Gas economy: city vs hwy driving, and efficiency mods

Digitalchaos

Registered User
With the increasing gas prices across the nation, I believe it would be beneficial to create a new thread about gas mileage. There are two specific issues dealing with gas mileage which I would like to see addressed.

First, city driving versus highway driving. As many already know, in many vehicles there is a significant difference in fuel economy when traveling short city distances versus long highway differences. Numerous issues have been sited for these differences, including energy wasting stop and go, idle time, efficiency of engine and various RPMs, engine temperature, and others. I would like to obtain information dealing with which of these factors, or perhaps even others, make the biggest contribution to the difference between city/hwy driving. Is there one big thing which causes the most difference, or are these differences really the result of many smaller factors combined?

Next, fuel efficiency improving modifications. This has been covered before, but I think it can always be beneficial to add more detail and acquire new information. Of all the variations which can be made to this car to alter it from stock form, including internal engine, ignition, intake/exhaust, drivetrain, suspension, tires, ect.. which things might have a positive effect on the efficiency of the engine or overall gas economy? What are the realistic differences which can be seen from these modifications? I know, for example, that exhaust modifications which increase airflow while maintaining emissions equipment have often been sited for increased fuel economy, though the difference is not normally discussed. Another example, how might changes to the ignition system, like relocated DIS module for less heat, different plugs/wires/gap, aftermarket modules, ect increase combustion efficiency, if they even can?

Just some recent thoughts resulting from my observations of ever increasing fuel prices, and confusingly watching as my last tank got over 300 miles in normal city driving..
 
Town: Everytime you use the brakes you are losing energy built up. (stop and go)
Solution, don't use brakes and you will get better economy. The same thing happens on acceleration, you want to maintain a constant speed.

The largest factor in a IC engine for efficiency (theoritical) is compression ratio. Increase it too far and you will get knock, but the most efficient engine will be right there. So if you want to ditch all you power with the blower and run 9.5 or 10.1 cr on premium go ahead.

I don't know where forced induction plays into this. If your motor was always making 10psi boost it is obvious you would be burning a ton of gas, but since the CR is up it is probably very efficient at the power level. Turbos are generally %10 more efficient on centrifugal SC's and 20% on Eaton type.
Intercoolers are where it is at for efficiency. Can't go too big. So for the most efficient motor if you could get it started would proabably be a 10 psi turbo on a 7 or less CR motor.

Oblivously stuff like underdrive pulleys, AL pulleys, AL driveshaft, lightweight spining or moving parts will give free horsepower which means more efficiency.
Which brings up another point it takes a lot less power to accelerate a light car, once moving it doesn't really matter. So intown you should gut your interior and pull the boom-boom box.

edit: Fill your tires up to max pressure will help, may be noticable in MPG in extreme cases. IE. try riding a bike with a flat tire, hard wasn't it.

Or you could just blow all of this stuff of like me and drive your car like you stole it. Cause it is FUN!
 
When you do things like intake and exhaust mods it alows more volumetric efficiency or more potential max power, that also means it has the capibility to burn more fuel. I'm not sure how much efficieny change you would see.

I suspose on the intake side of the stroke would go a little easier with a big intake and same for exhaust side. I can't imagine you would gain much. Spining our SC may amplify the results though.

The CR, and then parasetic drags, then car weight is where its at.
 
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