difference in drag during trips ~ aero or brakes?

[j12piprius]

On a long steep downhill, my previous top coasting speed was 49.1 mph which the car hit and maintained for 1/4 to 1/2 a mile. The top speed during the last trip was only 45.6 mph, a 3.5 mph difference. There was a similar difference in my mpg average for the trips in either direction.

                    october   november   difference
downhill coasting     49.1mph   45.6mph   - 3.5
east 125 miles        58.9mpg   54.2mpg   - 4.7
west 125 miles        60.1mpg   56.5mpg   - 3.6

What is the most likely cause of this? The only changes I made were (1) trimming the bumper cover and smoothing/covering the under part, (2) enlarging the vent, and (3) removing resistance from the brakes. However I'm still suspecting the brakes, maybe due to sticking front pistons. Two hours after getting home (I forgot right after), the temperature outside the front wheel covers was 4 degrees higher than in back. The setback is disappointing, and I'm wondering what to focus on next.

previous bumper cover


current bumper cover

[jakobnev]

Could be just lower ambient temperatures.

[j12piprius]

Temperatures were the same, in the 70s in mid afternoon.

I'd warmed the car up prior to starting, then driven 70 miles by that point.

Wind was negligible the previous trip, and slightly favorable during the last one.

[awcook]

Could be the different types of fuel that the pump uses, since it is getting colder out, the gas stations have been switching to different types of fuel, which isn't good for fuel economy.

[j12piprius]

I'm not sure about the differences in fuel, as I think they're not shown on the pumps, but that wouldn't explain the different speeds for dfco coasting down the hill, which felt like a major difference. The previous trip the car was going full blast, and the last time hardly moving along.

[aerohead]

If you had full instrumentation and data-logging capability we might be able to figure something out.There's just too many possible variables which would have to be nailed down to find the signal in the background noise.
You'd also probably want to do at least 10-runs each time and average the numbers,just as in coastdown testing.

[JRMichler]

Originally Posted by johnlvs2run

Temperatures were the same, in the 70s in mid afternoon.

I'd warmed the car up prior to starting, then driven 70 miles by that point.

Wind was negligible the previous trip, and slightly favorable during the last one.

Tire pressure?
Check brake disks/drums after driving several miles and coasting to a stop. They should all be at ambient temperature.

[2000mc]

Brake temps -
In diagnosing vehicles with a known but intermittent brake drag problem present, I have finally stopped using an infrared temp gun to try to help diagnose which wheel has the issue. It makes no sense to me why it doesn't help, but more often than not it will misdirect me.

[j12piprius]

Tire pressure was 52 psi for both trips.

Originally Posted by 2000mc

Brake temps - In diagnosing vehicles with a known but intermittent brake drag problem present, I have finally stopped using an infrared temp gun to try to help diagnose which wheel has the issue. It makes no sense to me why it doesn't help, but more often than not it will misdirect me.

What have you found to be most likely causes of intermittent brake drag?

[2000mc]

Caliper, flexible brake hose.
Do confirm where the problem lies before replacing components, I've worked on a guy's Colorado where literally everything in the hydrolic system was replaced except for the hard lines by another shop. Booster was the problem
Gm full size vans can get an unusually hard pull due to aftermarket pads, but they don't actually drag.


When you think it is actively dragging, is when you would want to be able to pick it up asap, to spin the wheel to check for the drag. Might not drag for whatever reason ( I think heat related usually ) if it sits very long.