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Actually it's inches of vacuum. Sounds like your 5th gear is tall enough for good cruising, but 4th and 3rd are better for acceleration.
Logically speaking 5th will be your most efficient gear.
There are two opposing but valid arguments about accelerating in 5th or the lower gears.
One rationale is get there in much less time using the lower gears then get into your high vacuum 5th gear cruise for max economy.
The second rationale is even though it takes longer to get there in the highest gear you should be using less fuel because even though the consumption is greater the number of revolutions of th engine is lower.
Best rate of acceleration by your vacuum gauge reading should be at about 2 inches. If you gearing is so tall that the acceleration is insufficient, then use the lower gear at 2 inches reading.
My VX had very tall gearing with 3rd gear red line at over 110 MPH. That is about the same as a lot of the performance Hondas in 5th gear, same as 3rd in the VX!
Think like the truckers who have many gears to choose from. If you get below 2 inches of vacuum, you are near WOT enrichment, so the lower gears will most likely get you to your desired speed with less overall fuel consumed.
Bottom line it really depends on the overall ratios of your gearing. Ideally (my opinion) most cars would benefit for gearing that allowed these speeds at max engine RPM.
1st--------30 MPH
2nd--------60 MPH
3rd--------90 MPH
4th--------120 MPH
5th--------150 MPH or more
In my 76 280 Z that was the ideal combination of power and economy. Fast 0-60 times with only one shift, as well as 30+ MPG at speeds averaging 55-60 MPH.
While different size and performance engines would probably necessitate a change in the top speeds in each gear, the relationship should remain the same.
regards
Mech
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