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Old 04-28-2009, 08:10 PM   #1
Blone
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Default good highway mileage, but bad city mileage

I have a 99 V6 prerunner SC URD 2.2 pulley. I am getting about 17.4 mpg highway, but in the city I am getting about 12.5-13 mpg, easy driving out of boost. what would cause such bad city mileage, but decent highway mileage?
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Old 04-28-2009, 08:40 PM   #2
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Originally Posted by Blone View Post
I have a 99 V6 prerunner SC URD 2.2 pulley. I am getting about 17.4 mpg highway, but in the city I am getting about 12.5-13 mpg, easy driving out of boost. what would cause such bad city mileage, but decent highway mileage?
I read an article about miles per gallon and fuel efficiency and i'll try and sum up what I read.

In theory, an engine, modded or not, is a air pump. It is efficient at a certain speed on the road. Also take into consideration, wheel weight, vehicle aero dynamics, etc. There was math involved over my head, but a vehicle on a highway is moving at a speed and rpm that is efficient, where as in the city, is isn't always efficient as it changes from stop lights, turns, slow downs, etc.

What helps me in the city is to pretend as their is an egg under the gas pedal and you don't want to break it. Gradually increase pressure, not boost from every light. Also, avoid braking and coast to lights so you don't 'stop & go' all the time.
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Old 04-29-2009, 09:00 AM   #3
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yeah i would say that sums it up hahaha
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Old 04-29-2009, 09:56 AM   #4
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Originally Posted by Blone View Post
I have a 99 V6 prerunner SC URD 2.2 pulley. I am getting about 17.4 mpg highway, but in the city I am getting about 12.5-13 mpg, easy driving out of boost. what would cause such bad city mileage, but decent highway mileage?
stopping and starting; inconsistent speeds (also not running at engine's efficient RPM)
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Old 04-30-2009, 12:47 PM   #5
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stopping and starting; inconsistent speeds (also not running at engine's efficient RPM)
A change in gears or smaller tires would get you back to normal in city. Ever heard the expression "highway gears" ?
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Old 04-30-2009, 02:41 PM   #6
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The truck has stock sized "31 tires, and the stock 4:10 gearing. I understand the engines efficiency at certain speeds, but I just dont understand how the mileage can be that low compared to other trucks city mileage with comparable mods. I would expect 15 city at the worst, but 12 is horrible especially considering the highway mileage is decent. It just seems like such an abnormally large difference between city and highway mileage. I have been driving very easy as well to maximize mileage, and I do coast as much as possible up to lights and such.

I know the guages arent entirely accurate on these trucks, but I recently filled up, and drove 200 miles round trip all highway, and the guage read barely under half tank at that point. as soon as I started driving in the city again it only took 25 miles to go from slightly under a half tank, all the way down to the first mark on the guage above the red empty mark (1/4 tank I think?). If I fill up, and drive only in the city I only get about 100 miles at the half tank mark every time!
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Old 04-30-2009, 03:38 PM   #7
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Do you have a piggyback? Could it be a bad tune? When did the low city MPG start? Do you have an AFR meter/gauge to compare city vs. highway driving? Do you have an OBDII tool to compare fuel trims?

Have you checked the obvious like exhaust and vacuum leaks? Just a wild guess, but here's my logic. Vacuum leaks sometimes only occur under certain air flow conditions, and maybe the post-MAF vacuum leak occurs only when in low throttle city driving.

Items like cats, oxygen sensor, and AFR sensor may generally increase your gas mileage on a high odometer truck like yours, although I don't see them as the cause of particularly low city driving MPG.
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Old 04-30-2009, 04:31 PM   #8
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Do you have a piggyback? Could it be a bad tune? When did the low city MPG start? Do you have an AFR meter/gauge to compare city vs. highway driving? Do you have an OBDII tool to compare fuel trims?

Have you checked the obvious like exhaust and vacuum leaks? Just a wild guess, but here's my logic. Vacuum leaks sometimes only occur under certain air flow conditions, and maybe the post-MAF vacuum leak occurs only when in low throttle city driving.

Items like cats, oxygen sensor, and AFR sensor may generally increase your gas mileage on a high odometer truck like yours, although I don't see them as the cause of particularly low city driving MPG.
I am using the split second unit. I do have and AFR meter, and the obdII scan tool. No exhaust or vacuum leaks that I can tell, I checked the vacuum lines pretty well about 6 months ago, maybe I should check again. The shop I took it to when I replace the cat (it was bad) said the truck runs clean before the cat, and nothing seemed out of the ordinary, i figured they would have found a leak somewhere... The MAF, 02 sensor, TPS sensor, Catalytic converter and anything that could link to this problem have been replaced recently. The low city MPG's started well over a year ago or more.
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Old 04-30-2009, 06:16 PM   #9
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And what did you change "a year ago or more?"

As I said, compare the AFR and fuel trims between city and highway driving conditions.

Hoses most commonly split and small vacuum leaks go undetected. These often cause intermittent and inconsistent vacuum leaks. Check to make sure you don't have a valvecover leak as this can also cause a vacuum leak.

It could very well be exaggerated driving habits too. Whether you're in boost or not, if you're using gas, you're going to have lower gas mileage. For instance, do you coast to a red light, or do you accelerate till you're near the intersection and brake? Do you immediately accerlate to 3000 RPM after the light turns green? Do you use your brakes a lot?
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Old 05-01-2009, 02:42 AM   #10
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Do you have a piggyback? Could it be a bad tune? When did the low city MPG start? Do you have an AFR meter/gauge to compare city vs. highway driving? Do you have an OBDII tool to compare fuel trims?

Have you checked the obvious like exhaust and vacuum leaks? Just a wild guess, but here's my logic. Vacuum leaks sometimes only occur under certain air flow conditions, and maybe the post-MAF vacuum leak occurs only when in low throttle city driving.

Items like cats, oxygen sensor, and AFR sensor may generally increase your gas mileage on a high odometer truck like yours, although I don't see them as the cause of particularly low city driving MPG.
X2, but I have 32"s and 4.10 auto and I usually get 100-125 to a half tank and only 200-230 to a full tank. I've got a heavy front bumper though. Did you get it real hot at some point and might have burnt a valve? Tranny problem? Maybe the impellers are getting hot in stop and go traffic?? Throttle body gasket leak or loose SC mount bolts could do that.
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