RPM and MPG

[jsw2233]

This is probably an entry level question on cars. Does fuel economy has a direct proportional relationship with engine rpm? I do two 30 hwy miles everyday. There are uphills, downhills,and flats, the mpg gauge reads reasonable: lower mpg when uphill, higher mpg when downhill. But the tech is sticked at 2000 rpm the whole time. This confuses me, will there be a different fuel consumsion on the same engine speed? Why isn't my tech shows a different speed going uphill/downhill? Is it broken? Even I was using cruise (at ~65 mph) and the hills are not super steep, I am still expecting different rpm.

Thanks!

[88civicb16]

well i can tell you this the higher the rpm the more gas you are using. And when you are in vtec it consumes very much more. I can account to this with my 88 civic. When i drive it nice i get around 26 mpg but if i spend alot of time doing alot 8 grand shifts (8000 rpms) i can watch the needle drop. Just think the more rpms you have the pistons are moving very much faster making your fuel injectors deliver fuel when need so at 2000 rpms it is doing it alot just not as much as if you where at 4000 rpms it would be twice as much. Sound liek to me you tach is stuck.

[Cary Swoveland]

...Does fuel economy has a direct proportional relationship with engine rpm?...

No. RPM is proportional to vehicle speed, so long as the transmission's gear is not changed. Therefore, as long as the gear doesn't change, if you maintain a constant speed, you will maintain a constant RPM, regardless of whether you are going uphill or downhill.

What changes, between driving uphill and downhill, is the amount of fuel you burn per engine cycle. When going uphill you need to depress the accelerator pedal more, and when going downhill you can ease off the pedal. The accelerator pedal controls the amount of air that enters each cylinder. (It would be more accurate to call it the "air pedal".) When more air enters, the fuel injection system adds more fuel, to achieve an optimal air/fuel ratio. The extra fuel is needed to generate more force on the piston, and thereby deliver more power to the wheels.

Cary

[jsw2233]

No. RPM is proportional to vehicle speed, so long as the transmission's gear is not changed. Therefore, as long as the gear doesn't change, if you maintain a constant speed, you will maintain a constant RPM, regardless of whether you are going uphill or downhill.

What changes, between driving uphill and downhill, is the amount of fuel you burn per engine cycle. When going uphill you need to depress the accelerator pedal more, and when going downhill you can ease off the pedal. The accelerator pedal controls the amount of air that enters each cylinder. (It would be more accurate to call it the "air pedal".) When more air enters, the fuel injection system adds more fuel, to achieve an optimal air/fuel ratio. The extra fuel is needed to generate more force on the piston, and thereby deliver more power to the wheels.

Cary

Thanks for the explanation. The impression that when I step on gas and rpm grows faster than speed should be due to a downshift. What you are saying that is with the same rpm, more air will burn more gas. Thanks again.:)

[sleeksilver]

well i can tell you this the higher the rpm the more gas you are using. And when you are in vtec it consumes very much more. I can account to this with my 88 civic. When i drive it nice i get around 26 mpg but if i spend alot of time doing alot 8 grand shifts (8000 rpms) i can watch the needle drop. Just think the more rpms you have the pistons are moving very much faster making your fuel injectors deliver fuel when need so at 2000 rpms it is doing it alot just not as much as if you where at 4000 rpms it would be twice as much. Sound liek to me you tach is stuck.

First off, VTEC on the K24 in the G2-G3 CR-V engages at 2200RPM so that is unavoidable. If you are at 5000RPM and off the gas coasting downhill you aren't using a considerable amount of fuel since the throttle is closed.

Second, I doubt his tach is stuck. RPMs remain constant when traveling at a set speed.

[88civicb16]

i don't know about you, but when I go up hill my car usually down shifts and my tach jumps. Or if the car starts to bog down a little you can watch the tach fall a little. Pretty sure you could watch tach movement.

[sleeksilver]

i don't know about you, but when I go up hill my car usually down shifts and my tach jumps. Or if the car starts to bog down a little you can watch the tach fall a little. Pretty sure you could watch tach movement.

That is going uphill, when the engine is under load.

Going downhill there will be no load on the engine....

[88civicb16]

This is probably an entry level question on cars. Does fuel economy has a direct proportional relationship with engine rpm? I do two 30 hwy miles everyday. There are uphills, downhills,and flats, the mpg gauge reads reasonable: lower mpg when uphill, higher mpg when downhill. But the tech is sticked at 2000 rpm the whole time. This confuses me, will there be a different fuel consumsion on the same engine speed? Why isn't my tech shows a different speed going uphill/downhill? Is it broken? Even I was using cruise (at ~65 mph) and the hills are not super steep, I am still expecting different rpm.

Thanks!

"But the tech is sticked at 2000 rpm the whole time"

So if he is going uphill the motor would be under load which would cause a down shift which would cause the car to shift to a lower gear and the tach to move.:)

so from this piece of the info the tach not moving on the uphill the downhill the flats the whole trip would tell you hey maybe it is stuck. Plus there has to be some take off point which would be starting in first gear and the car would have to shift into hopefully all the gears so the tach should move for this also.:)

Not trying to be rude, just sometimes there is more info in the question then some people pull out.

[gary696]

This is probably an entry level question on cars. Does fuel economy has a direct proportional relationship with engine rpm? I do two 30 hwy miles everyday. There are uphills, downhills,and flats, the mpg gauge reads reasonable: lower mpg when uphill, higher mpg when downhill. But the tech is sticked at 2000 rpm the whole time. This confuses me, will there be a different fuel consumsion on the same engine speed? Why isn't my tech shows a different speed going uphill/downhill? Is it broken? Even I was using cruise (at ~65 mph) and the hills are not super steep, I am still expecting different rpm.

Thanks!

What does the tach read at idle? 2000? If not, must not be stuck.