Coast down Speeds

[htvfd460]

What is the average coast down speed on a steep long grade hill?
For example going down a mountain hill in neutral my truck would stop acelerating at 65 mph, After dropping my truck its gone up to over 80mph in Neutral coasting.

I am trying to figure a general rule of thumb.

[MGB=MPG]

well it all depends on how steep the grade is and how much mass you carry .
...
on a 6% 90-100 maybe more .. watch out for that corner on the bottom though when your brakes get hot and start to fade.. it is not called deadmans curve for nothing

back when i ran with the wild bunch last century we would box the gears out when we tached out .. some call it *Georgia Overdrive*

[euromodder]

Originally Posted by htvfd460

What is the average coast down speed on a steep long grade hill?
I am trying to figure a general rule of thumb.

There isn't one.
It depends on the angle, on your vehicle weight, the aerodynamics of the vehicle, even the road surface.

When the aero and rolling drag equals the pull of gravity, the car will reach its terminal velocity.

I need around 5% gradient to maintain about 60 mph

For example going down a mountain hill in neutral my truck would stop acelerating at 65 mph, After dropping my truck its gone up to over 80mph in Neutral coasting.

Dropping, as in lowering the truck's ride height ?
That's a known drag reducing measure, so it's clear it reduced drag on your vehicle, and made it coast down faster until it reached its new equilibrium at over 80mph.

It's a serious increase in speed though.
Have you noticed a reduction in fuel consumption since dropping it ?

[BamZipPow]

How much did you drop it down by? Got any piccies?

What was the clearance before and after the drop?

[aerohead]

Official coastdown tests are conducted on level roads,so no 'official' data exist for downhill coasting.
That being said,under same-day,back-to-back downhill comparisons,if you could get to terminal velocity you could probably see a definite trend.
Safety should override all other considerations,so this would be problematical.
I have been doing 'informal' coastdowns on the last 1.1-miles of 60-mph highway road,from a hilltop,down a gentle grade to a creek bottom,then up other side to my turnoff.
With zero modifications to the truck,depending on weather conditions,my velocity has varied from 38-mph,to negative mph,stopping short of my turnoff by varied distances.All with no modifications whatsoever to the truck.
While it's an interesting exercise there's little I can make from the scatter-plot of data.
I'd recommend you scope out a level, lightly traveled stretch of road for testing.
From that we might be able to help.

[htvfd460]

On a 2.3% grade was my test. I gues I should have worded a bit differently. Terminal velocity is what I am looking for. (my T.V. Went up a little over 15 MPH. From 65 to over 80, with the cops in NY I didn't want to go further and get another ticket)
I did a 4" frront and 5" rear drop. Later because of chewing threw fender wells I added a 1.25" rubber isolated from a Superduty (with modifications). I went from stock avg 16-17 mpg all the way up to 20-23 mpg. (also tuned and k&n air filter) from about 300 miles per tank to over 400 miles per tank. My wallet gives me little kisses every night.
With the overall ride height I don't remember what the exact numbers were but with in a range of +- .25". I only really paid attention to the ride height while experimenting with + and - sized tires. Then back to stock. (it's nice having other vehicles to use for swapping rims and tires.)

[aerohead]

Originally Posted by htvfd460

On a 2.3% grade was my test. I gues I should have worded a bit differently. Terminal velocity is what I am looking for. (my T.V. Went up a little over 15 MPH. From 65 to over 80, with the cops in NY I didn't want to go further and get another ticket)
I did a 4" frront and 5" rear drop. Later because of chewing threw fender wells I added a 1.25" rubber isolated from a Superduty (with modifications). I went from stock avg 16-17 mpg all the way up to 20-23 mpg. (also tuned and k&n air filter) from about 300 miles per tank to over 400 miles per tank. My wallet gives me little kisses every night.
With the overall ride height I don't remember what the exact numbers were but with in a range of +- .25". I only really paid attention to the ride height while experimenting with + and - sized tires. Then back to stock. (it's nice having other vehicles to use for swapping rims and tires.)

On a constant 2.3% grade it would be the vertical vector component of the gravitational constant ( 32.3 ft/sec/sec ( 9.81 m/sec/sec)),factoring the vehicle mass and rolling-resistance,along with the air drag providing all the force to just balance the force due to gravity.
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* By lowering the truck you've altered the frontal area a bit and you could calculate the difference by subtracting the amount of tire face which is now shielded within the body due to the drop.
* You haven't altered the mass so that's a constant.
* the grade is the same so the push from gravity can be taken as a constant.
* The tires need to remain unchanged for at least one back-to-back data set or it will enter another variable into the mix.
* Total weight needs to remain constant.Same gear on board each time.Maybe a full tank each time as each gallon is about 6-pounds and directly affects momentum/inertia( the heavier truck will have a higher TV).
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*( from Hucho ) For top speed,velocity will vary at 10% additional velocity per 30% drag reduction.
If say your new TV is 80 mph vs 65 mph for that particular gradient,then 80/65= 1.23,or,a 23% velocity increase at 'fixed-power'.
23%/10% per 30% drag reduction = 2.3X 30= 69% drag reduction.So this suggests that only 31% of the original drag remains.
If the Ranger started at say Cd 0.45,the formula suggests that the drag was cut to Cd 0.139,if frontal area was unchanged.That's pretty low!
That's a very simplistic look at the Ranger.
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I hope you have a baseline for mpg without mods.If you'll track your fuel use maybe we can back-engineer your change.
10% delta-Cd = 5% delta-mpg @ 55 mph
10% delta-Cd = 6% delta- mpg @ 70 mph.

[htvfd460]

Aerohead those calculations are sexy! I can't wait till I get my new tires to do some testing! I tried to use the links to find my stock CD but my truck wasn't listed. Sins I have the 06 ranger STX that has a completely different front valance from the other packages offered from Ford and having the Mazda bed my truck's definitions are different. I would love to know the most accurate way to calculate my CD. Including the response from the grille and the body gaps.

[stovie]

on a 2.3% slope my jeep will hit about 65-70mph in nuetral witch i'd like to increase with aero mods so then i can do in gear coasting(my injectors shut off when i let off the gas in gear) cause in gear right now it only slows a little bit but by reducing the cd should allow me to maintain speed will in gear for better mpg's, it should also allow me to get better fuel economy going up hills. I'm between 15 and 20 mpg going up 6-8% slopes at 55-60mph

[KamperBob]

Originally Posted by stovie

on a 2.3% slope my jeep will hit about 65-70mph in nuetral witch i'd like to increase with aero mods so then i can do in gear coasting(my injectors shut off when i let off the gas in gear) cause in gear right now it only slows a little bit but by reducing the cd should allow me to maintain speed will in gear for better mpg's, it should also allow me to get better fuel economy going up hills. I'm between 15 and 20 mpg going up 6-8% slopes at 55-60mph

+1 Stovie

My ScanGauge shows the fuel injector shutoff effect while engine braking down hills. With my 4.7L idling at 0.5gph my coasting MPG at 60mph is 120mpg versus infinite (9999mpg on SG screen wit 0gph denominator). When towing trailers through PA mountains (many trips over 2 years) the strategy helped substantially.

htvfd460, IIUC you have both a Ranger and Super Duty and your circa 20mpg numbers would be from the latter. If so, those numbers look great. Rock on!