How does a topper affect fuel consumption?

EZ Livin · September 29, 2004

I was fascinated to read the effects on fuel consumption of tailgate up / tailgate down / box cover in this thread:

http://www.gm-trucks.com/forums/index.php?...topic=30042&hl=

and particularly in this link:

http://sitemaker.umich.edu/um.gm.truck

I was wondering if anyone has seen similar tests done on toppers?

Shaners · September 29, 2004

I think highway MPG's have improved in my truck, but around town is worse because of the extra couple hundred lbs.

1500CCZ71 · September 30, 2004

I had a cab high, aluminum cap on my 93 1500, std cab. I figured having the cap on cost me about 2 mpg versus having it off.

03GMCinBC · September 30, 2004

I notice a slight increase in MPG, around 2 per gallon. I did it more to secure the stuff in the back from the rain so any increase is a bonus and decrease is something I have to live with.

Scyry · September 30, 2004

My 1990 regular cab got about 1 to 2 mpg better on the freeway with the SnugLid I had on it. Cool thing was when you hit 75+ you could feel the truck squat down a little from the down force of the cover, was like a giant wing with the stink bug look of the 2WD's.

awdvkd · September 30, 2004

I never really bought in to this making a difference one way or the other. Then one night during a bad thunderstorm I observed my truck taking a direct head wind of about 70-80 mph in a heavy dowpour. It was just like watching it in a wind tunnel. You could see the wind forming a pocket directly behind the cab. The wind literally went over the cab and then it lifted about two or three inches, then exited over the tailgate. It never came close to the tailgate at all. I rally believe now that having the tailgate up is the best way. Just my 2 cents.

catiaeng · September 30, 2004

When I had my 94 Chevy and would travel on the hi-way to Detroit, I always put the tailgate down if not haulin things. I would get 17-19mpg while on highway. I miss that truck.

I have seen trucks break throught fog and smoke and you see the hole punched into the wind. Trucks with toppers had less disturbance, tonneau covers were better than nothing in the box. Now I pull my trailer and have a tonneau cover, but wish I had a topper for the long hi-way trips to remove the pocket of aire above the box.

The boss just walked in, not good to be posting all the time.

dannyukonxl · October 1, 2004

with the tailgate up, theres an airpocket that forms like awdvkd talked about, with the tail gate down you are trying to accelerate out of a vaccuum right behind the cab... with the top on, the air from underneath the truck flies up and fills up the vaccum made by the top, along with the air from the top..., i left the tail gate up anyways on my ram after a medium duty truck turned it into modern art piece... i figured it was much cheaper to have the hitch take a hit rather than the tailgate

EZ Livin · October 1, 2004

Thanks for the input, guys. My truck came with a soft cover on the box. I haven't had it long enough to do any mileage comparisons.

From the results in the University of Michigan website, I found it fascinating that the sloped cab addition actually increased drag. I'm really curious to know if a well designed topper could actually have less drag than a box cover. I'd also like to know if adding a spoiler to a bed cover would reduce drag more than the cover alone.

I think I will pose these questions to the team at UofM to see if they are planning any further testing.