Poor MPG?

[jd4242]

I'm running 31s on 4:10 gears, it's seemingly a good balance.



I have actually, which brings me to my next post, I've cleaned the MAF in this car twice, and I looked at the live data this morning, both the upstream O2 sensors were switching properly, nothing out of normal, so I thought about it, and decided that since the old air filter didn't seal perfectly, box was a bit loose, new one seals fine, that it might be an issue with the sensor itself since the the box hadn't been sealed, so I took some old junk wheels for scrap and picked up a $5 sensor out a 99 Sport with 136K miles, cleaned it, and installed it, and it's seemingly fixed, doesn't smell rich and feels a lot smoother and has more power at road speed, so the old sensor had to about be damaged. I figure the MPGs will go up now since in theory, the sensor was causing a false lean.

Have you adjusted your speedo for the gears and tires??

 

[RandomNerd2000]

Have you adjusted your speedo for the gears and tires??

To be fair, it came with 31s, so I'm not 100% sure if it's been done or not, nor do I know how to do it, so no.

 

[jd4242]

To be fair, it came with 31s, so I'm not 100% sure if it's been done or not, nor do I know how to do it, so no.

From the factory ? ? thought most came with around 28"ish..easiest way to do it is with a hand held tuner

 

[jmvb]

With 31 tires, and 3.55 gears, the speedo is 10% down of the actual speed.

I measured this with a GPS.

In my country, we use KM/H, but I'll try to make a conversion

If the speedo says 56 MPH, actual speed is 62 MPH

 

[jd4242]

With 31 tires, and 3.55 gears, the speedo is 10% down of the actual speed.

I measured this with a GPS.

In my country, we use KM/H, but I'll try to make a conversion

If the speedo says 56 MPH, actual speed is 62 MPH

So you taking that into consideration when doing the milage ? ?

 

[jmvb]

@jd4242 yes, I do

It is something that I don't understand about the gears relations

With my actual 3.55 gears, if I drive to 60 MPH, my tachometer is just under the 2000 RPM
If I rise the gears to 3.73 or 4.10, at the same 60 MPH, the RPM will be more or less than 2000 RPM?

 

[jd4242]

@jd4242 yes, I do

It is something that I don't understand about the gears relations

With my actual 3.55 gears, if I drive to 60 MPH, my tachometer is just under the 2000 RPM
If I rise the gears to 3.73 or 4.10, at the same 60 MPH, the RPM will be more or less than 2000 RPM?

It would be more..BUT the load on the engine would be much less and take much less power or fuel to get to 60..in city for stop and go driving you would gain alot..the added load on the engine trying to turn those larger and heavier tires hurts mpg drastically..also puts added stress on the drive train

 

[RandomNerd2000]

Actually, looking at it, I could just use the GPS my phone has since it shows how fast I'm moving and find out. I know it's not too badly off because I haven't been pulled over yet. I'm personally a fan of 3:73 gears for the 31s, but I'm running highway and like the get up and go in the city. The engine RPMs oddly being higher don't seem to hurt the fuel economy. If you're mostly a highway driver, go with the 3:73s, if you're mostly city, go with the 4:10s, if you're half and half, I'd probably do the 4:10s because it's a 2 ton SUV and you don't need but so much speed, and you'd have great acceleration.

 

[jmvb]

I think that I actually have 3.73 gears

I did the maths

Like I said, when I drive to 60 MPH, the RPM are just down to 2000, lets say, 1900RPM

1900 (RPM) : 0.75 (5º gear ratio) = 2533 shaft turns by minute


2533 : 3.73 (diff ratio) = 679 wheels turn by minute


C= πd (C= circumference, d= diameter)

31 (tires size) X 3.1416 (Pi) = 97.38" tires circumference length


679 x 97.38 = 66129 inches by minute


66129 inches = 1,0437027 miles

1.04 x 60 minutes = 62,4 MPH, which is exactly 100KMH, like my GPS says

 

[RandomNerd2000]

Well there's an about sure way to find out, crawl underneath the back and on the rear differential housing, there's gonna be at least one tag, my 01 has two, one will have something like 4L10, which in my case is 4:10 gears with limited slip, 3:73 with limited slip would be 3L73, in your case, your code shows 3:55 gears, and that's more likely than not what's under there if the truck was stock when purchased, plus considering your fuel economy.

 

[jmvb]

I'll check it out tomorrow, but

1. 1900 (RPM) : 0.75 (5º gear ratio) = 2533 shaft turns by minute
   
   
   2533 : 3.55 (diff ratio) = 713 wheels turn by minute
   
   
   C= πd (C= circumference, d= diameter)
   
   31 (tires size) X 3.1416 (Pi) = 97.38" tires circumference length
   
   
   713 x 97.38 = 69482 inches by minute
   
   
   69482 inches = 1,0966225
   
   1.09 x 60 minutes = 65,4 MPH, far than 62 MPH, which is exactly my gps 100KMH

 

[RandomNerd2000]

So, I've got closure for this thread, solution? Simple, failing plug wires and a dirty MAF, both seemingly helped it and I'm doing over 20 MPG on the highway now, plus it feels completely smooth sitting in the seat at red lights now. I think the plug wires, although not dated that old, weren't a good set and were just failing from being junk, they weren't anywhere near as heavy as the replacements, plus they were seemingly a bit short.