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Old 01-24-2016, 05:57 PM   #1 (permalink)
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rpm reducing

hi- mmonce has touched on a worthy idea-though not exactly as coasting........i have a 1995 ford ranger 4x4 with the 3.0v6 auto. o/d. this engine turns at a whopping 2500 RPM when at 60 MPH...it would seem 90% certain that there would be large savings on long trips if i had a device that i could set at whatever rpm i chose following the final hwy speed shift point....say 1200rpm?

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Old 01-24-2016, 06:11 PM   #2 (permalink)
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Not quite sure I follow.
Your finally rpm at cruise is a product of 1. gearing and 2. load.
we have discussed on here about increasing the diameter of the tire (not the wheel) to lower the rpm at cruise. I did this on my Infiniti Q45 and saw some nice results. I increased the tire diameter about 6%. The trick is you need to spend a lot of time at cruise to offset the little bit of mpg loss while driving around town.
2. load - the energy needed to keep your vehicle moving down the road at the same speed. Aero plays into this. Rolling resistance also (air up the tires to above 40psi.)

Point is, you cant arbitrarily set the rpm to what ever you want with your current set up.
I just remembered you have a 4x4. I would imagine the rear gears are high and not really designed for a lot of freeway driving.
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Old 01-24-2016, 09:45 PM   #3 (permalink)
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If you are considering gearing your truck for 1200rpm at 60 mph, you are making a mistake. I think 1800 is about as low as you will want to go. Even then you will have to downshift to accelerate in a reasonable amount of time.
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Old 01-25-2016, 05:16 AM   #4 (permalink)
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Exclamation

thanks- lost my sense of gearing for a bit there.
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Old 01-27-2016, 09:01 AM   #5 (permalink)
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The aero on your Ranger is likely a nightmare. Before dropping $1000's on changing your gearing - which would almost certainly have a good result, no denying - you can get pretty good results with a few aero mods. If you don't go offroad much - and few people really do - you could do a lot with a simple air dam. If you're worried about it getting torn off by deep snow, make it one that's easy to remove and reinstall.

My truck has 3.73 gears and I am certain it would be happier on the highway with 3.55, maybe even 3.35. It just sounds buzzy. But I'm too cheap to do it.
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Old 01-28-2016, 12:24 PM   #6 (permalink)
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Elhigh, can we calculate what your cruising RPM would be with 3.55 and 3.35 gears and mileage improvements?

I cannot imagine the ROI would be reasonable, I am just curious.
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Old 01-29-2016, 02:37 PM   #7 (permalink)
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Standard tire size = 195/75 R14
790 revs. per mile

Truck's rear = 3.73:1
W56 5th = 0.85:1


Overall top gear ratio = 3.17:1
engine revs = 2507

With 3.55:1 rear = 3.02

= 5% rpm reduction
engine revs = 2385

With 3.35 rear = 2.85

= 10% reduction
engine revs = 2251

The 22R isn't really powerful but makes good torque for its power, and makes it pretty low in the rev range. It's a super flexible, meaty engine down low if you don't ask too much of it. Puttering along at 50 in top feels great, 60 feels thrashy. Having 3.35 gears would move my cruising speed down into less buzzy territory.

As to what that would do to my MPG, I can only speculate. It's carbureted. I can only record tank-to-tank, there's no instant display that really helps. Even the vacuum gauge isn't much use, it keeps wandering up to where it shows 10" when the engine is off. But I know that this is a pretty light truck, I never drive very fast and rarely have a large load (firewood once in a while, the wood splitter once in a while, Lowe's once in a while). I've had it since it was new.

ROI = never. There is no such thing on a truck this old, there's just wrenching because it's more interesting than watching TV.
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Old 02-09-2016, 12:58 PM   #8 (permalink)
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Quote:
Originally Posted by rngr4142 View Post
...i have a 1995 ford ranger 4x4 with the 3.0v6 auto. o/d. this engine turns at a whopping 2500 RPM when at 60 MPH...
In my 2013 Honda Fit with manual 5-speed, cruising at 100 km/h (62mph) - which is the legal speed limit here - makes the little 1.5 L engine spin at 3,000 RPM. If I drive consistently at this speed, slow down a bit as climbing hills, and coast downhill, I can easily hit 40-45 mpg over an 1 hour drive.

My point is that RPM's aren't everything. If you have an instant fuel consumption display, it becomes very clear that fuel consumption depends on the load more than RPM's. I could be wrong, but I think engine load is affected by many factors such as grade/slope of road, air temperature, wind speed and direction, weight of car and all the stuff in your trunk, type of transmission, 4x4 drive, tires (type and pressure of tire), drag forces / aero of your car, type of engine oil, etc...and of course how many cheeseburgers you had that day.
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Old 02-09-2016, 01:24 PM   #9 (permalink)
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Quote:
Originally Posted by Andrei_ierdnA View Post
In my 2013 Honda Fit with manual 5-speed, cruising at 100 km/h (62mph) - which is the legal speed limit here - makes the little 1.5 L engine spin at 3,000 RPM. If I drive consistently at this speed, slow down a bit as climbing hills, and coast downhill, I can easily hit 40-45 mpg over an 1 hour drive.

My point is that RPM's aren't everything. If you have an instant fuel consumption display, it becomes very clear that fuel consumption depends on the load more than RPM's. I could be wrong, but I think engine load is affected by many factors such as grade/slope of road, air temperature, wind speed and direction, weight of car and all the stuff in your trunk, type of transmission, 4x4 drive, tires (type and pressure of tire), drag forces / aero of your car, type of engine oil, etc...and of course how many cheeseburgers you had that day.

Yes and no. Assuming a fixed power need to travel at a fixed speed, increasing RPM means you'll have lower percent load at the same speed (because the engine can make more power at higher RPM), and low load causes parasitic losses (mostly vacuum/pumping). Higher RPM also has exponentially increased friction = more parasitic losses. Sometimes this is partially offset by an engine being more efficient at higher RPM, but ideally you want to be geared so that you're at high load at the lowest RPM possible when cruising, to maximize efficiency.

EDIT: My previous car had a very, very short transmission. The 1.8L DOHC engine spun at ~4500RPM at 72mph, and at this speed, I usually got about 30mpg. If geared differently, let's say closer to 2500RPM, it's likely I'd have been closer to 45mpg, because of the tremendous decrease in pumping and frictional losses.
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Old 02-09-2016, 02:43 PM   #10 (permalink)
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Decode the axle code on your sticker near the drivers door striker. Something like a 308 to a 335 would be good.

regards
mech

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