Why is my Gas Mileage so bad? '66 Skylark w/ 300.

My '66 Buick is my daily driver. I take good care of it, but it's nothing fancy.

I recently decided to track my mileage for the last 3 months and I'm getting really crappy (I think) MPG. My setup is pretty mild, but I'm only averaging 12mpg. That's UP from about 8-9 MPG on my old carb.

I've done everything I can think of to balance decent performance and good driveablity/mileage.

Stock "freshened up" 300 with a '65 cast-iron 4-bbl intake
2 1/4 Dual exhaust (no headers)
Electric Fan
200-4R overdrive trans (stock Monte Carlo converter)
3.08 rear POSI
500 CFM Edelbrock (New) using the out-of-box jetting (I tried jetting a little richer but didn't get a performance payback. I have not tried jetting leaner)
MSD ignition and plug wires, running a pertronix ignitor.

Car runs dead-nuts at 180deg. Throttle is crisp, no stumbling, choke works great on cold mornings. Engine idles like a Lexus, no smoke, no oil burning. I have an oil accumulator to prime the engine on cold-starts. I'm also running a 3/8 phelonic spacer to keep the carb from getting too hot.

I have a pretty short commute to work, no freeway. A bit of traffic but nothing major.

Given my mild rear gears and 4-spd overdrive transmission, I'm failing to see why my mileage is so bad. With my combo I was expecting at least in the 17MPG range. Even if my carb is *mildly* rich using the edelbrock stock jetting, it can't affect my mileage THAT much, could it? Any jetting problems that would affect mileage so signifigantly would also produce a drivability problem I would think.

Is there anything I'm overlooking? I don't want to granny the car out just for good MPG, but given the investments I've made I was hoping for a better return (at least from the 200-4R!). Still, I want to make sure I'm turning over every stone.

Thanks!

 

With a short drive to work, is your choke not pulling off quickly enough? I've found that my old cars never do well with just around town driving...Max on these things is 18-20, but that would be strictly freeway. In fact, in colder weather, my Skylark is likely to do 12-14...

 

In town, short trips, ideling, 15 mpg or so. out on the road 17-18 .
The phenolic spacer is cutting milage, because the fuel system is designed to warm the gas as it goes thru the carb making it 'flash' quicker. If I remember right the intake on that engine is heated by coolant. Make sure that all water passages/ heat riser passages ect are open and working and like has already been mentioned ck the choke settings. I set a choke where a 1/8 drill will just fit between the lower edge of the choke plate and the carb body.
Run at least a 180 deg. thermostat. and make sure the thermostat is not stuck partially open.

 

That car was designed to run on real gasoline not the cat pee that we get now when we fill up. That is probably at least some of the problem.

 

What do you get on a trip? Try running for an hour and recheck your MPG. Running around town and short trips create too many varibles to accurately check mileage. I find seeking the best mpg as a baseline is better for tracking mileage. And I imagine its pretty cool in Kirkland this time of year, which will also worsen your mpg. Mpg was less important when gas was 22 cents! And that's my 2 cents.
- Bill

 

the guys are right about all they said BUT here is my 2 cents timing is very important Frist you have to checked it at idle(vac line pulled and pluged ) next with the vac line off checktiming as you install the vac it should jump8 t0 10 degrees////////then with the light still on the marks increase the throttle to 3000 and the timing should advance another 10 degree.....ifthat is the case then the dis is advancing fairly correct (i did this off the top of my head so i could be off a few but you get the general idea/////////////////////////////////////if it is ok then try a OLD trick if the timing should be 3deg set it at 6 and drive it and see if it pings/////itmight but only if you push it in high/////if itdoesnt try it and i bet the miles will be better luck to you pete

 

Wow thats surprisingly low mpg.

When I got my 76 century is 4,000 pounds with Buick 350 it got 10 city and 13 hwy (80,000 miles bone stock). After I cleaned out the fuel tank and lines, removed carbon from pistons and head, rebuilt the carb and power timed it I got 14 city 22 mpg hwy. I've since changed to a larger cam and still get 20+ on the hwy (100,000 now).

 

Sean, is that mi./US gal or mi/Imp gal?

 

you said msd, is it a dist or just a box? what is the trigger if its the box? if its a gm dist. check your vaccuum advance can and make sure it is holding vac. if it isnt get an aftermarket adjustable one and tune per directions, if it is ok, set total timing and disregard initial for the moment. total with vac and mechanical advance should be 34-38 degrees TOTAL all in by 2500 rpm. also a set of gears may be in the works as th "lugging" talked about earlier is a definite gas guzzling scenario. you want you engine to be working in its optimum power band for torque, thats what makes you go thats why you have a buick! a stock 1964 300 with 273 gears and a st300 trans got around 14 in town and 20 on hiway, I had a 401 and got 20 on the hiway so its all in the combo. if you have no vac advance on the distributor, GET ONE no vac is for race cars and or big cams!:TU: :beer

 

forgot also edelbrocks seem to be set up for 350 chevys get out the book it came with and set it up for 2 steps leaner in cruise mode and leve the power mode alone and try it also the vac advance line goes ont the passenger side nipple not the drivers side one I see that all the time.:3gears:

 

Thanks guys. My timing, etc. should all be in top shape but I'll check again. I haven't checked it for sure in a while. The MSD box is triggering a stock disty with a pertronix. I'll double-check vac advance and all that stuff this weekend.

re: lugging. This *was* a problem until I got the new Edelbrock carb. My tranny place had fabricated a 200-4R bracket onto the stock carb (which was an auto carb off a 401 that I had re-jetted) which was causing early shifts, but with the Edelbrock carb I was able to use the engineered piece from bowtie overdrives that got the shifting just right. Also, I installed an electronic speed-sensitive lock-up controller so 4th doesn't lock-up until 50mph. During normal driving conditions I'm never below 1500 RPM (that's the lowest) and don't really feel any lag.

I did order a set of rods that are one step leaner than I'm running now. I'm going to give those a try this weekend and see if it helps.

 

US gallons

 

You have got to be happy with those milage figures, Sean. All that Buick power and good milage too! 1500 rpm might be a little low to get good milage. I know that my 97 GMC gets better milage at 75 mph (2200 rpm) than it does at 60 (1650 rpm) and I think that that is because it is just getting into the fat spot on the torque curve.
Marc

 

Wow, lots of great advise posted guys!:TU:
I got about 14 mpg in my '66 Special in mixed hiway/suburban driving.
I lost about 2 mpg when my distributors vacuum advance developed a leak.
It's mostly stock, with dual exhaust, 4 bbl, and petronix added. Rear is only a 2.79, but the larger 255/60-15 tires throw off my speedo a bit...
I installed a 3-speed manual setup last summer which is ALOT more fun that the 2 speed auto!:laugh:

 

Once I get the 200 r4 and rebuilt 71 with poston intake and headers I won't be worried about millage, I'll be haulen but!

Chevy SB like the rpms, almost need a 4:10 gear to feel like a Buick with highway gears.

 

Cars of those years were built to cruise the highways at about 2500rpm at the speed limit or a little above, so 70-80mph. Everything was designed to maximize torque right about there, so lots of times when guys try to improve mileage by changing the axle gears or adding overdrive they end up with a loss. Something to keep in mind. I would think you'd want to try to stay between about 2200 and 2600 rpm at your own comfortable cruising speed.

Jim

 

I'm going from 2.56 with th350 to 4.11 with the 200 R4.

 

I'd ditch the edelbrock for a Q-jet first. Then the 3.08s with a .67 overdrive is too much. Alot of people have a notion that in order to acheive the best posible mileage, the motor needs to be turning very low rpms. which is simply not true. engine efficency is where its at. as mentioned, try to keep the rpms around 2000-2500 cruizing. I would only use the overdrive at speeds above 50. I do the same in my truck- 3.73s, .70 overdrive (700r4) 30 inch tall tire. I get better mileage and the it revs about 2500 or so. you could consder a swap to 3.42s or even 3.73s.

 

Replacing the edelbrock with a Q-jet isn't possible since there isn't a q-jet manifold for the buick 300.

Agree on the gears - problem is I can't find a 3:23 posi gears + carrier for my 8.2 rear (they don't make them new) and 3.08 was the largest that fit in my carrier.

The good news is that 4th only comes in around 40 mph but regardless the 3.08 keeps my engine right about 1500 all the time. Sounds like that might be part of the issue.

 

I didn't read every word that has been posted, but are you sure that your odometer is correct?

Kurt