Have a 2.73 ratio rear end in 1948 Chevy which is a gas hog. Can I change the rear end to a higher ratio and get better gas milege?

 

2.73 is already pretty high. I'd look at tuning your carb and consider an overdrive transmission(if you haven't already).

 

Hello, I am a mileage freak myself,

Here are some tips for mileage
1. vehicle weight is king of mileage. For ex. 3,000lbs should be able to get 25-33mpg whether its a 4, 6, or 8 cylinder engine matters little. Maths involved in converting fuel to distance ask about weight, not engine size.
2. Reduce Rolling Resistance. i.e. tire pressure, alignment, tire composition, good brakes that do not drag, wheel bearings, CV joints, U-joints, etc
3. Rotating weight/efficiency - use the lightest wheels, tires, driveshaft, rotors, flexplate, a high efficiency converter with lockup, etc

Finally, the engine/'trans
-use an overdrive transmission
-use a modern fuel injected engine with reasonable compression, from the year 2001+
For example, 2002 Chevy V8 from Silverado/Tahoe/Escalade/Denali etc truck, 4.8L or 5.3L can be found for less than $500, will support 500-600 horsepower with a turbo in a daily application at 23-28mpg (3200-3800lbs), is extremely reliable to 150,000-250,000 miles.