Cough, Splutter - How Much MPG?! 😱

[teamzr1]

Graham,
I suggest you verify what rear end diff ratio the car has and also tire wheel size as 2,700 RPMs just cruising at 60 MPH sounds
high if having like a 3.0x gear

Use my calculator and formulas on my website at

Cal & Formulas

Match up your 1973 with the GM specs to see what gear ratio was stock ( not knowing if some owner changed it)
and then what the tire/wheel size is to calculate RPMs over MPH

 

[GCorvette]

There is certainly someting amiss there Graham - even my 489 with a comparitively huge 950 cfm double pumper does 15/16 at similar speeds. If you are using the correct diameter tyres (27.4 ish tall) 70 mph should mean a 2550/2600 if equipped with a 3.08 rear end. Is it an auto box?. That should have at least another 5 mpg in it at cruise speeds. Has it got a/c? are you using it?

It is an auto.
The A/C (which is excellent) I kept off on this trip to see what the true figures were.
Just checked the tyres, 26" tall. Hmmm
48mph = 1580rpm (from photo)
65mph = I recall around 2700 (possibly less)

 

[teamzr1]

What size does the tire say on sidewall and wheel size ?

Stock wheel and tire size

 

[GCorvette]

What size does the tire say on sidewall and wheel size ?

Stock wheel and tire size

View attachment 17952

255/60R15 102V GALAXY R1 RWL 60kg 110.70 442.80 20.0%
Height: 70 Width: 70 Depth: 27

 

[teamzr1]

Looking at the 1973 with auto tranny and came with A/C came with a 3.55 dif gear



With 26-inch diameter tire that then is 13 inch radius so at 60 MPH put RPM right around the 2,700 RPM your seeing

 

[GCorvette]

Looking at the 1973 with auto tranny and came with A/C came with a 3.55 dif gear

View attachment 17953

With 26-inch diameter tire that then is 13 inch radius so at 60 MPH put RPM right around the 2,700 RPM your seeing

View attachment 17954

Thanks for that, so the tyres are probably not the issue.
I guess then potentially carb related & not being mechanically minded, is there anything else for me to consider?

 

[teamzr1]

Thanks for that, so the tyres are probably not the issue.
I guess then potentially carb related & not being mechanically minded, is there anything else for me to consider?

If MPG is lower than it should be

1. Type of gas you are using, if it has a percentage of Ethanol, will reduce MPG
2. Dirty aircleaner and/or Carb
3. Jetting of Carb maybe running too rich
4. Engine timing

Find a shop that can stick a probe in the muffler tip and see what the AFR is,
should be about 14.7:1 (except when doing WOT)
The lower that value is, the richer the AFR is

 

[GCorvette]

If MPG is lower than it should be

1. Type of gas you are using, if it has a percentage of Ethanol, will reduce MPG
2. Dirty aircleaner and/or Carb
3. Jetting of Carb maybe running too rich
4. Engine timing

Find a shop that can stick a probe in the muffler tip and see what the AFR is,
should be about 14.7:1 (except when doing WOT)
The lower that value is, the richer the AFR is

"Type of gas you are using, if it has a percentage of Ethanol, will reduce MPG"
A mixture of 97-99 Octane was used at various stations. I know the 99 Octane I use is a maximum of 5% ethanol, the rest I don't know.

I will of course have points 2 - 4 explored.
Thank you.

 

[Forrest Gump]

Thanks for that, so the tyres are probably not the issue.
I guess then potentially carb related & not being mechanically minded, is there anything else for me to consider?

255/60/R15 tyres are 27.0" diameter, not 26". The larger diameter will actually be helping you though with a lower rpm for a given mph.

 

[Nassau65]

That is pretty horrendous MPG, considering your mileage was overall figures. As mentioned before, when I was using Corvettes and other vehicles with a 350ci motor in regular use in London, ( inc “the city) I was getting 15/16. ) on nice runs motorways and A roads 18/20.

 

[Roscobbc]

I seem to recall American car magazine 'period' road tests on C3 and C2 Vettes with HO 350 horse 327's with very low equivalent to big block mpg's.
Question for you - if you remove a spark plug after a cruise run what colour are the points and central insulator? - are we seeing a plate coffee colour? - is there signs of sooting-up anywhere? Presumably you are 'brimming' the tank when recording fuel economy? is the speedo and mileometer accurate? - can you do a GPM compared run or better still do a 30 mile run in the Vette and then a current vehicle and compare the miles shown.

 

[Nassau65]

I remember in the early 80’s a couple of friends were running Granada Ghia’s with the 2.8 in them ( sapphire models blue/silver) . I used one on one occasion and thought it wasn’t a lot better than a 350 going into the city and back from north London. Probably better on a run, but I was stop/start all the time.

 

[GCorvette]

Presumably you are 'brimming' the tank when recording fuel economy?

I was indeed.

is the speedo and mileometer accurate? - can you do a GPM compared run or better still do a 30 mile run in the Vette and then a current vehicle and compare the miles shown.

I think the mileage is correct based on pre-tour mileage estimates, but perhaps something to check.
The only difference was the fuel cost twice the estimate (there was also all the recent fuel hykes adding to that though.)

 

[Nassau65]

The cost of fuel to feed these old big engined V8’s will certainly have a bearing on their usage nowadays. Weather it will affect their value in the future, only time will tell. People won’t want a car that they can’t use how they would like to use.

 

[teamzr1]

Some info for 1973 MPG

Based on data from 4 vehicles,
18 fuel-ups and 2,490 miles of driving,
the 1973 Chevrolet Corvette gets a combined Avg MPG of 11.94 with a 1.44 MPG margin of error.

Mileage Tests

Might make sense of this ?

Euro Mileage

 

[Nassau65]

That should equate to 15mpg ish on a UK gallon. I suppose the 1974 and upwards 350’s were a little bit more economical as it was after the 1973 oil crisis, and US manufacturers tried/did get a little more MPG from them.
Before the first oil crisis nobody cared about MPG in the US.

 

[GCorvette]

That should equate to 15mpg ish on a UK gallon. I suppose the 1974 and upwards 350’s were a little bit more economical as it was after the 1973 oil crisis, and US manufacturers tried/did get a little more MPG from them.
Before the first oil crisis nobody cared about MPG in the US.

So are we now saying the 1973 just isn't economical and the 12.21mpg is accurate?

 

[James Vette]

The cost of fuel to feed these old big engined V8’s will certainly have a bearing on their usage nowadays. Weather it will affect their value in the future, only time will tell. People won’t want a car that they can’t use how they would like to use.

The future sucks.

 

[GCorvette]

The cost of fuel to feed these old big engined V8’s will certainly have a bearing on their usage nowadays. Weather it will affect their value in the future, only time will tell. People won’t want a car that they can’t use how they would like to use.

Did I not hear that F1 were working towards a "100% sustainable ‘drop-in fuel’ – meaning it can be used in a standard internal combustion engine without any modification to the engine itself?"

 

[Nassau65]

So are we now saying the 1973 just isn't economical and the 12.21mpg is accurate?

Not really sure. My dads new 1970 impala bought from Lendrum & Hartman used to do 15/16 around North London according to him. The 73 Impala convertible was about the same.
the Corvette I assume would/should have been doing about the same.
I used mostly rubber bumpered versions when I started driving so were post 73.