VCCA Home Forums TECHNICAL - 1945 AND NEWER 1958-1960 gas mileage

Hi everyone, I have a newly rebuilt 1958 chevy 235 with a T5 trans and 307 rear-end, and I'm only getting between 10 and 13 miles to the gallon (10 in town and 13 on the road); Seems to me with this combo I should be getting considerably more to the gallon. any ideas? valves are adjusted at 1/2 turn and timing and points and plugs are correct to the best of my knowledge. Thanks for your advise...olgzr

Did not have a '58 235 but did have both a '56 with overdrive and a '57 three speed. If I remember the '56 had a 4.11 rear ratio and the '57 was a 3.55. Both got over 20 mpg at highway speeds and 15+ in town. Based on that I suspect that you are not turning the engine fast enough to produce an efficient amount of power and torque. Secondarily I would check the carb, manifold to determine if you are producing an optimum gas/air mixture. Changing the fuel delivery system without expert engineering is problematical. The resistance and part fit of a new rebuilt is also a contributing factor.

Possible that you have no vacuum to the carburetor power piston and its in the rich position at all times. The vacuum is supplied by a notch in the insulator block under the carburetor. This notch must align with the vacuum holes in the bottom of the base. May be blokced by a gasket also, and no gasket is required at this point. Also the location of the notch was changed in the later years. I would check this first..It will cause the mixture to be very rich.

I drove my '55 235 3 speed 3.70 ratio over 2700 miles to the anniversary meet in Grand Junction. Overall mileage, 19.7. Temps over 100 and altitudes up to 11000 feet. Speed 60 - 65 mph...The 235 is a 67000 mile 51 year old, untouched original engine - used 3 qts of 10 - 30 oil.

Joe, I thought your 1955 was a V8.

I averaged 17.5 for 3000 miles on my 1950 untouched Power Glide (50,000 miles) and 2 qts. of oil.

Ask Imfalco about his problem in Colorado. He exchanged carburetors and had the vacuum passage blocked of as mentioned above. Was so rich that it fouled the plugs and hardly ran at the high altitude.

Check that the vacume advance is working or installed properly. If it is not functioning you will also notice a lack of power.

What do you mean by a T5 transmission? You mentioned that the rear end ratio was "3:07" (there was no 3:07 ratio that is why I ask) did you mean 3:08 or 3:70 (3:73)

Have a Happy Thanksgiving,
John

Hi Chevgene, I checked the vacuum holes in the carburator and they are clear, I will check the vacuum advance next to see if it is working properly. My spsrk plugs are nice and white so I don't think it is running rich. Any more ideas on what to check for? thanks

But does the hole under the base of the carburetor align with the notch in the insulator block? Also I have seen the power pistons stuck in the full power or rich position due to "ols age" or some one failed to install the return spring.

With all the ratio and transmission changes is the speedometer/odometer correct?

Hi Chevgene, Finally got back to checking for the mileage problem that we discussed and yes the two holes in the carb block are open. However I did find that my vacuum advance is not changing the dwell when I increase the rpm's, I am getting vacuum from the port on the carburator but the vac. advance is not working, Is it likely that this is causing the low mileage? Thanks for your advise. P.S. My speedometer is pretty accurate, I installed a ratio box.

the vacuum advance doesn't affect the dwell, it advances or retards the timing

Whats the proper way to check for a defective vacuum advance anyway?, I normally use a dwell meter and then raise the rpm's to see if the dwell increases. Is there a better test? Thanks

On November 24th "Dads 31" made the following posting:


:) :grin:

You can check the vacuum advance by observing the movement of the shaft on the vacuum advance when the RPMS are increased quickly. You can also attach a timing light and observe the reaction when the engine is given a quick acceleration.

If the vacuum advance and mechanical advance are not working, YES, it will lower your gas mileage.

The timing mark moves when the rpm's are increased, but the shaft(lever) on the vacuum advance does not move, I beleive that the timing is advancing due to the mechanical part of the distributor, I tried a hose on the vac. ad. and it still doesn't move, so from all appearances the vac. adv. is not working. Correct or not? Thanks

For a final test. Disconnect the vacuum line from the vacuum advance. Now connect a small hose to the vacuum advance. Place the free end of the hose in your mouth and "suck". After you have removed all the air pressure place your tongue over the end while still sucking. The hose should be attached to your tongue by the vacuum.

You can also accomplish this test by runing the engine and perform the suction. This should change the timing.