Gas Milage Question

[Scootin159]

Which gets better milage:

3000rpm in 3rd gear w/ about 10% throttle

-or-

2000rpm in 4th gear w/ about 90% throttle

 

[JC]

I think you'll find 3000@10% throttle gets better mileage....though 2000 or lower is best for mileage, at 90% throttle you're getting into the power circuits of the carb, which will
kill the mileage. At 10% you're just on main metering which would be a lot leaner.

JC

 

[Scootin159]

<< I think you'll find 3000@10% throttle gets better mileage....though 2000 or lower is best for mileage, at 90% throttle you're getting into the power circuits of the carb, which will
kill the mileage. At 10% you're just on main metering which would be a lot leaner.

JC >>



It's fuel injected if that makes a difference. Now I know my examples are a little extreme, but what is the general idea behind gas milage & rpm. I know lower rpm & lower throttle will always give you better gas milage, but what mix is generally best (since 1000rpm & 0% throttle isn't really an option to get anywhere).

 

[JC]

Under 2000RPM is generally best for gas mileage. Fuel injection fuel curves are similar to a carburetor's, so that shouldn't make much difference, except maybe better atomization at
lower (all, really) RPM.

 

[Scootin159]

would it make a difference to where my car is geared for high RPM's? Like idle is about 1200 & 70mph in 5th (highest gear) is about 3000.

 

[JC]

At those speeds, 3000 in 5th is prolly better than say, 4000 in 4th. At 4000RPM you're likely in the fat part of the fuel curve too. But if you're using 90% throttle to stay in 5th, lighter
load in a lower gear might help.

 

[Scootin159]

I'd say 40% in 5th, about 10% in 4th @ 70mph

Yeah, peak power is about 3000-4500 (redline@5500). It's a weak engine (~60hp), so I tend to stay in that 3000-4000 range until I reach 'crusing speed', and then I drop the rpm's to ~2000. I just kinda wondered as to if I was killing my gas milage by letting it get into the higher rpm's (I know floring it @ 4000 isn't good for milage, but I'm going about 50% throttle).

 

[yellowvespa]

The answer is simple, but because it's new year eve and I've had a few I have to ramble a bit first.

Years ago, before computers and .. well.. before most of you were born, Pontiac made a car. It was called a 2+2 .. it was a huge long car and was powered by a 421 CI engine with three 2 barrel carbs and many had 4 speeds. Gas was cheap back then but this thing was so bad on fuel that Pontiac had an option. It was an instant readout mileage meter. It was in the dash next to the speedo. Big dial with a red, yellow and green zone. How did they get an instant readout without computer technology? Simple.

Fuel consumption is directly dependant on throttle opening, manifold vacuum and RPM. There are other factors but these are the big three. The best gas mileage is in direct relation to the highest manifold vacuum reading. As you step on the gas, manifold vacuum will drop thus by monitoring the manifold vacuum one can in a sense, monitor gas mileage. A steady light throttle will produce a high manifold vacuum and high gas mileage. Step on the gas at the same speed and gear and the vacuum will drop, so will your mileage. A vacuum gauge is a simple installation and can teach you to get mileage like you read about in the test magazines "how I got 36 mpg in my V-8 Chevy pickup".

Bottom line. Light throttle and comfortable engine RPM (each engine is different) will result in super gas mileage. Vacuum gauges are inexpensive and simple to install.

 

[Sachmho]

60 hp??? wtf... a golf cart?

 

[ander]

Good info...I agree with all of it.

 

[Scootin159]

<< 60 hp??? wtf... a golf cart? >>



Pretty much...1988 Pontiac Lemans. Thanks for the info everyone. That makes sense to me that a steady throttle will give good gas milage. Just curious though, what would you do on a car with a turbo/supercharger? In those cars there should be a area of high pressure in the manifold & not low pressure like in 'normal' cars. Would you measure the vaccum in the air intake prior to the turbo/supercharger?

 

[Cooltech2k]

<< Under 2000RPM is generally best for gas mileage >>



Sometimes, But Mine Gets it's best mileage about 4k....

 

[thomsbrain]

My Jetta seems to get 30 MPG no matter how I drive it.
4200 RPM @ 80 MPH in 5th. (vroooom! "i'm deaf!")
They should have put a taller final drive in that thing.

 

[Scootin159]

<< My Jetta seems to get 30 MPG no matter how I drive it.
4200 RPM @ 80 MPH in 5th. (vroooom! "i'm deaf!")
They should have put a taller final drive in that thing. >>



I agree, but after thinking it over, my car came out during the 55mph national speed limit, and 60mph is just about right in 5th gear. Now however I could use a much taller final drive (3500rpm @ 75mph in 5th).