Poor Mileage - Cure or Sell?

[scott313]

My son purchased a 74 Challenger when he was 12 years old (with his own money). He and I have been working on it continuously with new suspension, rear end, transmission and 360 motor. He is now 17 years old. Unfortunately, it has one major problem that is causing him to consider selling it. It only gets 6 to 7 MPG. Most of his driving is around town going to and from high school and work (average speed is 35-45 mph). I have driven it with the same results (no lead foot at all). Below I have listed the vehicles specifications.

I had it dyno tuned (Superflow Chassis Dyno) and there was no improvement. The dyno showed peak RWHP of 303 @ 5700 rpm and TQ of 336 Lb/ft @ 4100 rpm. (Note: I have Performance Trends Engine Analyzer and it calculated the peak TQ @ 394 lb/ft – 3700 rpm and peak HP @ 344 hp – 5200rpm. This program has been consistent with the builds/dyno articles in Mopar Muscle. This should put the RWHP @ 240 to 260. We are getting considerably more.)

If you can suggest anything we might do to get the mileage up to prevent selling his dream car I would be very appreciative.

Thank you,

Scott Hamilton

360 Motor:
4.040 Bore
Stock Stroke & Rod
Probe Pistons 12351-040
-14.5cc Dish
+.005 above deck

Heads: Edelbrock 60779, 63cc Chamber, 2.02”I / 1.60”E

Head Gasket: FelPro #1008 4.18” Bore, .039 Compressed Thickness

Camshaft: Hughes HEH 1523 AL
.470 I / .506 E
215 I / 223 E Duration @ .050
112* Lobe Separation
109* Installed Centerline (Per Cam card) and Degreed
Opening @ .050 tappet lift Closing @ .050 tappet lift
I = -1.5°BTC I = 36.5°ABC
E = 46.5° BBC E = -3.5° ATC

Manifold: Edelbrock Performer RPM #7176

Carb: Edelbrock Thunder AVS #1806 650 CFM

Transmission: 727 (stock gearing), Hughes Converter apx. 2000 stall (actual about 2200)

Rearend: 8 ¾ 3.55 Posi

Tires: 25.5” tall

Calculated Static Compre

[345Dart]

In town driving - I doubt it will get much better.
Taller tires might help a bit - 255/60/15s are 27" tall but at 40mph you should only be in the 2000rpm range. As long as it is running decent - plugs are not fouling, etc. I suspect that is all you will get.
I get 4-5 at best with 4.30 gears and 27" tall tires.

PM me if you ever do decide to sell it. By coincidence a friend of mine is looking for a 74 challenger.

[1973Swinger]

I agree with 345dart, you arent going to get all that much better than that. Really though, what did you expect? That combo isnt meant for economy. You could always throw a /6 together and drop that in it, or put the 360 back to stock and a 2bbl,but them you would lose all of the power that it has. Personally, Id get a 4 banger econobox for everyday driving and save the Chally for weekends and cruising.

[Dick]

The mileage sounds about right for that package.

My only concern would be using it as a D/D, especially to HS and work. It isn't like you can go and pick up a replacement if something was to happen to it. Being that it represents a father/son project covering a span of time and is also quite rare today, I would vote to use it sparingly. (If there WAS a vote. )

[JLM440]

Thats the price you pay when you wanna play

[sabbfan]

Buy a cheap daily driver and use the challenger for fun stuff. You'll regret selling it because of mileage.

[Satty71]

You could try the spreadbore route and higher gears but it won't have the puch off the line and wouldn't be nearly as fun to drive. My ride has a TQ and 2:71 gears and gets awesome milage but the launch is none to exciting, but the highway is awesome. Being on a military base average speed is 25-35 mph so gas lasts forever just using the primarys. Call the sizzler, he could probably give you a better estimate of fuel economy with the swap which would help you decide.

[TK]

Drop the heads, RPM manifold, and lighten the cam, and you might get 10.

You guys are missing something about gears though, LOWER GEARS get better MPG around town, because the engine takes less power to push the car, lower gears get better MPG's until RPM's get too high. Around town, that isn't a problem.

[cutting torch]

Scott313, I would suggest this: you buy it from him! You can keep it in the family, and if (when) he wants to buy it back, you can sell it back.

In the meantime, YOU can have fun with it!

torch

[dodger1]

I guess I'll concur with the rest of us here. I have a MILD 360 (maybe 275 hp at the flywheel) in my 3700 lb. Mirada. With the stupidly tall 2.24's originally in the car it got as much as 20 mpg on a long trip. Now I have 2.94's and I get around 12 in town and 16-18 hwy. Thats with a TQ, stock converter, 220/228 Mopar cam, 9.0 to 1 flattops and stock (1978)truck heads. So gears and a TQ might help you a bit but I surely wouldn't want to back off much more on such a nice investment in your time and $$.

[scott313]

Thank you for your suggestions. We were trying for a 300HP car, fun but not over the edge. Simular to the HP motors back in the day. Total around town driving I used to get 10 - 12 mpg in my Dad's Cuda I had to pay for the gas). I like the idea of the TQ and we are going to try an OD transmission. Thanks again!

[jd-indy]

In my opinion, you should be getting better mpg than you are getting. I have a 360 dart that gets at least 12 mpg. I am using 2.02 j-heads and have a smaller cam (stock 340) and I use an emissions jetted edelbrock 600 afb. One thing to remember is that the Edebrock heads don't have any manifold heat riser passage. This can lower mpg when the motor is warming up. If you are using stock exhaust manifolds that still have the heat riser valve, the exhaust on the passenger side has no where to go when the valve is closed. That will kill power and mpg. If you are running any kind of choke make sure it is opening all the way . I would try a different carb if you have one handy, my lean jetted 600 gets way better mpg than my performance jetted edebrock 750. If you can get a hold of a re-jetting kit, I would try to lean out the front jets on the carb using the fattest metering rod in the kit and work from there. A closer to stock cam would really help mpg and also low end torque, but I really do like your cam choice for street power. I also run the performer rpm manifold, but I left the exhaust flowing through it, I run an insulative spacer for the carb. A hotter thermostat may also help mpg. Fuel injection would be the ultimate mpg upgrade.

[nhdriver]

I think it it would be an expensive waste of time trying to wring an extra 2 or 3 MPG out of this car. Keep it home in the garage & save it for an occasional nice sunny day. Get him a 4cyl econo car. With the gas money saved he can buy gas for those fun days in the Challenger.

[dodger1]

If you do go with an A-500 (904 OD) or A-518 (727 OD) that'll give you a final drive of 2.45 or so and combined with a TQ just might work, but it WON'T be apparent at low (city) speeds as much as on the highway. Satty 71's comments certainly apply to these proposed changes.

[twistedpipes]

IMO. you dont buy a 74 challenger to build as a father/son project then complain about gas mileage? then ask a question cure or sell based on gas mileage??? I believe it's obvious that dad and son didn't bond to well doing this project,because any stand up father would assist his son in purchasing a cost effective economy car(after seeing that his son wanted to build with his dad at the age of 12) hope your head comes out to see the light of day some time soon. My dad and i had a similair sitiuation we sold our 68 truck we built when i was growing up,now 15 years later dad is gone and so is truck,what is left to say I built this with my dad to give to my kids? food for thought.

[Dick]

The more I think of this, the more I remember when I was 17, if I had a car with 300rwhp....................................

If it was a car built within the last 10 years I wouldn't give it much thought, other than the ticket potential. But a Challenger is a very special car that is rare and very costly to restore or buy already completed. Add to that the F/S build up element and it just doesn't seem that this car should be used as daily transportation.

Too bad you guys didn't build a Camaro at the same time.