what constant mph gives optimal fuel economy?

[BAM_wants_an_Aero]

Hi,

I was wondering if anyone knew what the optimal speed is for a 9-5 Aero to get maximum fuel economy? For example the owners manual for a ford focus 1.8 diesel states that for maximum fuel economy, you should go at 56mph at 2000rpm in 5th gear. I looked through my Aero manual and whilst it did give optimal gear change times (3rd at 25mph, 4th at 40mph etc.), it didnt seem to list an "overall" speed/rpm combination like the ford.

Out of interest, the best ive got is doing a steady 70mph (using cruise) on the motorway which gives me a solid 46.3 mpg. Citywise, im scraping 30-31 mpg if I drive conservatively (which is hard in this car lol).

Anyone got any ideas?

[DriverEd]

Generally, the lowest RPM in top gear will yield best efficiency. My guess would be around 90 Km/hr.

[Cambio]

It would depend on how slow/fast you want to go. I get 30-32mpg at 85mph (136kph). Good enough for me. I have gotten 35+mpg at 65mph (104kph).

[aeropilot]

Originally Posted by BAM_wants_an_Aero

I was wondering if anyone knew what the optimal speed is for a 9-5 Aero to get maximum fuel economy? For example the owners manual for a ford focus 1.8 diesel states that for maximum fuel economy, you should go at 56mph at 2000rpm in 5th gear. I looked through my Aero manual and whilst it did give optimal gear change times (3rd at 25mph, 4th at 40mph etc.), it didnt seem to list an "overall" speed/rpm combination like the ford.

Out of interest, the best ive got is doing a steady 70mph (using cruise) on the motorway which gives me a solid 46.3 mpg. Citywise, im scraping 30-31 mpg if I drive conservatively (which is hard in this car lol).

Depends on whether it's an auto or manual.

If an auto, 56mpg won't be very economical, especially using cruise control, as at that speed the auto will kick down a gear too easily when wanting to accelerate for inclines etc, rather than on a manual where you would just use the torque in 5th.
On my Sentronic auto I find I get better economy driving in M+ mode on a run. Never use the cruise as a result on the 9-5. Used to use it a lot on my old manual 9k Aero.

I'd say anything between 70-80 would be at it's most efficent for econony on a run, as youve already deduced.

If you are getting 30-31 for city driving your doing well........I've never got better than 23-24mpg for London 'city' driving in mine. I can get that up to 26-27 if staying out of too much stop-start traffic.

[II Kings 9:20]

With a 2.3 lpt auto, my best is 37mpg with the cruise on 65mph.

[i want an aero]

best ever was 40mpg on the m5 to bristol at a tad under 70 with the cruise on, was so bad being passed by everything

[Jafar]

this is so weird. im getting like 16-17 mpg within the city in my car, and i drive conservatively MOST of the time. im not getting any leaks or anything. there must be something wrong with my car.

[Kermit]

If you have a 5 speed manual and you're getting 16-17mpg around town then you do have a problem, what do you get on a run?

[stipud]

I get about 13.3l/100km (17mpg) driving hard with sport mode on in the city (with a 4spd auto - often locked in second/third gear), and 8.0l/100km (30mpg) on the highway, cruising at 135km/h (~85mph) with 4 in the car.

I hope my conversion is right.

[born again]

I think we may have some apples and oranges going on here. Are the UK/Ireland posts in Miles per "Imperial" Gallons and the US posts in Miles per US Gallons? These are not the same and give different values. Conversions are about:

Miles per Imperial Gallon = 283/(L/100km)
Miles per US Gallon = 238/(L/100km)

Incidentally, the best highway mileage is determined by two things: the Cd or aerodynamic drag coefficient, and the net efficiency of the drivetrain. It will be somewhere between 55 and 70 miles per hour. A more aerodynamic car will have a higher optimum speed. Rolling friction has a minor effect at highway speed.

I haven't found the 9-5 to be all that great on fuel, averaging between 10 and 11 L/100km. Best 8.2, worst 12.5.