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I'm getting mixed messages on what the most fuel efficient speed is to drive at, assuming a clear road.

Around 50 mph is a figure I see a lot, though I recently read an article on research on this topic where a variety of vehicles were researched, and it found that the optimum speed is much lower, as low as 20 mph for some vehicles. It basically said "the lowest speed at which the engine still runs smooth". This indeed is also the impression I have from driving my car.

By the way, I drive this:

http://th.88dbmedia1.jobsdb.com/db88uploadfiles/2006/11/26/8CAE4E92-C72B-4BC3-8BFC-863E3FF620FD.jpg

3.0 liter turbo-diesel engine that's pretty efficient. I try pretty hard and get about 40 mpg. (Mine is the same as pictured, though without the plastic bug deflector and window coverings)

So obviously this truck has more air drag and is a heavier vehicle compared to most cars in Europe (Not in the USA though :) )

I don't know if a mpg indicator can be fitted to this particular vehicle, but I do feel that about 30 - 40 mph is most efficient.

Thoughts?

Generally, the optimum speed for MPGs would be at the lowest RPM for your highest gear, without lugging the engine. With an automatic, you would try to keep the RPMs just high enough to prevent downshifting at every slight rise you come to.

Similarly, with a manual transmission, you would not want to downshift to keep the engine from lugging, and so would have to keep your RPMs up high enough to prevent having to do that.

Because of different gearing, different vehicles get there best MPGs at different speeds, but rarely is this at speeds over 50-60 MPH.

Best advice, use your built in instant MPG guage or purchase a SGII and see where is optimal for your car.

However, I personally don't see what the difference it between 20 & 50...neither is appropriate in places where the other is. You would never catch me doing 50 in a 30/40 zone, nor would you catch me doing 20 anywhere I could legally do 50....actually maybe you could, I was on a 55 mph old highway without any traffic and really just enjoying the ride & scenery, but I was also doing a massive range P&G, plus I did accelerate back up to speed whenever traffic was coming up from behind.

Automatic - a speed just above the top gear shift point is usually best. On my minivan, that point is 45mph, so 50 is a good target. Any higher and the mileage starts to taper off.

Manual - the optimal speed is, quite simply, "slower". Slower than you drive now, slower than you think the car wants to go, slower than is acceptable on many roads. On the World Fuel Economy Championship, I used a top speed of 25mph for my P&G cycle. The average speed was somewhere in the teens.

Best advice, use your built in instant MPG guage or purchase a SGII and see where is optimal for your car.

Would I be able to fit that to my car? It's a 2000 Thai built Isuzu 1 tonne truck.. Is there any way to check if this can be built in?

However, I personally don't see what the difference it between 20 & 50...neither is appropriate in places where the other is. You would never catch me doing 50 in a 30/40 zone, nor would you catch me doing 20 anywhere I could legally do 50...

I don't have that issue. ;) This is Thailand, and I do my daily commute on an almost empty 6 lane ring road that isn't really a highway (there's U-turn gaps every couple km) and doesn't really have any kind of speed limit (either maximum or minimum) that's enforced in any way.

So if I stick to the middle lane I can drive at pretty much any speed I fancy; (Can't do the left lane as traffic suddenly merges into there. ;)

This shows where it is:

http://sportstracker.nokia.com/nts/workoutdetail/index.do?id=313605

(Used a GPS also to check if the odometer of the car is accurate; even a fancy trip / mpg computer would give you incorrect results if the odometer is off. )

Optimum Speed, the first and fastest thing you can do to help that truck get better MPG is buying a cover for that pickup bed of yours! That or remove the tailgate as soon as possible. Either one of those two things will remove the "parachute" effect your tailgate is causing right now. Im sure you know your diesel truck better than any of us up here in North America who dont see that vehicle. Although I wish we did.