your power-timing will probably drive the slanty into detonation pretty easily. It's a long long way from 32 to 50, never mind from 20 to 50, and in either case, it will suck gas.
The device is easy to use;
Just drive it up to your cruising speed on a flat level hard road, and wait for the SPEED to stabilize. Then dial in 3 degrees and wait. If the speed goes up,drop it back to your starting speed and repeat. Keep doing that until the speed no longer increases. If it eventually slows down, then back up 3 degrees.
If you run out of range, you will have to advance the D manually, and start over.
You will have to remember to dial it back down, whenever you change throttle setting, and especially if you have to gas it, or climb a hill. If you hear it rattle jump on the device! If it continues, back off the throttle until it stops. The device cost me about $170 back in the early 2000s. IDK what they cost today. Mine paid for itself very quickly. I even used it to dial in my power timing. And my PT (Part Throttle) timing. And I used it to figure out if carb changes were in the right direction.
Pump your tires up too.
On the way back, I followed a late-model Corvette who was caroosing, so didn't get quite the mileage. She was geared 85=2100
The factory long-stroke slanty can do very well cruising at low rpm, from point to point. But as soon as it gets into stop and go driving, it all falls apart. With hiway gears she just can't get the ignition timing she needs.
If it was me, I'd be installing an adjustable, stand-alone, dash-mounted, dial-back timing box; and tune it as you drive. Not for vacuum, but for highest speed at smallest throttle opening. There is NO OTHER Way to get the ignition timing right.
Let's say;
With 2.76s and 27.7" tires, 65=2250 at zero-slip. She could be wanting over 50 degrees. But your factory D will be lucky to make 20 plus your initial. Factory timing is around TDC so 20 it is. If you advance the unmodded D to 12*, you might then get 32. But your power-timing will probably drive the slanty into detonation pretty easily. It's a long long way from 32 to 50, never mind from 20 to 50, and in either case, it will suck gas.
The device is easy to use;
Just drive it up to your cruising speed on a flat level hard road, and wait for the SPEED to stabilize. Then dial in 3 degrees and wait. If the speed goes up,drop it back to your starting speed and repeat. Keep doing that until the speed no longer increases. If it eventually slows down, then back up 3 degrees.
If you run out of range, you will have to advance the D manually, and start over.
You will have to remember to dial it back down, whenever you change throttle setting, and especially if you have to gas it, or climb a hill. If you hear it rattle jump on the device! If it continues, back off the throttle until it stops. The device cost me about $170 back in the early 2000s. IDK what they cost today. Mine paid for itself very quickly. I even used it to dial in my power timing. And my PT (Part Throttle) timing. And I used it to figure out if carb changes were in the right direction.
Pump your tires up too.
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cruiser
Well-Known Member
225 slant six engine, stock A-904 tranny, 2.76 rear end, stock one barrel Holley carb, stock ignition and coil, timed to 10 degrees BTC. Very stock and original.
How many miles are on the engine? When was the last tuneup with new plugs, cap/rotor, wires, air filter, PCV valve? Does the engine fire right up when starting, or does it crank and crank before starting? Have you pulled a plug to see what the condition is? Plugs will tell quite a story!
Coffee&Cars
Well-Known Member
Well it's about 1/2 mile per gal....LOL
Back to your original question. The OEM tank is listed as having a 18 gallon capacity. If your tank is 16 gallon it is the wrong one but as long as it fits it really isn't a issue. Just a shorter driving range.
thesiren74
Well-Known Member
I do a 800 mile trip once a year in my Duster with a big block i usually top it off at about180 miles just to be safe never ran out. Bring a gas can.
Excellent! Thanks.
What size tires?
Do you know how many rpm’s you turn on the Hwy.?
Recurving the distributor and resetting it a little higher can help a bit. Also, ignition is a key source of mileage. You did t mention the year, if it has points are electronic ignition. I like the Chrysler chrome box, but there hard to find. A FBO box is also really good. An overall winner when there right is a MSD unit. There multiple sparks at low rpm’s do help a smidge with more mileage. Open up the plug gaps to about .050.
After these little tricks, (and I’m sure there’s more that others will chime in with...) some of the other items can be a little pricey. They would be be change parts. There is a tiny bit more mileage with a lock up converter trans.
A 904 version is available. It would probably also come with a deeper first gear. That will wake up the “get up and go factor” in first. Then it will act normal until you pop into OD. But that’s pricey and not the end of it since you would have to shorten your driveshaft.
All in all, that’s a serious “Ka-Ching” expenditure with little in return.
I know there has got to be someone here that has a super tuned /6 with cheap tricks for cruiser and his car.
Princess Valiant
A.K.A. Rainy Day Auto
Your dad must have had some pixie dust fuel additive. I-25 is exactly where I do a lot of commuting.
I'm betting slowing down might up the mpg but I still don't see it going from 16.5 to 26 by dropping from 85-90 to 70 mph
yellow rose
Overnight Sensation
Best /6 I ever had was a 170. It blew the doors off the 225 it replaced and got better fuel mileage.
85-90 ? You are about topped out, huh? LOL
Actually, his Valiant Signet did 112 top end on I-25 (according to the speedometer). Who knows by radar..... I think he averaged about 70 mph traveling to work on that interstate.
Have you considered a lock up 904?
I cannot say the same. This 225 I have now could be mistaken for a V8 idling and when you put your foot in it.....but of course soon as the throttle is cracked, you know "what" it is. lol
Princess Valiant
A.K.A. Rainy Day Auto
I have an od 833 and I don't like it. So I'd switch to a 1 to 1 4 speed before going auto. People think an od is going to save fuel and be the magical solution ....... yea if you drive like miss daisy to piggly wiggly.
I'm usually kicking down to 3rd which is 1 to 1 just keep it responsive and full of pep. When you up to OD, the engine sort of bluh on its face and you're not going to maintain 75+ in OD. also have 2.94 gears out back so if someone is going to have an OD, then 3.23 gears or Lower is probably going to work better, otherwise you're going to be constantly kicking down and keeping the RPMs up in order to get power and maintain speed and the mpg is going to go straight down the crapper.
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