Classic/Antique Car Repair: rear axle ratio, checking axle ratio

Question
What was the stock rear axle ratio for a 6 stick c-10 chevy truck I now have a 350 - 350 set up but I get about 5 miles a gallon need a better ratio.  Thanks please recommend a good ratio

Answer
You can check your rear axle ratio by jacking up one side of the truck, putting the gearshift in neutral (BLOCK THE WHEELS AND USE A SAFETY STAND!).

Then, slide under the truck and make a chalk mark on the driveshaft where you can see it from the side of the truck.  Then release the parking brake and turn the wheel that is off the ground 2 complete revolutions exactly, while you watch the chalk mark on the driveshaft.  I'm guessing the chalk mark will go past the initial point slightly more than 4 times.  If it does, it has 4.10 gears.  Whatever you count- that's the axle ratio that's in it now.

It probably came with 4.10 gears originally - with the automatic, you should be running 3.73 gears.  Any truck salvage place will have the right rear axle for you.  You will improve the mileage about 10% by changing to 3.73s.  

6 MPG is 454 territory - something must be bad wrong with your engine or the tuning of the carburetor and/or ignition - or else you have a really lead foot!  Take it to a professional tune-up shop and have them check it out!

Dick