The Stovebolt Site Forums The Shop Area Engine Shop 1950 3100 - 261 Gas mileage?

Have a 1950 Model 3100 with a 1960 261 in it. Single barrel R carb and with a 4-speed tranny. Engine was redone this summer and have driven only a handful of times so far. Issue is fuel gauge isn't working. I have suspect tank sensor but haven't gotten around to replacing yet. I HAVE though run out of gas once (when I notice gauge was bad), and today a 2nd time.. so time to get all this fixed :-).

Problem is, I was assuming at least 10mpg, but after having put 10 gal in it the first time I was tracking miles each drive to make sure I wouldn't run out again... i.e. should have been good for 100-125 miles or so. I got 72 miles when it ran out the 2nd time.... equating to 7.2 mpg. This seems to be about 1/2 what I would have expected and what I'm reading in various posts and threads. Mechanical pump, no fuel leaking, and no excessive idling or anything, engine runs smooth, good vacuum (tuned for max vac), and RPM low-mid (guess 500-600, haven't tested it yet). No excessive smoking going down the road or sitting idle.

Assuming 7.2 mpg sounds way off... any ideas what could be consuming so much or a better way to gauge consumption ?

Thanks all.

I get between 16 and 19 MPG out of my 41 (sedan).
That's with a hopped up 261, Fish carb and overdrive though - so your mileage (literally) may vary.

And no, 7.2 doesn't sound right at all. Something must be wrong.

Timing not set correctly - too far retarded.
Wrong sized tires - too small diameter
Under-inflated tires
Poorly functioning carburetor
Worn out engine
Brakes dragging
Rear end gear ratio?

Maybe the odometer reading is way off.

Definitely fix the fuel gauge problem.
The correct "single R" (assuming one-barrel Rochester B) carburetor for a 261 is hard to find. If your carburetor is for a 235 engine, it may run rich.
Tuning for maximum vacuum is a shortcut method to getting the engine to run. I would tune-up with dwell meter and timing light.

Tuning to the original specs is IMHO just a starting point to get her running - it doesn't mean you are set for the best mpg/performance.

For example: today's fuel is totally different from what it was back than (it's way less volatile today), so it need's more advance to get max fuel economy, but it also can tolerate more advance before it starts pinging.
Personally I prefer manifold over ported vacuum, when it comes to mpg - idling, starting and temperature also benefit big time from manifold vacuum.

Also, setting your dizzy with a timing light under no load in your garage, does only work for idling. For every other rpm, it will not be optimal.
Since your mechanical advance is fixed to your rpm, there is actually only one rpm where you can set it to optimal timing (and that only under a certain given load).
Let's say you ether have an old worn out engine, or a hopped up one with a ton of speed parts - there is only so much you can do with changing weights, springs, and vacuum canister.
The only way you can tailor a curve that really fit's you engine is IMHO a full programmable dizzy.

The 261 I just installed in my 1-ton gets 10+ MPG running over the mountain pass to the next town and back. It has 2 Zenith 228 carbs and a 250 degree Crower cam. Stock advance curve. With the camper on making 8500lb. gross it gets 10- over the same route. Engine has only about 250 mi on it so it may be a little tight yet. The only time I've gotten way sub 10 MPG was pulling an 8,000 lb. trailer through mountainous terrain. Then it was ~9.3

There's definitely something wrong with your setup. Perhaps it's too rich as has been suggested or more likely the ignition timing is off. Setting the timing for maxium vacuum will get you way over advanced with all the problems that brings on. Static time it to the ball in the flywheel as a starting point.That will get you an initial spark lead of 5 degrees. Also check the plugs for gas fouling from a too rich mixture.

Stovebold 6, I think you have that backwards. Today's fuel is MORE volatile than it used to be. That's why it evaporates out of lawn mowers and carburetors if it sits for a few days or a week. Some of today's ethanol fuels can vaporize at temps as low as 90 degrees.

Stovebold6, don't eat the whole cow in one bite.

Joe is working on a problem and we are hoping to get some feedback, so we can move on to the next step.
I agree that today's fuel is different and that we can compensate for it after we are satisfied that the engine is running decent.
It is too early to begin discussions about programmable distributors and such. It adds confusion at this point.
Joe could have a major issue i.e. diluting crankcase oil with gasoline.
I for one need time to think. I'm senior now and it takes me longer to get the hood open and find where I left my tools.

My 1954 3100 with bored 20 over and a t5 transmission and a nova rear end with 2.73 gears, I swore for years that I was getting 20 miles to the gallon on trips. NOT SO. when I figure my electronic speedometer was miss programmed. NOW around town 13 14 on a trip 15. So I don't kee[ a record and jus put gas in as needed.

Hi all.

Sorry for the delay in getting back on this point.... too many vehicles to keep up with (granddaughter, Son, Daughter, my few, car shows, etc.,etc.), not to mention non-auto related things that also have to get done. I appreciate all the feedback though. Today I'm finally back to working on my more urgent issue, getting my gas gauge to work so I don't run out of gas AGAIN. New tank sensor fixed that aspect but that wasn't all of it... so working up the chain. Once fixed I'm gonna go back and retest speed on truck's speedometer with a GPS based app on my phone, I recall a difference when checked previously, but don't remember the specifics.... but it definitely was not off by 2x though or anything. The other obvious thing will be to do the active timing gun timing, and look again at the carb settings. Will post an update (hopefully this week), assuming I get to work on what I want to work on :-(

A cowboy way to set your timing. find a long steep hill put the truck in 3rd or 4th and lug it down going up the hill. if it pings, back the timing off a little until it stops pinging, and run it. if going up the hill and i doesn't ping. advance it until it does, then back it off until it stops run it.
Jay D.

Jay D Does anyone have a recording of what pinging sounds like? Is it pretty loud and easy to notice?

It sounds like a little guy with a tiny hammer banging on the inside of the engine trying to get out.

Lol thanks KL

Too rich
Spark not fully advanced by cruising RPM
No vacuum advance
Choke does not fully open
Power system opens too soon

Getting back on subject....


I wonder if the flywheel is the original one? Some of the old flywheels had the ball set at 2 degrees BTDC. If you have one of those flywheels an you set the timing to the ball, it will be wrong.

Also, check operation of the choke with the air cleaner installed. At one point, the air cleaner was interfering with the operation my choke

Find it hard to understand the mileage issue with out knowing more of the basic information such as..... tire height or tire size, rear end gear ratio, MT or loaded, flat or hilly and at what speed.......


My stock rebuilt 261 will give me 12 mpg driving at 32 mph with 2000 rpm in high gear with 6.5 rear end ratio....tires are 10:50 x 16 at 36 inches high....... running a Carter YF and a paper filter instead of oil bath ......... happy with anything over 10 mpg. Oddly enough, the mileage barely changes whether I am doing 2 wheel drive or 4x4 on dirt roads at same speed.... suckes gas real good in low range 2:1 in 2nd and third........ oh top speed is 44 mph in hi gear on flat pavement.... 2900 rpm..... front end is square like a brick.....

I've regularly been getting 13-14 mpg in a stock 1950 3100 with an un-rebuilt 216. Tires are about 30" diameter. Driving is a mix of 30-40 mph roads and 50-55mph freeway driving.