3 mpg gain by cleaning up the undercarriage.

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ALS

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May 28, 2003
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Most of us want better mileage but many never really try.
Well I decided to pick a few brains of people who are a little obsessive when it comes to getting better gas mileage.
Well here is what I did and what it got me. Yes there are pics at the end. Maybe it will give you guys an idea for a project for the winter.
The car is a 1997 Volvo 960. I ran my usual base line route I always take after making any kind of mpg mods to the car. Speed is held to 65 mph and tires are inflated to 40 psi.
My base mileage on this run was an even 28.0 mpg which was run in July.
Well to the meat of the facts. At the front I enclosed the openings between the bottom of the fenders to the edges of plastic splash pan with aluminum boxes and ABS plastic covers. This is an open area under the battery and the corresponding passenger side. The reason for the boxes is in case I need to change the bulbs in the fog lamps I can just pop off the cover to get to it. Also I had to move the horns because they were in the way of the mod.
Heading towards the rear I cleaned up around the independent suspension and installed a steel air deflector plate which is attached to the bottom of the suspension cross member. This was done to keep the air from being pulled up into the IRS frame work while it is flowing under the car. From there I ran an aluminum plate all the way back to bottom of the rear bumper. I did leave the rear muffler exposed due to the heat.
All in all I got an astounding 3.0 mpg increase with this mod. This has been probably the best mpg increase from all the mods I have made.
When I bought the car Feb 05 I got 25.3 mpg on the highway and today 31.0 mpg. 5.7 mpg or 18.4% increase is nothing to complain about. Plus I figure next spring I'll spend a little time cleaning up the rest of the under carriage. That should be worth another 1/2 to 1 mpg.
For those that are interested in all the mods.
This Volvo RWD car weighs in at around 3500 lbs and has a Cd of .36
I started with an Auto-Rx engine cleaning. A clean engine is an efficient engine. That helped me get the car to 26.3 mpg, Then I swapped out all the fluids for synthetic and I saw 27.4 mpg. I then increased the tire pressure from 36 psi (factory) to 40 psi and saw 28 mpg. With the addition of the belly pan mods I now get 31 mpg.
My Scan Gauge matched the gas pump when I filled up with 8.238 gallons of gas for 255.7 miles. And that was not with LC FP60 either. I forgot to throw it in the tank when I filled up at the start. I do run LC20 in this car BTW.
Pics of the mods and car
 
That is very cool man! I've been wanting to do that for a while and you did a very good job. Did you bend sheet metal and rivet it on your own or buy pre-fab stuff?
 
I had one of my buddies friends do it. He builds street rods and restores Corvettes for a living. There is no way I could have done such a clean install. If it doesn't involve duct tape, a screw driver and a hammer I'm some what clue less when it come to stuff. I'm mechanically inclined at repairing things but body work and stuff that is this time consuming I just pay someone else to do it. They will do it right the first time unlike me.
 
Very nice job.

Not trying to say that your mod doesn't make a differnece, but please remember that ambient temperature does make a difference as well.

"My base mileage on this run was an even 28.0 mpg which was run in July."

It's a lot colder now so you are getting much denser air which means better compression.
 
The temperature out side at the time during the last run was between 55 and 65 degrees. It was maybe 10-15 degrees warmer in July. That would not make that much of a difference, plus I'm dealing with winter gas now. Denser cooler air would cause the computer to compensate by adding more fuel to the mixture.
 
I can't wait to see what the real difference is. I'm going down to Naples to visit family for the holidays. I found a great road to test fuel mileage on. It is pretty flat and in the morning the temps in late Dec should be around 60 degrees in Fl. The road is route 41 between Naples and Miami. A two lane with a 60 mph speed limit all the way across. I ran this road back in June doing some MPG testing and got some very interesting numbers. And there is always I-75 between Naples and Fort Lauderdale with it's 70 mph speed limit.
Around me there are just so many hills it is tough to maximize your fuel mileage.
 
The denser winter air causes more aero drag so I agree things should get even better next summer.
 
LOL. When I read your topic title I thought at first that you got a 3mpg improvement by washing the undercarriage with a pressure washer. Now that would be an easy mod!
 
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How would adding weight increase gas mileage?




because by covering things it decreses drag and everyone knows drag is a killer on gas mileage. why you think everything on a aircraft is covered and streamlined.
 
If this is the case, why wouldn't the car makers equip their cars the same way? They would surely like to increase the gas mileage of their products.
 
Quote:


LOL. When I read your topic title I thought at first that you got a 3mpg improvement by washing the undercarriage with a pressure washer. Now that would be an easy mod!




So did I, I though oh gawd, another clueless BITOG MPG story. His sounds little on the optimistic side but believable.

Any pictures?
 
Sounds ridiculous but I've found my mileage to be better in the summer with the A/C switched on during highway runs than in the dead cold winter on the same stretch of road (250mile trip). I thought it might be the denser air meaning more aerodynamic drag?
 
I'm getting WAY better mpg now, after fixing brakes, adding a front air dam, larger, well worn tires, and 5w20 motor oil. I recently scored nearly 33 mpg at 73 mph, over an 800 mile trip. That's WAY above the EPA highway mileage rating. Distance and speed measured with GPS.
 
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If this is the case, why wouldn't the car makers equip their cars the same way? They would surely like to increase the gas mileage of their products.




Take a look under most contemporary vehicles. Pretty much a flat clean view under the car from front to back. The exhaust snakes through the tunnel ...typically wraps around the gas tank. Nothing protrudes. Now it's not totally smooth, like our OP's ..but way clean.

They could go more in depth ..and sometimes do. How many here whine about having to remove some belly pan to change the oil
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It takes time and money to engineer underbody plates that decrease drag but don't add too much weight to negate the benefits. Manufacturers do do this when it counts - take a look at the bottom of a Lotus Elise.
 
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