Which car consumes less fuel on highways, V6 or V8?

It's depends on the siz of piston 

Consider the factors weight ratio, mechanical losses etc. V6 will offer more fuel efficiency on highways.

Toyota V8 engine is best. Comparatively to other v8 and as well V6.

Dpeneds on who is driving the V6 and V8. A wise driver will need less fuel to cover the same distance in V8 or a V6 compared to the idiots on the road who rev a lot.

All cars start the journey with the effort to over come its total waight, then less powerr is required to keep the car in motion.

to compare fuel consumption for two deferent engines, many factors are required. but if you are reffering to identical cars with only the engine deferenct size , the answer is that the V6 consumes less fuel as the V8 is higher in weight and need more power to overcome its weight, even at highways.

Thank you for the question

it all depends on the car because waight and the way its drivven play a big part

For less fuel consumes the best Engine is v8

Car consumes less fuel on highways, V8.