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**98Camaro3.8** · 08-05-2004, 08:54 PM

Your engine does not know or care how fast the car is going, only how often it has to inject fuel to be burned. MPH does not affect how much fuel is burned, but RPM does.

On a side note, engine load also affects the amount of fuel burned. For instance, you can be driving at, say, 50 mph in either 4th or 5th gear (5 speed). You will be turning fewer RPMs in 5th gear, but it will take more throttle to sustain that speed. Since the engine is under more load, it will require more fuel. I'm not sure which of those 2 is the most fuel efficient.

**93camaro_zzz** · 08-06-2004, 01:31 AM

Obviously your thought is correct. You are covering more distance so your economy will go up. But like 98camaro3.8 said sorta. It takes more power out of your engine to keep your 3500 pound car going 70 (mostly because of wind resistance) than to keep it moving 45. but you will get better fuel economy at 70 than at 45 in about 99% of the cases. Now RPm definetly is a factor, the engine is sucking in more air so fuel is also added to compromise. Now past 70 mph, is where I am thrown in the dark.... I think for example my car for instance.. I was actually wondering if I got better or worse mileage at 90mph thgan at 70mph. I actually think thatr since wind resistant really comes into play. I mean really, comes into play. I think its probably a curve in relation to speed, wind resistance, and rpm. And I have come to the assumption that the curve tops out at probably 60-70 mph. Hope this answers alot of your question. Sorry for so long. Good luck,

**V6Bob** · 08-06-2004, 10:01 AM

Air resistance is the biggest factor. Because of that slower is almost always better for gas mileage.

Engine friction and "pumping losses" count too. At a given speed the highest gear that will run smoothly will give you the best mileage.

"What I want to know is, am I burning the same amount of fuel while I'm doing 45 MPH in 3rd gear as I am doing 70 MPH in 4th gear at 2K RPMS?"

No. Because the air resistance is hugely greater at 70, you'll be pressing down harder on the gas and burning a lot more fuel.

"If I shift to 4th Gear (OverDrive) while doing 45 MPH I drop down to 1.5 RPMS... does this mean I'm saving gas?"

Yep. Less friction and pumping losses in the engine.

**93camaro_zzz** · 08-06-2004, 02:08 PM

Originally posted by V6Bob:
"If I shift to 4th Gear (OverDrive) while doing 45 MPH I drop down to 1.5 RPMS... does this mean I'm saving gas?"

Yep. Less friction and pumping losses in the engine.

In theory. You dont have as much power at 1500rpms than you will at 2000 rpms. So even though 2000 RPMs is 33% faster engine speed, at that lower rpm, you will also be having to push the pedal a little farther. Its a toss up, lower engine rpm, but not as much power. Its just like saying you have a 6 speed camaro and you think going down the road in 6th gear at like 45 mph and like 600-800 rpms is efficient. Not exactly. Just my thoughts. ;)

**Need4Camaro** · 08-07-2004, 07:10 PM

So which would get me the better MPG??

- 45 MPH @ 2K RPMS in 3rd Gear?
- 45 MPH @ 1.5K RPMS in 2nd Gear?
- 70 MPH @ 2K RPMS in 4th Gear?
or
- 80 MPH @ 2.5K - 3K RPMS ???

**V6Bob** · 08-07-2004, 07:56 PM

"Its just like saying you have a 6 speed camaro and you think going down the road in 6th gear at like 45 mph and like 600-800 rpms is efficient."

You can take any argument to an extreme. Which is why I said "highest gear that will run smoothly". The question was 3rd (2K) or 4th (1.5K) at 45. Better mileage in 4th, absolutely.

"- 45 MPH @ 2K RPMS in 3rd Gear?
- 45 MPH @ 1.5K RPMS in 2nd Gear?
- 70 MPH @ 2K RPMS in 4th Gear?
or
- 80 MPH @ 2.5K - 3K RPMS ???
"

Of these, 45 MPH in 3rd is best. Then 45 in 2nd. The increased air resistance blows 70 and 80 out big time, the decreased rpms make the higher gear better at 45.

Then 45 in 2nd, 70 in 4th, 80 in 4th.

**93camaro_zzz** · 08-07-2004, 09:23 PM

I know this is just a discussion, but I think honestly that 70mph at 2000 would beat 45 in 3rd at 2000. Just because the more amount of ground you are covering would outweigh the increased fuel consumption. Just my opnion though. Dont want to make an arguement.

**Turbo V6 Camaro** · 08-07-2004, 09:51 PM

well it takes our cars 11.9 HP to sustain 55 MPH

so weather your 2nd or 9th grear you only need 11.9 HP

but in second the motor EFF% is WAY WAY jacked up be like 20% or so were in 5 gear for a 5 speed it more like 90%

i once drove my car around with out ever using 5th gear and i lost 5 MPG (city/highway)

to a point Lower Rmps is better, i did this one time with a $6000 GM scaner (cuzin works for gm i got the hook up)

at 60 MPH in 4th gear i had ~2500 RPM (this was almost 2 years ago numbers may be off lol) and i think it was 25% on the TSP
then in fith gear (i used cruse control and set it at 60 MPH exacly (using the real time scanner read outs) i was at like 2000 RPM at lie 15% TSP

so the car is really making more with what it has because of the gear. forth gear is a 1:1 crank turns one time so do the tires, 5th gear is .68 i think. so crank turns .68 but wheels turn 1 full turn. so really if you kept it at the same RPM you was in 4th gear, you go faster in 5th gear at the same RPM you was at in 4th

to make it easy say your going 1500 RPMS at 60 MPH in 4th gear and it 1:1.
now you shift it 5th gear and your only turning at 750 RPM but going the same speed because your 5th gear is a .5:1 ratio so for every turn on the crank your tires turn 2 times or go twice as far.

it just like lifting 500 pounds with a pully sytem so really the load on the motor is less, the motor is doing less work

(i had to do this in class one time lol )

oh yeah theres a point were the motor has to do more work becuase you are going to slow for the gear (TRYING TO TAKE OFF IN 3RD GRE FROM DEAD STOP)

[ August 07, 2004, 11:17 PM: Message edited by: viper04af ]

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RPM - Gas relation question

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