what is optimal speed for good fuel economy ?

2003 Jetta TDI (Automatic).

What is the best highway cruising speed for having the best fuel economy ?

as well, what about city driving.

Or is there a magic number, that if you could drive at that speed no matter where you are, you are getting the best fuel mileage ?

Depends on the highway, and other road users.

Typically you dont have much choice about the speed you travel as every one else on the freeway is going speed limit plus 10.
In Vancouver I drive the upperlevels highway which has a speed limit of 56 mph (90km/hr) for a while, and then 50 mph (80km/hr) down the Cut, and then 44 mph (70km/hr) over Second Narrows Bridge, then its back to 50 mph, then 56 mph. So you can guess at the actual speed travelled.
Those are lower speeds than people in other parts of the world encounter on the highways.

Anyway, given that situation:
My tdi is consistently giving +/- 61 mpgUS.

I think Jonathan Bartlett attained 80 something mpgUS in a contest, by driving 42mph, which was amazing.

Most typical highway speeds return mpg's in the upper 50mpgUS

hth.

It is true, usually people do speed +10 on roads. I tend to drive about 105-110 km/h, and keep up pretty good. Pretty happy with my consump, but could be better. I have just hit a current high of 49 mpg. I know a few reasons it is lower, so I am happy with the number as is. I think I could get over 50 without blinking if I changed a few things...and will do so eventually.

Anyhow, as for your question. Typically I have heard something around the 88-92 km/h mark is about max fuel economy. There are other numbers around which state max is around 60 (due to wind resistance and all that).

But basically, just drive what you are comfortable driving. If you don't feel comfy driving a little slower, than drive a little faster, and still enjoy the great fuel economy. Just think...if you would have bought a 2.slow, you would have had to fill up half a tank ago.

Im my Passat the furthest distance per volume of fuel is produced at about 35 to 40 mph in 5th, high 90's mpg. The contest mentioned had a prescribed route and minimum and maximum times per lap. I went only fast enough to not exceed the maximum time alotted, about 47 mph. The 10 mph difference between the speed driven that day and the speed of highest consumption efficiency, resulted in a 6 to 7 mpg drop or from 97+ to just over 92 mpg. Speed, even at these relatively low rates, is a major factor in distance per volume. The contest organizers converted all fuel consumption into BTU/mile (or kW/mile for electrics) and resulted in the same heat energy consumption rate as 80+ miles per gallon of gasoline.

According to this graph taken from fueleconomy.gov, 55 mph is the fastest you can go with an acceptable fuel economy.

It's not always easy to drive 55, but it's possible to go to the speed limit and accelerate when needed.

My last 2 tanks I got 1200 miles driving to the speed limit or slower.

When going downhill I leave the car in gear and the speed could go up to 70-75 mph (around 120 km/h).

I use engine compression when coming to a halt, because doing so doesn't use any fuel at all.

I close all windows when driving and tilt the sunroof slightly when needed.

Taking the roads less travelled allow you to go the speed limit or lower, look for 'em!!!

Don't know if this is correct or not but according to my dealer Transport Canada measures hwy mileage during testing in top gear on a flat surface at a steady 78 kph for all vehicles to achieve the rating of the vehicle, 64 mpg for the TDI. Not sure why 78 kph is the choosen # but he thought he read somewhere it is the overall avg. hwy. speed in Canada, which seems absurd unless rush hour is factored in.

Answer the the question is:
As low as you can drive in highest gear without lugging the engine. Typically about 30 mph.
Is this comfortable or sensible? Hell - no, but it is the answer. Adding speed from this increases the fuel consuption too.

I made my personal TDI record last Monday.
Passat over 70 miles doing 24 mpg!!
OK, average speed was 107 mph.

Don't know if this is correct or not but according to my dealer Transport Canada measures hwy mileage during testing in top gear on a flat surface at a steady 78 kph for all vehicles to achieve the rating of the vehicle, 64 mpg for the TDI. Not sure why 78 kph is the choosen # but he thought he read somewhere it is the overall avg. hwy. speed in Canada, which seems absurd unless rush hour is factored in.

Click to expand...

The Transport Canada test simulates a variety of speeds and the average speed in (I think a 16 minutes stop to stop cycle) is 78km/hr. They publish 64 miles per imperial gallon which is actually 15% downgraded from the actual figure attained to allow for "normal" drivers. So the real figure is 75mpgIMP. Of course you can do better than that!
hth

The Transport Canada test simulates a variety of speeds and the average speed in (I think a 16 minutes stop to stop cycle) is 78km/hr. They publish 64 miles per imperial gallon which is actually 15% downgraded from the actual figure attained to allow for "normal" drivers. So the real figure is 75mpgIMP. Of course you can do better than that!
hth

Click to expand...

US EPA fuel economy numbers are also derated (by 22%). The EPA's test averages 48mph (not a constant speed, but no stops except at the end). Since to get the reported 49mpg, the actual test had to give 63mpg, driving a constant 48mph on flat ground without wind should give around 63mpg or even better.

I always try to drive downhill for the best mileage. Going uphill uses way too much fuel. When I get to the bottom, I just call AAA and get a tow back up.