Fuel Consumption at high speed

It's a bit more complicated than that. Basically, fuel consumption is more directly a factor of engine speed/rpm and throttle opening. We'll leave out throttle opening to keep things simple. A 3.5 liter at 4000 rpm will consume more than a 2.4 liter at 4000 rpm.

Now, if you add in gearing, then things get more interesting. a 3.5 liter, mated to say a 6-speed manual, cruising at 85 mph is turning over at a somewhat leisurely 2600 rpm. A 2.4 liter mated to a 5-speed manaul cruising at 85 mph may be turning over at 4000 rpm. The gearing is chosen considering an engine's torque curve, the 3.5 liter having the better one. Due to better gearing and more gears, a 3.5 liter can approach the 2.4 liter's fuel consumption and economy. Generally, though, if everything else is equal, the bigger displacement always consumes more.

CD

Thank you. What would be answer for Automatic Cars? I guess I will need to ask what would be the RPM of a 3.5 litter car vs

2.5 litter car when the speed is 85 mph?

Thanks,

galsaba Codifus wrote:

Automatic or manual, the same rule generally applies. It's about the gears and gear ratios. Automatics will always be less efficient than their manual counterparts because;

1) they're heavier 2) Their fundamental technology, like the torque converter, is in-efficient until the tranny "locks-up"

As for your question of what the engine speed would be for a 3.5 compared to a 2.5 at 85 mph, you'd have to look at a particualy model of car. Not all 3.5s spin at the same speed. It all depends on the application.

Lets' take Nissans for example. There the Nissan Murano with the VQ35

3.5 V6, mated to a CVT. Then there's the 350Z also with the VQ35 3.5 V6, mated to a 6 speed manual. 95% the same engine yet in the 350 the Vq35 produces 300 HP and in the Murano it produces 245 . . . .I think. Because the Murano is a heavier vehicle and is designed for more trucklike duties, the tranny will be modified with shorter ratios (or is it longer?). Top speed of The 350Z is something like 155 mph whereas the Murano is probably limited to 130 mph. All these factors influence what type of gearing would be used on a vehicle. Despite these 2 vehicles having pretty much the same engine, their ratios and subsequently fuel economy ratings will be very different.

At this point you would get out your calculator and find the exact ratios for each vehicle to find our answer. I only vaguely know how to do it, but I do know it involves some number crunching. Usually manufacturers publish the gear ratios in the transmissions. You can get that info and go from there.

Best thing to do as the other poster suggested, is to look at the published fuel consumption figures on the websites he mentioned.

CD

A simpler answer for you is that I used to have a 1.6 manual Primera Hatch (115 PS) and I now have a 2 litre estate (150 PS). The 2 litre is quite a lot thirstier, perhaps 15-20%, in all driving. I don't think that's down to the body type. It takes a lot of concentration for me to get the MPG meter to show 40, and I think it lies to me; the old one I just drove it (no meter) and usually topped 40.

Of course the 2L goes rather better!

Andy