better mpg cruising in 4th ? -5spd

decipha

Registered User
yesterday i had to bring a friend to the northshore for a job interview, to get there i took the 'causeway' which as some of you may know is the longest continuous bridge over water in the world, anyway, in 5th gear with my laptop hooked up in closed loop, my injector pulsewidths averaged at 5.5ms some where in the neighborhood of 2k rpm for the first 12 miles, soon as i hit the halfway mark on the bridge, i dropped her down to 4th still in closed loop and my average injector pulsewidth dropped to 4.4ms around 2500rpm, this was all at a 70mph cruise with cruise control on

so what im getting at is cruising in 4th yielded better fuel economy, this is all in my bone stock blue 92 5spd, unfortunately i can't go by actual distance since my little plastic odometer gear broke

anyone else observe anything like this ?
 
My experience was run in 4th until 65mph then shift to 5th.
When I was trandfered from Hawaii to New England, we drove from Cali. @75 on the flats, it avgd. 34-36 mpg (calculated at each fill-up). Bone stock '93 with a full trunk and back seat with the wife and the child. I was driving a '95 Dodge Daytona L4 with a 5 speed. With just me and two large mutts, I was losing mpg above 68 mpg.
MikeH
 
You are incorrect sir. Because the gear ratio difference between 4th and 5th is 1.00:0.75, then it follows that if rpms in 4th are 2500, then without changing speed, rpms in 5th will be 1875rpm. If pulsewidth is 5.5ms in 5th then to consume the same amount of fuel in 4th you'd have to see 5.5 x .75 = 4.125ms to be getting equal mileage. 5th will always generate better mileage on an SC.
 
The pulse width may be smaller in fourth gear, but it's happening more often as you're turning higher RPM's. :rolleyes:
 
im my sc i shift from first to third to fifth and i average 29 mpg ... best i got was 32 once one a long trip
 
im my sc i shift from first to third to fifth and i average 29 mpg ... best i got was 32 once one a long trip

God I hope I get that kind of mileage with mine. Coming from an F150 that got 17MPG and then a Mustang that got 20MPG.
 
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