Slowing down going down a mountain

[Wiltan365]

I have a 2021 f150 2.7l xlt. It has a 6 inch lift with 35-12.50-r20lt nitto ridge grappler and the air dam is removed. I live in michigan and recently took a trip to Florida and drove through the Carolinas. I was going down what I believe was a 5% grade and the truck was slowing down going downhill without me applying the brake and without adaptive cruse on. I decided to put it into neutral to see if maybe it was downshifting without telling me. That wasn't the case. Then I was thinking that maybe it was because of the air damage being removed but in my thinking, going down a 5% grade nothing should slow down without the brake being applied. Just trying to make sure nothing could be wrong with the truck. So I'm just confused and trying to make sense of it. Any help would be appreciated.

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[Oxford_Powerboost]

What speed were you running? Wind resistance on these things is huge at highway speeds.

 

[Wiltan365]

What speed were you running? Wind resistance on these things is huge at highway speeds.

I was doing roughly 70-75

 

[BMWtoF150]

I live in upstate SC and travel that mountain pass regularly. It is 7% grade. My 3.5 eco does not lose any speed. In fact it will speed up a bit. My suspension is stock.

 

[Kanuck]

Rolling resistance of the tires?

 

[EricR]

On one hand:

Tall lift, no rake, no air dam, plus big meaty tires - it can all add up.​

On the other hand:

5-7% is a significant grade.​

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Maybe a brake pad dragging?​

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Under-inflated tires would be a culprit. I read elsewhere the ridge grappler should be run at 52 PSI (ymmv - literally and figuratively).​

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What MPG did you get in the flat?​

Consider my rambling as a bump to this thread. Hopefully others with mods similar to yours will chime in with their experiences.

 

[Wiltan365]

On one hand:

Tall lift, no rake, no air dam, plus big meaty tires - it can all add up.​

On the other hand:

5-7% is a significant grade.​

​

Maybe a brake pad dragging?​

​

Under-inflated tires would be a culprit. I read elsewhere the ridge grappler should be run at 52 PSI (ymmv - literally and figuratively).​

​

What MPG did you get in the flat?​

Consider my rambling as a bump to this thread. Hopefully others with mods similar to yours will chime in with their experiences.

I was running 60 in the front 50 in the rear. According to the tires themself max is 80psi. In the mountains was getting 16ish before the mountains I was getting 19ish. No brake dust on rims so I don't believe it was dragging.

 

[DcnPat]

Just for a data point, I just did a trip through the PA mountains with several 4% or greater grades. My powerboost would speed up on the down hills even when charging the hybrid battery.

 

[CycleChem]

Although not an engineer (so feel free to correct me if wrong), I do still remember a little bit of my college physics classes, so just did a little napkin math (with some assumptions - no engine power (neutral), 70 mph, 4600 lb truck, Cd of 0.49, published dimensions + 6 inch). It is possible for that truck to slow down on a 5% grade due solely to air resistance. However, it was really close, so any engine power should have been enough to overcome it. In fact, using the estimates, it would have only slowed down to 62 mph and then maintained that down the rest of the way. Or if the grade changed to a little over 5%, then it could have maintained 70 mph.

For my quick math, I only focused on air resistance and not rolling resistance, so adding that in as well would mean it would slow down even more. And of course, change any of those numbers (headwinds, tailwinds, weights, dimensions, grade, engine power applied), and the results are different. For example, with the powerboost at an estimated 5500 pounds, the additional weight ensured that it would overcome the air resistance and speed up.

 

[Kanuck]

Just today had a talk with a friend who drove down to Vegas and he had friends came along in their truck. F 150, year unknown, 3.5 EB, 6 inch lift, 35x12.5x20 tires and non stock exhaust. Not towing and only 3peoplein thetruck with their lugage.That truck over heating going over the mountains and could not climb the grades. They ended up turning around and went home. I don't know what the grades were but that truck, stock, should have climbed those grades easily.

 

[Jackpaul]

I have a 2021 f150 2.7l xlt. It has a 6 inch lift with 35-12.50-r20lt nitto ridge grappler and the air dam is removed. I live in michigan and recently took a trip to Florida and drove through the Carolinas. I was going down what I believe was a 5% grade and the truck was slowing down going downhill without me applying the brake and without adaptive cruse on. I decided to put it into neutral to see if maybe it was downshifting without telling me. That wasn't the case. Then I was thinking that maybe it was because of the air damage being removed but in my thinking, going down a 5% grade nothing should slow down without the brake being applied. Just trying to make sure nothing could be wrong with the truck. So I'm just confused and trying to make sense of it. Any help would be appreciated.

have you found an answer? My truck does the exact same thing and I could not figure out what’s wrong. this is the first threat that even addresses it.

 

[Ford Motor Company]

have you found an answer? My truck does the exact same thing and I could not figure out what’s wrong. this is the first threat that even addresses it.

Hello, Jack! Send us a private message with your VIN and the name and location of your local Ford dealer. I’d be happy to see what I can do to assist with this concern. Thanks!

 

[Wiltan365]

have you found an answer? My truck does the exact same thing and I could not figure out what’s wrong. this is the first threat that even addresses it.

I haven't found an answer yet.

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