Gears/Economy question

Say you're in 5th gear going 30mph (about 1500rpm) and you want to accelerate up to 60mph. Is it more economic to floor the throttle all the way up to 60mph in 5th gear, or is it worth changing down to 3rd and getting up to 60mph and then putting it in 5th? I usually do the latter anyway because of boy racer reasons but I was just curious, especially in smaller engines, I was thinking it might put more strain on the engine keeping it in 5th. Also having extremely low torque in 5th gear is a problem for smaller engines which is why I rarely accelerate in 5th because it's extremely slow, whereas if you have a bigger engine it doesn't really matter. Thanks

Reply 1

BigYoSpeck

19

Original post by Xopher_

Say you're in 5th gear going 30mph (about 1500rpm) and you want to accelerate up to 60mph. Is it more economic to floor the throttle all the way up to 60mph in 5th gear, or is it worth changing down to 3rd and getting up to 60mph and then putting it in 5th? I usually do the latter anyway because of boy racer reasons but I was just curious, especially in smaller engines, I was thinking it might put more strain on the engine keeping it in 5th. Also having extremely low torque in 5th gear is a problem for smaller engines which is why I rarely accelerate in 5th because it's extremely slow, whereas if you have a bigger engine it doesn't really matter. Thanks

I probably wouldn't drive at 30mph in 5th gear unless I was on a long flat or declining road with no traffic. 1500 rpm is fine for an engine under light load, but you shouldn't be giving it any more than light throttle to sustain the speed, and certainly not trying to accelerate from that low an RPM. On most roads that are 30mph you aren't likely to be maintaining a consistent speed long enough to justify the gear change from 4th.

It would vary wildly depending on the car, but typically in the rare circumstances I would be doing 30mph in fifth I would probably drop to 4th up to about 50mph, then up to 5th to continue up to 60mph if I were going for economy. Or if I was overtaking someone bumbling along on an NSL road I'd be into 2nd up to the redline of the car.

Reply 2

Xopher_

OP

20

Original post by BigYoSpeck

I probably wouldn't drive at 30mph in 5th gear unless I was on a long flat or declining road with no traffic. 1500 rpm is fine for an engine under light load, but you shouldn't be giving it any more than light throttle to sustain the speed, and certainly not trying to accelerate from that low an RPM. On most roads that are 30mph you aren't likely to be maintaining a consistent speed long enough to justify the gear change from 4th.

It would vary wildly depending on the car, but typically in the rare circumstances I would be doing 30mph in fifth I would probably drop to 4th up to about 50mph, then up to 5th to continue up to 60mph if I were going for economy. Or if I was overtaking someone bumbling along on an NSL road I'd be into 2nd up to the redline of the car.

Thanks for the info :smile:

Reply 3

Joinedup

20

I've got an instantaneous MPG meter in my car, flooring it in top gear from low engine speeds isn't efficient. you're better off changing down and giving it a moderate amount of gas pedal

Reply 4

NX172

17

Original post by Xopher_

Say you're in 5th gear going 30mph (about 1500rpm) and you want to accelerate up to 60mph. Is it more economic to floor the throttle

No. It's called lugging your engine. It can be very harmful to your engine.

Reply 5

Treeroy

18

It completely depends on the car. Does your car have a turbocharger? Is it petrol or diesel?

In my old car at 30mph it was not happy in anything over 3rd gear, my new car can easily idle along in 5th gear.

Speaking just generally, if you have a naturally aspirated engine you will probably need to drop a gear in order to accelerate properly. Whereas if you have a turbocharged engine you'll probably have more low-down torque and less of a need to change gear. Again it will depend on the car and the car's engine.

(edited 6 years ago)

Reply 6

Dysf(x)al

20

Original post by Xopher_

Say you're in 5th gear going 30mph (about 1500rpm) and you want to accelerate up to 60mph. Is it more economic to floor the throttle all the way up to 60mph in 5th gear, or is it worth changing down to 3rd and getting up to 60mph and then putting it in 5th? I usually do the latter anyway because of boy racer reasons but I was just curious, especially in smaller engines, I was thinking it might put more strain on the engine keeping it in 5th. Also having extremely low torque in 5th gear is a problem for smaller engines which is why I rarely accelerate in 5th because it's extremely slow, whereas if you have a bigger engine it doesn't really matter. Thanks

It depends on a lot of factors such as how quickly you want to accelerate and your engine's torque curve (this varies hugely depending on whether it's a petrol or diesel).

In general, "lugging" (flooring it at low RPM) is a bad idea as it can damage the engine and will deliver very poor acceleration.