Fuel economy at different highway speeds

corbanbrook · Post by **corbanbrook** » April 3rd, 2006, 11:13 am

Last week I drove from my home in kitchener, ontario to beautiful montreal, quebec, approximately 600 km away (375 miles).

During the trip I knew I would be filling up on gas a few times so I decided to calculate my MPG at different sustained highway speeds. On the way there I drove during the night (8pm->3am). Not much traffic and could afford to drive at higher speeds. On the way back I drove during the day, it was nice and sunny and I wasnt in any rush so for the purpose of the experiment I drove the speed limit (100kph/62mph). Below are the speed brackets of sustained highway speeds and their respective fuel economy rating.

100kph / 62mph = 38 MPG
130kph / 80mph = 30 MPG
150kph / 94 mph = 26 MPG
170kph / 105 mph = 24 MPG

The highway is very hilly and it was quite windy. During the tests I sped up down hills and tried to ease off the gas uphills so that my target speed was maintained as i crested the top of the hill. I am confident that I will be able to reach 40MPG on flat roads during less windy driving conditions.

Although driving the speed limit offered incredible MPG I doubt I will do that again. The driving was tedious, and at times felt unsafe when old ladies and transport trucks were passing me on the 2-lane highway.

When travelling at higher speeds I did have to slow down for trucks swapping lanes and then accellerate again to reach my target speed. This will damper the MPG. Trucks travel in packs driving closely behind each other so that they reduce their drag and increase fuel economy, they take turns being the lead truck, but pass very slowly.. trucks drive at around 110kph and they will pass each other at 112, on a 2 lane highway this blocks off the lanes for a couple minutes as one slowly passes the other and I got stuck behind trucks more then a few times.

Overall the driving was pretty uneventful.. and running the MPG readings was the only thing I had to cut through the boredom. Its funny what little games you come up with in your head when you have to drive for 6 hours straight.

DJGypsy · Post by **DJGypsy** » April 3rd, 2006, 12:57 pm

awesome reasearch. i like how you documented everything about your trip. you wouldn't happen to be an engineer, would you?

corbanbrook · Post by **corbanbrook** » April 3rd, 2006, 2:06 pm

No, just a lowly computer programmer...

glad you liked it.

torpedan · Post by **torpedan** » April 4th, 2006, 1:22 am

If you really wanted to try and milk it for whats its worth you could consider installing a vacuum gauge, its a constant reminded of the load situation on the motor and lets you learn to drive in such a manor to reduce load and improve gas milage.

Hadrien · Post by **Hadrien** » April 4th, 2006, 12:05 pm

Wow those are some great mileage numbers!! I cannot seem to crack the 30MPG mark yet with my girlfriends 92 MX-3 with the 1.6L SOHC engine and 5SP.

It has fresh oil and filter, a new Purolator air filter, and new PCV valve. The plugs, wires, and cap all look new, but maybe I should replace them anyway? Any other suggestions?

I plan on cleaning the throttle body soon accoriding to the writeup on here, however I cannot find a part number for that gasket inside the throttle body that you have to take out so you can get in there with a wire brush. Any one know that part #? I cannot find it at rockauto.com, onlinemazdaparts.com or anything.

Sorry if my plea for help was a thread jack, that was not my intention.

torpedan · Post by **torpedan** » April 4th, 2006, 5:55 pm

Doing all that is good measure if you dont konw when it was done last, although having made a 2000lb Mx-3 get 8 MPG I can freely attest the most important thing to getting better gas milage is sitting in the drivers seat.

corbanbrook · Post by **corbanbrook** » April 4th, 2006, 7:30 pm

Hadrien: about 2 months ago i was getting about 17MPG. Something things I did initially were new cap, rotor, oil filter, air filter, sparkplug wires.. and that really didnt seem to help all that much.

After replacing the O2 sensor, water thermos ensor (engine coolant sensor) and putting in a premium fuel filter and premium Splitfire sparkplugs, I finally saw my MPG jump. If i would put my finger on it i would say new o2 sensor and coolant sensor were the culprit.

As I understand it, the responsiveness of these sensors fades overtime and the efficency of your engine will suffer.

I have the same 92 SOHC 4Cyl 5speed as your gf and my clutch has about a month left on it before it absolutely needs to be replaced and I was able to acheive higher MPG then the 35MPG a brand new MX-3 should get.. so if my car can do it certainly you can as well.

Nice thing is all of these parts are easy to install yourself and source for cheap.