I have a 1970 442 has 3.23 gears 2200 stall 455 with trw pistons E heads performer intake 750 quick fuel carb and a comp xe274 hydraulic flat tappet cam what kind of fuel mileage should it get I drove it to a car show this weekend and got 8.15 miles to the gallon mostly highway miles 2800rpm and 68 mph

 

If you fine tune it a bit and drive with an egg under the gas pedal, you may get it to 10/12 on the highway.

 

I was just trying to see if that's right or if my carbmneeds some work runs perfect but i just thought it would be closer to 10-12

 

My 72 H/O with 3.23 and 455 and a somewhat warm cam will get 10 mpg at 80, 12.2 mpg at 70 and 16ish at 60. RPM really matters.

 

I might need to take cover off rear end mine at 70 is turning 3000rpm sounds like its screaming id be afraid to ride 80 mph

 

I WOU:LD NOT take the cover off at 70. That would be messy and expensive...Plus hanging off the suspension at that speed will be tricky.
Kiddin aside...Quick n dirty method to see what gears are in there is to count tire to drive shaft rotations. It will get you in the ball park on the rear ratio,

 

lol yeah that would be a little tricky at 70 mph I will jack it up and spin Tires and see what it is but I'm pretty sure when they installed the posi they told me it was a 3.23

 

A 3.23, with normal tires, will get approx 2800 at 70. Converter slip varies.

In my defense, I had to follow a crazy woman to a car show at 80 mph. Funny thing is, it got the car warm enough to finally suck in enough coolant when it cooled down to super fill the radiator from the overflow jug, which I like.

 

that's where I'm at 2800 is like 68 mph in my car so it's probably a 3.23

 

8mpg is pretty bad, especially with 3.23s. I'd expect 10, minimum. What do the plugs look like? For comparison, my 550ish hp 468 with 3.70s and no overdrive gets 10mpg all day long.

 

not sure I will look at them tonight I thought it was bad also

 

What size tires? Myself and others see around 250 to 300 rpm of slippage at highway speeds. Sounds like the carb needs tuned with a wideband and maybe some timing adjustments if a recent tune up was done.

 

I agree, RPM matters. My 455 with E heads, 3.23, stock replacement shallow dish pistons, quadrajet, unknown cam, gets similar mpg except at 60 I get about 14.5 mpg at best.

 

My 70 455 Stock with E Heads 4 speed with 3.08's 15" wheels has got the best at 11 MPG +- Highway 75 mph -- definitely at 55 it would have been much better

But beating on the car around town I have gotten 7 MPG +- (lol I like the sound of the Q Jet Secondaries !)

 

I'd say you're leaving a lot on the table. With 3.50 gears, my 470 Booick will get 13.5 cruising 55-60, it drops to about 11-12 going 75-80. Thats turning 3400 rpm. Thats a pretty hot engine too. 8 mpg is nuts, you must be wayyyy rich.

 

Yeah that's why I was asking what guys with similar cars where getting I thought it should get at least 10 mpg at least

 

Also, keep in mind that the blend for "winter gas" is slightly less efficient than the "summer blend."

 

I'd guess with high confidence that your tune is so far off that you're also losing lots of horsepower. Buy an AFR gauge- they are invaluable when tuning.

 

Lower stall or OE converter will help mileage.
Had similar mileage issues with my 70 442 when I changed the carb and installed a higher stall converter. Tried to blame it on the carb jetting but stall speed ended up being the main culprit .
Mine has 3:42 , I would bet the 3:23 ratio would keep the rpms in slipping range more of the time.

 

DOes the converter slip when you aren't accelerating? It's my understanding that the converter doesn't slip when cruising.

 

There are many experts here with more knowledge than I but as I understand, the converter always has some slip, pretty much always unless it has a lock up feature. That being said the higher the stall, the less efficiently engine rpms are “ converted” to the trans input especially in the lower rpms. Once your engine rpm is at or above the advertised stall speed the slip is minimal but if you think about how much of your driving is below that rpm, more slipping is what is designed. Big reason why trans coolers are strongly recommended with higher stall converters.
That is my opinion..........your mileage may vary😊

 

Before figuring fuel economy confirm odometer accuracy. Go on an interstate and ride for ten miles using mile markers in comparison to your odometer.

 

How are you guys measuring the distance travelled and fuel used?

 

This depends upon the way the converter is built. Quality converters don't slip excessively. For a while, I had a 2.56 rearend under my car, so a lot of my driving was below the 2500 RPM stall rating. I was curious, so I tracked the engine RPM at various speeds and calculated the converter slippage. I found what olds 307 and 403 posted, the converter slippage was around 250-300 RPM when cruising at 1800 RPM or at 3200 RPM.

 

Converters are load sensitive, meaning the less load on the transmission, the lower the stall speed.

A good quality converter won’t slip or feel mushy at part throttle. The 10 inch Coan converter in my car will flash to 4200, but drives very well in daily driving.

The Buick Grand National came from the factory with a converter that would flash to 2800, you can bet your bottom dollar GM would never have signed off on that if it would cause drivability or customer complaints.

 

Before getting too deep into this, it might not be a bad idea to verify the odometer is accurate. You can use a speedometer app to verify speedo accuracy, and use the mile markers on the interstate.

If you haven’t done any tuning to the carb, it’s almost certainly jetted on the rich side of the calibration chart.

Get a wide band and calibration kit and be prepared to spend some time getting it tuned correctly. Keep some detailed notes, don’t change a bunch of things at once. Make sure the ignition timing curve is correct, especially the vacuum advance.

 

I agree the accuracy of the Speedo and fuel is the first step.
Beyond that if the converter is slipping 250 to 300 rpms or more, at cruise, would that not be roughly 10 to 12 % of engine rpms wasted on slipping which roughly accounts for missing 1+ mpg at the expected 10 to 12 mpg ?
I haven’t ever felt a higher stall converter slip just driving normal, as much as I have noticed when it starts to spin tires at or very near the rated stall speed on launch.
Even my switch pitch on high stall (3600) still “ feels” normal under light throttle.
Mileage changes if I cruise in low stall (1800) as opposed to high stall. Not much but definitely the numbers discussed here.
I’m 4500 ft above sea level so I’m sure that is a factor as well on the expected mileage .
I still could be wrong but....

 

When my Vista Cruiser was new, I did something similar to what Kenneth did. Here's the result. This is with a factory converter, which is tight. Regardless, you can see that the percent of slip keeps going down, but the slip rpm keeps going up as speed is raised. This will be the general result for a non-lockup converter.

 

I have an Autometer GPS speedo and their fuel gauge. I calculate the mileage from fuel added. No, it's not precise, but it's pretty close, close enough that i can estimate range to empty.

 

Is this a one time measurement for mpg or an average of multiple trips?

 

one time I'll try it again this weekend checking into a wide band there's so many not sure witch one to get

 

I like to see an average of multiple trips. One trip could be far out for whatever reason.

 

Cool chart VC455 ! My head can’t get around 3355 rpm at 100mph ! In a Brand new Vista Cruiser ! Imagining my 6 year old self in the back seat! Hopefully not the very back. Would want to see out the windshield .

 

the GPS speedo is accurate, but what are you using the fuel gauge for?

you should be using the fuel used from the pump reading when you refill….start with tank full, then refill after the drive. The refill amount is the exact amount used.

 

Well, yeah, I'm not a complete moron, only about 1/3. I use the fuel gauge for distance to empty based off the mpg calculation.

 

When we brought our W-30 it's freshly rebuilt 455 got 20 mpg with the following. Completely stock with the exception of the cam which was a 350 2 barrel cam. Now this mileage was at 60 mph, at 65 mph the mileage dropped to 18.5 and at 70mph the mileage was 17mpg. But from a dead stop and just flooring the gas pedal you would get double stripes on the road for 5 houses and an incredible amount of tire smoke. I removed this cam in favor of the W-31 cam and seen the expected dropped in mileage. The best was then 14.5 with the AC on.

 

No offense but I love comments like this. Completely stock well except this and that. So it isn't stock at all. A W-30 with a 350 two barrel cam??? So far from what a W-30 was built as.

My ALL original 70 442 was anywhere from 8-18 depending on what fools I was up against. Usually pretty consistent 12 mpg.

I didn't buy my car for the gas mileage. 🤔

 

“I didn't buy my car for the gas mileage. 🤔”



Should have been the second post !
I like to refer to this as smiles per gallon.

 

No offense taken, it's was just 1 of the reasons why we brought it as our 1st car after getting married. We wanted to right the injustice that was done to the car and having only paid $1000.00 for the car. It was so much better installing the W-31 camshaft.

 

I am a little confused. You had a W-30 or W-31?