Vacum, gearing, and MPG
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Member
Joined: Feb 2003
Posts: 67
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From: Oakland CA
Vacum, gearing, and MPG
Been thinking about this quite a bit lately-
Could it be possible that a 3.55 rear end could net better mileage than a 3.08 considering the engine works less per stroke to turn the wheels with the lower gear?
I've been thinking of installing a vacum guage lately to see how hard my motor's working. Depending on the circumstances, a motor can be more fuel efficeint turning more RPM than fewer as long as it's not loaded as much, right?
For instance, I have a motorcycle from '96. That bike model's rear end, through '99 was geared lower than the 2000 to current year bike. The newer model bike's final gearing in top gear is higher than mine. It's a common upgrade to take a rear end from the earlier bike and put it on the newer model to get it to go as fast as the earlier model.
Higher gearing, but slower terminal speed. Lower the gearing to make it faster. Better efficiency with lower gearing.
Given the same driver and the same rig, the same fuel on the same road, could it be possible to get better mileage (lets say up to 75mph or so) with a slightly lower gear than the 3.08?
Comparing who's best MPG with anothers' best MPG just won't tell the tale here. Too many variables. Fuel quality, ride style, road conditions, ambient temperatures, the vehicles' state of tune...
This should get interesting here.
Could it be possible that a 3.55 rear end could net better mileage than a 3.08 considering the engine works less per stroke to turn the wheels with the lower gear?
I've been thinking of installing a vacum guage lately to see how hard my motor's working. Depending on the circumstances, a motor can be more fuel efficeint turning more RPM than fewer as long as it's not loaded as much, right?
For instance, I have a motorcycle from '96. That bike model's rear end, through '99 was geared lower than the 2000 to current year bike. The newer model bike's final gearing in top gear is higher than mine. It's a common upgrade to take a rear end from the earlier bike and put it on the newer model to get it to go as fast as the earlier model.
Higher gearing, but slower terminal speed. Lower the gearing to make it faster. Better efficiency with lower gearing.
Given the same driver and the same rig, the same fuel on the same road, could it be possible to get better mileage (lets say up to 75mph or so) with a slightly lower gear than the 3.08?
Comparing who's best MPG with anothers' best MPG just won't tell the tale here. Too many variables. Fuel quality, ride style, road conditions, ambient temperatures, the vehicles' state of tune...
This should get interesting here.
Technical Article Contributor
Joined: Dec 1997
Posts: 9,417
Likes: 11
From: Windsor,Ontario,Canada
Changing the gears in the rear change the torque to the rear wheels. Going to a 3.55:1 from a 3.08:1 will increase engine rpm at the same mph but will increase the torque to the rear wheels. If you do nothing to increase the efficiency of the engine you may well end up just consuming more fuel at the same speed because of increased engine rpm. If on the other hand you are in a situation that you are constantly flooring it to maintain a given speed than you may find that changing gears will increase fuel efficiency.
Example; with 3.08 gears and 4R70W transmission 255 70R16 tires you will be turning 1447 rpm @ 60 mph in O/D. With 3.55 gears you will be at 1660 rpm. Just with that example you are turning 213 more rpm with the 3.55. Because the engine torque is almost the same at these rpms I would be willing to bet that at these speeds the 3.08 will be more efficient.
Mow at 100 mph the 3.08 will turn 2411 rpm and the 3.55 will be at 2779. The 3.55 is putting out more torque from the engine than the 3.08 and also because of the torque multiplication to the rear wheels I would be willing to bet that the 3.55 will be more officiant because of the increased drag at that speed.
Regards
Jean Marc Chartier
Example; with 3.08 gears and 4R70W transmission 255 70R16 tires you will be turning 1447 rpm @ 60 mph in O/D. With 3.55 gears you will be at 1660 rpm. Just with that example you are turning 213 more rpm with the 3.55. Because the engine torque is almost the same at these rpms I would be willing to bet that at these speeds the 3.08 will be more efficient.
Mow at 100 mph the 3.08 will turn 2411 rpm and the 3.55 will be at 2779. The 3.55 is putting out more torque from the engine than the 3.08 and also because of the torque multiplication to the rear wheels I would be willing to bet that the 3.55 will be more officiant because of the increased drag at that speed.
Regards
Jean Marc Chartier
Member
Joined: Dec 2002
Posts: 82
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From: Clarkston, MI
An engine's fuel efficiency closely follows it's torque curve. The closer you operate the engine to it's peak torque RPM the better your fuel economy will be.
You don't say what engine you have, but take a look at it's torque curve, it will tell you a lot. If you tow, also look at a non-OD calc.
Tom
You don't say what engine you have, but take a look at it's torque curve, it will tell you a lot. If you tow, also look at a non-OD calc.
Tom