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Originally Posted by 67ChevyRedneck
That's the second or third time someone has mentioned figuring mileage with a vacuum gage. I know what its for and I believe, can't remember exactly, but I think a 350 should pull between 12-16" at idle speeds. SO, what are good and bad readings going down the road??? If this really helps I might add a vacuum gage to my cluster....
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Take it at the Intake,,Manifold Port that goes to the Power Brakes. When your engine is operating at it's peak mileage it will also be at the peak Vacuum Reading.
I used to have a 64 El-camino with a 283-/308 gears/ Powerglide tranny.. I took a trip from Washington to California, I forgot the mileage that I was getting, But it wasn't too bad.. ( or so I thought) Going through southern Oregon Northern California, I got in behind a 18 wheeler that was running Fast. I pulled in behind and stayed there getting close to 100 MPH. for a long way ( No Andy, I didn't have a camera :p ) My mileage Jumped up a BUNCH ??? I had put in a Console from a 63/64 Pontiac Grand Prix.. It had the Factory Vacuum Gauge.
I took notice that the vacuum gauge was reading more vacuum ?? Then When I got back to Earthly speeds in Sacramento, Again watching the Vacuum gauge I noticed that the vacuum increased when I put the Tranny into low gear while driving in Town traffic at respective speed.
Moral of this story ? You need to find the operating Efficient RPM of your engine at what speed you intend to run it the most. One way is to install a Vacuum gauge, and going down the road a slower then normal Speed, change gears in the tranny and see where you get the highest reading and at what RPM. This is where you want it running, for where you do your major driving. Or take it out on the highway and stand on it in high gear watching the Gauge, again, when the vacuum reaches it's highest reading, (slower speed or faster) that's where you want your final rear end gearing set to at the speed your going to 'normally drive.'
There's a couple of ways to do this to set it permanently. But the most feasible is Rear end Gearing, and tire size.
PS: I traded the Camino off before I had a chance to change the gearing...