which uses the most petrol???

nursemolly

if i take a route somewhere which is 25 miles but only takes 30 mins as its a faster route, compared to to a route which is the same destination but is shorter in mileage but more cross country and takes longer in time.
which would use the most petrol??
thanks
molly
xx

rdwarr

Hi Molly,
I would say the second one. Normally the slower you go, the more economical your car is. 100 miles at 50mph will use far less fuel than at 100mph even though it takes twice as long.

kaya

the route that involves the most stopping and starting will use the most fuel, cruising at 55mph is the most economical, slowing down causes huge amounts of pollution, thats why speedhumps should have been removed years ago under an e.u directive

tomstickland

The route with the least speed changes (hills, junctions, corners) etc will be best.
Fuel consumption does increase with speed, but acceleration is the most demanding activity. Every time you brake or engine brake then you are throwing away energy which has to be made back up by burning some more fuel.
Sitting at a steady 70 on a level motorway allows 50mpg, whereas going up a hill might drop consumption to 20mpg and accelerating goes down to 12mpg.

hawk

rdwarr wrote:

Hi Molly,
I would say the second one. Normally the slower you go, the more economical your car is. 100 miles at 50mph will use far less fuel than at 100mph even though it takes twice as long.

Er Ive never heard of that before I always thought that even if you drive at faster speeds as long as your not over revving your ok.

nursemolly

thanks for the advice. so basically going the longer but faster route will not use any more petrol.
cheers
m3 here i come ....lol

rdwarr

hawk wrote:

Er Ive never heard of that before I always thought that even if you drive at faster speeds as long as your not over revving your ok.

There are two constant speed mpg figures quoted for cars. One at 56mph and one at 75mph. 56mph always gives better economy.
See here for more details on this.

chrisw

hawk wrote:

Er Ive never heard of that before I always thought that even if you drive at faster speeds as long as your not over revving your ok.

Air resistance also increases disproportionately with speed. I think it's the cube of the speed (or it may be the square), ie doubling the speed means 8x the drag.

bunking_off

rdwarr wrote:

There are two constant speed mpg figures quoted for cars. One at 56mph and one at 75mph. 56mph always gives better economy.
See here for more details on this.

Yes, but mainly because cars are specifically engineered to give a flattering reading at the 56mph reading, because it's the one that's always quoted in adverts.

As others have said, what matters is not particularly the speed, but the number of stop starts. For evidence of this compare the constant 56mph figure in a typical car ad (constant speed) with the urban one (which is a simulated mix of stop start). I've got a dynamic mpg feature on my in-car computer, and I can be getting 4mpg pulling briskly away from a junction, versus 30+mpg at constant 80 on the motorway...yes, my car has a big engine hence 30+ versus 50+ in a small car...

tomstickland

Yes, drag forces increase with speed squared, so twice as fast means 4 times as much drag force. The power requirement increases with speed cubed, but since the car is travelling faster, then the amount of time spend at that speed decreases by 1/speed, so the mpg should scale with speed squared.

However, the power requirements due to drag are quite small compared with those for acceleration. A car probably only needs about 30BHP when cruising at a steady 70mph.