mpg on a '75 Clamshell-What should I expect?

I have a 1975 Caprice Estate wagon with a 400 small block. My engine was rebuilt about 29k miles ago; last tune up less than 15K miles (3 years) ago. I'm finding my city mpg to be single-digit and freeway mpg to be between 10-12. I typically drive with no more than one passenger and very little cargo; A/C often on when freeway driving. I typically use 87 octane fuel. Is this "normal" or should I expect better mpg?

 

I don't have a clue. Except my 73 Gran Torino with the 400 got 16 highway.

I think if you "super tuned" it. Meaning check and adjust everything that you may pick up some. Leave nothing untouched. PCV, carb, timing, plugs, wires, filters and that's all including the ones in the air cleaner. Check for carbon builtup especially if you have an EGR. I think 12-13 highway is a tad low but I don't know Chevys that well.

 

Take a look at the Mr. Gasket Advance Ignition Curve Spring Kit for the chevies. Might give you a 10 to 15% boost in HP, Torque and MPG. For under $10, and a couple hours, you can't go wrong:

http://www.mr-gasket.com/ProductsLi...ID=5282&majID=525&selection=16&minselection=3

 

If it is a 2 Barrel you won't get much better. You can get a lot better with a correct Q-Jet 4 barrel.

 

Admittedly, I'm not much of a mechanic. I can perform an oil change and do very light body work, but I've reached that point where I am eager to learn more about what's under the hood. So thank you for your pointers and insights!
Since I don't truly know when my wagon had its last tune-up, (I've only put 10K miles on it since I purchased it two years ago), I suppose that's a good place to start. It's possible it hasn't had one since the engine rebuild. Anyway, thanks again...I'll post an update when I know more. :2_thumbs_up_-_anima

 

This is a DIY for the Advance Ignition Curve mods. It's on a Ford, but the methods are valid for GMs.

http://www.reincarnation-automotive.com/Duraspark_distributor_recurve_instructions_index.html

 

That seems a bit low to me. That thing I would think would be getting 18-20 highway, and probably 10 maybe 12 city. Just a guess off of my caprice and my folkses 77 ltd.

 

That is low, I would say. My '88 307 which is really bad in the mpg figures because of all the emission stuff, got about 13mpg average. A friend of mine had a similar car but with a rebuild 400 block (alu heads etc about 350hp) and with his driving style he still got 15-16mpg on LPG!

 

20w50?

Hmm, just had another thought, question actually. I'm using 20W50 motor oil. Could that have an adverse affect on mpg?
My first road trip with my wagon was when I purchased it. That trip was nearly 800 miles. At that time I got about 17 mpg highway, in triple-digit summer heat with the A/C running.

 

I am suprised some of the Clamshell owners haven't spoke up yet. But from what I can remember your MPG is about what I would expect from a 73-75 Clamshell with some options, You have to keep in mind that Gas milage wasn't really an issue for most of us in the USA untill the 1st Oil embargo in Oct of 73. Remember that is the begining of the 74 model year. So not much time to modify the designe of the 75 cars.
My Father had a 72 Caprice wagon, with a 402 BB, 3.23 axle. Good car, but I can remember him complaining about 2 things about it. 1st that the tailgate & rear storage area was narrower than his 69 Kingswood (his flat bottomed row boat wouldn't fit flat in the back) And the fact that his MPG was 8 in the city & 11 on the highway. Remember that you are dealing with a 5000lb car, and they really didn't get any real improvements in MPG until the 76 & 77 model years.
My 78 Impalla with 350/350 SBC & 3.08 rear axle gets 12 city and 19 highway MPG. All the previous advice posted here is solid, I just wouln't hope for more than 1 to 2 MPG increase. If you do have a 2bbl carb, a properly set up 4bbl Q-jet will increase you MPG on the highway by 1 to 2 MPG.
What rear gear do you have? And I would start looking at installing a 7004R OD trans, if you wan't any real increase in MPG. but that is going to cost you between $800.00-1500.00 depending on what mods need to be done to install in your car (driveshaft, a 4bbl carb is almost manditory with a OD trans swap. Much easier to adjust the TV cable.).

Good luck with your car, with todays $4.00 a gallon gas prices, it is much easier to justify the expense of a OD trans swap on any older car that is driven regularly. I am seriously considering it for my 78 Impala, that is driven daily in good weather.

 

I do have a 4 barrel quad, yes.
Friday I pulled one of my spark plugs out to check for carbon build-up and/or fouling. It didn't look bad, but enough carbon build-up for my taste to warrant a tune-up, which I'll do next week.
Should I stick with AC Delco plugs? Someone suggested I try Bosch. Despite my wishes and intentions, most of the driving I do in my wagon is "city-like" rather than open freeway, thank you L.A. traffic. I virtually have to leave town to find an open freeway it seems. :banghead3:

 

Go with AC Delco Rapid fires you will be a lot happier with those than Bosh. But if you really want to boost the mileage and don't mind spending the money. Go to Summit Racing and look up ignition parts and get a Accel HEI coil,module, Cap rotot. MSD is better along with Taylor wires and E3 Spark plugs which you can get at a local parts store like Auto Zone or O'Rilleys.

If you have a Chevy 305 or 350 get a set of headers,Hi Flo cats (EBAY) and true dual exhaust. If you have a Olds 307 you can't get headers.

 

It is hard to tell; the B.O.P. clamshells came with 455/4 engines. My Wagon f.e. does less than 10mpg with a slightly tickled engine and a luggage cover (at $10.50 per gallon )

 

Don't we love European gas price . Ah that luggage cover. Would love to make a copy of it, if you don't mind.

Again back to my previous owner of the 76 Buick I bought (yep just sent the big check for the car). She got 12mpg in a mixed driving conditions (bit highway bit city etc). This is not bad for a gigantic car, it's after all an engine that is strangled by fed emission regulations, pulling a 5000+ car . That is in my view pretty decent mpg for a engine of that vintage..

Beside the normal tuneup and making sure everything in in tip top condition there isn't much you can do to get things better than that. Unless we start to modify. (Will not rant about what you can do? There is a lot according to hubby -- I'm the body girl in our family -- Anyways can write about it if any of you want to listen to the rant )...

Cheers Dyna

 

Oh yes you can! Check out the Advance Ignition kits from Mr. Gasket! Should improve from 5 to 15 percent! They make them for the GM models.

http://www.mr-gasket.com/newproducts/

Page 98 in this PDF catalog (5.7 MBs:
http://go.mrgasket.com/newproducts/CAT_PDF/74345G.PDF