Hi all, It's pretty well accepted that the greater vacuum reading you can get while cruising around the better your economy will be. But I'm wondering if there are other factors to consider. For example in my car if I'm doing 40MPH in 3rd, I'll pull 20 inches of vacuum. But if I change into 4th (700R4), RPM will drop about 200RPM and vacuum will drop down to 16 or 17 inches. If I were to force the car to drive around in 2nd gear I can pull roughly 22 or 23 inches of vacuum at the same speed. Obviously, revving the car to get the vacuum up isn't going to be the best way to get economy. So, is there a rule of thumb that can be applied when deciding what gear to use vs vacuum? I guess doing some consistent driving and watching how much fuel is used would be a good start, but my fuel sender in the tank pack up a while ago so I wouldn't have a clue.

I had a vacuum guage in my truck and attempted to drive with the vacuum as high as possible in the highest gear possible to see the difference in fuel consumption. Around town keeping the vacuum up with a feather touch on the gas and pissing off other traffic my fuel economy went down from 14 to 12-13. Thats driving as light as possible. On the highway in 3rd (3 speed) with a 3.42 ratio rear I went from 17 to 24 MPG on a 120 mile round trip but I was a pain in the arse to drive like that. It has been proven that keeping your 700R4 in 3rd around town below 45 MPG does help reduce fuel consumption because it's not bogged down as much and keeping it from shifting up and down saves fuel.

High vacuum means lower relative load. Mixture ratio is leaned under those conditions. But that doesn't tell you how much fuel is being used. A fuel meter would be the right way to know whether a lower gear with higher vacuum or higher gear with lower vacuum is using more or less gas.

Well I guess diff ratio has something to do with it as well. I have my 700-R4 converter locking at about 48MPH to extend the life of the converter instead of having it lock too early. Driving at highway speeds in 4th with converter locked sees my setup pulling 16-17 inches of vacuum - does that seem low? I wonder if fiddling with advance and spark parameters of the TPI setup would alter things.....?

Can you point me in the direction of how driving in 3rd below 45MPH reduces fuel consumption? I'm curious about it.
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Tell me more about the fuel meter please. Where could I score one of these from, and how do they work?

Fuel flow meters are connected in the fuel line(s), and they measure the rate of fuel consumption directly.

I thought about getting one until I just saw the price of them, ouch!