What mileage should I be getting?

[Tom73]

Tom, how do I find the final drive ratio? Are there any markings on the diff to tell me what's inside?

It is on the bottom next to the spring mount. A two letter code. AW for 3.08; LR and AX for 3.36; AA for 3.55; and AB for 3.70.

These are all listed on The '73 Spot

tom...

 

[JJS]

Don't want to know

I KNOW there's no way I'm getting 10 miles to a gallon in my '71 LT1. I've never actually figured that exact mileage that I get, but I guess it's because I really don't want to know.

Just recently I read an article about someone going to Carlysle, and I was wondering how many tanks fulls it'd take me to get there from MA.

 

[slyone1000]

1971 LT1.........complete roller motor, 750 double pumper, Holley 110 GPM Fuel Pump, Reprogramed Turbo 400 Trans, Hooker Header Pipes, 4:11 gears...........I am lucky if I get 6 MPG.........I have something wrong with my right foot. It gets heavy and feels over weight, every time I get in it.......LOL

 

[Chas]

yo Tom....

i noted today that at 70mph im turning 2700 rpms....and if i take my foot off the throttle i have 18 lbs of vacuum...
does this say anything about my gearing in the rear end?
I was told if had that much vacuum it probably had a cam...that 350's in 73 would only have 10 to 11 lbs of vacuum....its about 15 lbs at idle....
thanks for the feedback...
Chas

Michael,
It is the overall gearing that makes the difference. On my 90 the engine is turning about 1800 rpm at 70 mph. On the 73 the engine is doing about 3500 rpm at 70 mph. It is all a matter of gearing.

tom...

 

[Alan76]

I average on the Hwy 16 MPG in my 76 and around 19 in my 73
around town about 12 and 14 MPG

 

[bossvette]

I have a 383 with TPI and a TKO OD trans I get 11 around town and 15+ on the turnpike I would like a little more gas mileage but with 30lb injectors and 58 mm throttle body I don't think it going to happen

 

[iron cross]

Milage

10 miles to the gallon is very poor. The combination has to be wrong. Some posts have indicated that the axle ratio will affect milage. True, but very little. My 61 315 hp FI car with with a 4.11 got 20 mpg at above highway speeds and the 62 FI 360 hp piece with a 4.56 gets 16 mpg. My 68 L88 gets between 8-10 with a 4.11. The 68, I expected and always from new had poor milage, but the other two small block cars, injected performed as expected. Without other information gentlelmen, normally only the ultra high HP cars are 10 mpg pieces. Even bone stock L72 pieces should get above 15.

 

[BeaterShark]

I tracked the mileage on my car for about 3 months. It pretty much varied from 14 - 16 MPG. The engine is a 388 with a Q-Jet carb, AFR 190cc heads, and a 236/242 solid roller cam and a Muncie 4 speed with 3.36 rear end.

I was driving the car daily during the time and my city/hwy MPG was about the same. Also, I did not baby the car, but did do a bit of driving during the 8/5 o'clock traffic where I was limited to how much I could get on it.

Prior to this engine, I would get about 10 - 12 with the mostly stock 300 hp 350, so I don't believe my odo is off enough to skew my numbers.

 

[captras]

I have a 75 Convertible, L48, with TH400 Auto, 31,000 miles. Just bought the car. 1st tank I drove 287 miles on 18.9 Gals.....15.18 MPG!

 

[mike weyman]

i find it hard to believe the fuel economy is so low on these cars.
i had a 82 chev van (former ambulance)with a 1972 350 sbc.400 turbo tranny,limited slip posi.if i drove it nice(egg between foot and pedal)i would get about 18 miles per gallon.hard to believe an aerodynamic car,same motor,same tranny,gives less mileage.
the very small primary ports on the rochester allow you to squeak along on little fuel,the bottom doesn't fall out till you nail it.
all i can figure is you guys have your foot in it a lot.and i want you all to know that as soon as mine's running i will be putting my foot into it too.lol.mike

 

[captras]

Mileage

i find it hard to believe the fuel economy is so low on these cars.
i had a 82 chev van (former ambulance)with a 1972 350 sbc.400 turbo tranny,limited slip posi.if i drove it nice(egg between foot and pedal)i would get about 18 miles per gallon.hard to believe an aerodynamic car,same motor,same tranny,gives less mileage.
the very small primary ports on the rochester allow you to squeak along on little fuel,the bottom doesn't fall out till you nail it.
all i can figure is you guys have your foot in it a lot.and i want you all to know that as soon as mine's running i will be putting my foot into it too.lol.mike

Actually, this is very conservative driving. About 1/4 of it was in town driving. Has alot to do with rear end, and no overdrive. At 70 MPH I am running about 2800 RPM. On my 2000 Chevy, at 70MPH, I am running about 1500 RPM. I looked at the mileage estimates for the 75 with the L48 and 15 MPG is right in there. Just a price we have to pay for the fun. Of course, that is why I will only be driving weekends!!!

 

[The73vetteman]

i find it hard to believe the fuel economy is so low on these cars.
...........hard to believe an aerodynamic car,same motor,same tranny,gives less mileage.
............
all i can figure is you guys have your foot in it a lot.and i want you all to know that as soon as mine's running i will be putting my foot into it too.lol.mike

The C3 might look aerodynamic but I'm pretty sure that it's not. I think I read that during development GM found the hood acted as a giant aerofoil and at speed would tend to lift the front wheels off the ground! I believe that's why our cars have that huge spoiler underneath the nose.

Has anyone else heard or read this?

And yes, I do have my foot on it a lot. There is no sweeter sound than my '73 running on WOT!

 

[gooney0]

The73vetteman is correct. The C3 is not very aerodynamic. The rear spoiler also takes weight of the front wheels. That is why they got rid of it in 74.

But the C3 is better than the C2 It's even worse.

As for the ZZ4... my MPG is probably about the same as with the stock 200hp motor ('72). Not great, but then the old engine was rated at MPQ. {miles per quart of oil}


-Gööney0