Hot Rods 14 mpg

i just came back from a 300 mile trip in the 36 suburban . i only got 14mpg running 70 . i was thinking it should be getting 20 + it has a 60 over 350 with a 650 edelbrock L79 camshaft 700r4 trans 373 rear gears with 235 70 15 tires

 

My expectations wouldn't be that high. Would like to see a pic of it.

 

Given your rather short description 14 gph seems very good to me since you are running a carburetor. The aerodynamics of a brick, traveling at 70 mph with a automatic transmission you are lucky to get 14 gph in my opinion.

 

You're pushing a brick through the wind at 70 MPH, slow down to 60 and you'll probably pick up a couple MPG.

Swap the E-carb for a well-tuned Q-jet and you'll likely see a gain.

Having the distributor curved to match your engine and drivetrain would be a plus as well.

You are running vacuum advance on your distributor, correct?

 

pic of the suburban.

 

sounds about right. Drop the rearend gear to around 3.0, and you might get a few more mpg. I get 16-18 on the road in my 57 suburban with a stock 350 with Qjet, TH400 with a Gear Vendors overdrive, and 3.08 rear gears.

 

the vacum advance is hooked up . my 29 delivery with a 350 trans and 273 gears gets about 17

 

For reference: I drove from Oregon to Omaha in an 80 Malibu with a 355, small cam and headers, 3:08 gears, and 28 inch tall tires. I averaged 17 MPG and got 19 MPG on one tank of gas. I used 91-93 octane premium. Carb was a fresh Q-jet.
The 19 mpg stint was on 2 lane roads at 55-65 mph. This showed me wind resistance began to be a factor above 60 mph.
Your 14 MPG figure probably won't improve until you slow to 60 and [depending on camshaft] go to some higher gears and/or taller rear tire.

 

I can't imagine getting 20+ mpg with that combination. I'd consider 14mpg a gift. How did you figure?

 

What rpm's were you turning at 70.
There might be room for improvement, but I wouldn't expect much more than a couple mpg.

 

You could buy a new suburban and get 20, and have it depreciate 15k the first year and 7k per year after that...

in other words, who cares if it only gets 14 mpg?

 

Nice looking suburban,but with the aerodynamics of a barn I would think the gas mileage would be pretty good at 70 MPH.

I guess it's just me but for the most part hot rods and gas mileage are not synonymous. HRP

 

A one point difference in axle ratio would surely explain some of the difference. And then there's a thorough flogging of the carb and ignition timing to consider. How much manifold vacuum do you have at idle and cruising speed?

 

I agree - with that running gear in that vehicle , I wouldn't expect much more mpg .

 

Related mileage wise but not hot rod related. I pull a fifth wheel with a duramax, to go 150 miles at 70 I'll use around 1/3 of a tank of diesel, slow down to 55, mileage increases dramatically, to 1/4 tank for the same distance. The faster you go, the harder the engine has to work to cut the wind, simple stuff really. However.....70-80 all day getting poor mileage in my 36 pickup is perfectly acceptable. 14 mpg is really pretty good.

 

Put a dixe cup on a pole out in front of you to break the wind.

 

I bet if you dropped to 60 mph you would get 2 or so more mpg. I tend to just go fast and not worry about it.

 

Got a Nailhead with a TH350 and 3.08's, got 18.4 mpg this summer going through the cities, MN. my avatar.

 

That's a cool Rig! I'd still drive it anywhere even if it only got 10 mpg! But like others have said, a little higher gear would help and probably not be too noticeable in first with that r4.

 

I've got a ZZ4/700R4 in my convert with 3.55 gears, 27" rear tires. Running a 600 Holley,timing pulled back to 10 initial, stock vac advance HEI. I ran back to western MI from Columbus OH one Sunday morning at 60-70 pretty much flat country. Averaged right around 25 mpg. Last trip to Bonneville pulling a Mullins trailer was 18.5 overall for about 7500 miles. About half at 70-ish, the rest on two-lane 55-60. The mountains out west pulled some mileage out for sure. The old 350/250 combo with 3.08 gears got 16.5-17.5 at best on flat land.

It's a brick and probably does as good as it ever will. There is room for improvement in that Suburban but 20 might be hard to find.

 

my stock injected 'non hamb friendly' 89 and 91 burbs get 13 to 16 on the open road. but i don't push them hard either. my daughters '01 with the 5.3 LS would get 18 with her 75mph driving. her 2014 4x4 denali gets 14.

 

I'm no guru or anything but I think a big problem is the camshaft selection, as stated by someone else a good q-jet may pick up a mile or two ... Years ago alot of guys had really good luck with , flat tops , 882 heads edelbrock performer intake, and the small performer cam... That combo seemed to provide very good grunt from 1,500-4000 rpm guys in 70s 1/2 ton trucks claimed as good as 16-18 mpg one guy said 20s but I think he embellished a little

Best I ever got with a sbc were both Tbi trucks , my 91 Chevy with 350 5spd3.08 gear got 22 pretty consistent highway and my grandfathers new at the time 93 350/700r4, 3.73 4x4 got about the same he got an identical truck in 94 or 95 after Chevy stopped using the 700r4 and never got better than 16-17
( fuel was different then too I suppose)

I think in the 20s may be possible but guys alot smarter than me would spend an aweful lot of time on tuning the combination and probably driving by a vacuum gauge

I think where your at with what your running is pretty good

 

My little 51 Vicky
has a lumpy little 350, Edelbrock carb, 700-R-4, 3:30 and 28.5 tall rear tires. Freeway speeds are always 70+ for us. Average for me is 20-22 mpg. I learned just because it shifts into Overdrive that's not all there is. You need to be sure the Converter is Locked up also. I have a manual switch for that. I don't use it around town. Sounds strange eh? Prior to adding the switch I was getting 16-17 mpg. Go talk to a Tranny guy that knows his stuff.
The Wizzard

 

That is a REALLY good looking truck! I think you can pick up a couple MPG but not 6. That visor is like a giant air brake on front. My experience with my old 54 Chevy sedan with a 350 was 18-20. Our 76 Blazer with a 350 ran its whole life, loaded or not and got 14mpg.

 

There are lots of variables when it comes to gas mileage on a rig and even pretty close too identical rigs.
According to this calculator (note it uses loaded tire radius rather than diameter) your truck should run at 2235 or close to it rpm at 70. https://wahiduddin.net/calc/calc_speed_rpm.htm
Since it sounds like a maiden voyage or close to it for the burb it may be time to do some fine tuning on the whole rig including:
Making sure the carb is jetted and adjusted for best all around performance including gas milage.
Ignition is set right including vacuum and mechanical advance.
Tire pressure is set at the proper pressure.
Front end is aligned properly as a bit of drag from having the front end out of alignment a tad not only wears the tires but eats up gas mileage.

Then you get into the how correct is the speedometer and odometer thing. Were you actually running at 70 or where you running 75?
The guys are right on speed differences causing big changes in gas mileage too. On a road trip to Texas we figured out real quick that we lost 5 mpg running at 80 on the 80 mph speed limit roads over running 75 mph. Simply meaning that you have to find that particular road speed where you can get the best mileage and still get down the road at a reasonable rate.

 

You bring up some very valid points Mr48chev. These are things I deal with all the time. I'll also add my Motor builder also did my tune up after brake in and on his Dyno. We tuned it with my headers and head pipes with 3 different types of Mufflers to find what worked best for my motor. We all know that not all dist. are created equal so getting one correct for your vehicle needs isn't just taking a New one out of the box and lineing up the marks. I also have a local ACE alignment guy. Most drivers might get an alignment with each set of New tires. You might want to re think that. Motors aren't the only things that need tune up's . A healthy car is a true pleasure to drive and a Hot Rod can be that too.
The Wizzard

 

I get 20+mpg in my 35 phaeton, 5lt, 700R, 3.54:1 235-75-15 tyres. My car weighs a lot less than the sub! Awesome car by the way. As HRP stated, these type of cars and economy don't always go hand in hand. A reduction in rear end ratio may improve things slightly as well as cruising at 60mph.

 

I think you should let me fly down and pick up the Suburban and drive it around the Carolina's for a year or two. I'll tune on it and bring it back when I get it up to say, 25 MPG.

Great looking truck.

 

Here we go again. Change that cam out for a 250*-260* XE Comp Cam and change the gear to a 3.00 - 3.25 and fine tune the Eddy. You'll be good for about 18-19 mpg. Won't have any rumpety-rump though.

If ya make enough torque, ya can make a brick fly! LOL

 

My '37 pickup gets terrible milage at 70 to 75 mph, slow down to 60 and it starts improving, a lot. With a 250 inline six, dual W-1 Carters, T350 automatic, 3-08 gears, 235/75R15 radial, at 60-65 mph, it will get 19-20 mpg, at 70 figure 15 tops. My little motor just doesn't have to power to overcome the wind resistance. It will run 70 all day, just takes way more throttle opening to do it.
These trucks have terrible wind drag!
Joe