Poor Gas Mileage

Recently purchased a 99 3.4L RWD Auto. 220k. running on 265/70/17
I have completed a host of deferred maintenance.
Spark plugs
Thermostat
Coolant flush
Crank and Cam sensors
Drive shaft bearing
Fully clean throttle body and IAC (new gaskets on both)
Cleaned MAF sensor.
New air filter
Oll change with filter
greased every zerk I could find..

Just did the math on my last tank and it came out to 10miles/gal.

CEL works but not illuminated. No Codes

Where should I start to get better mileage?

edit for more info:
45 mph
1450 rpm
Stft -0.78
Ltft 2.34
Maf 14.81 g/s

idle
705 rpm
Stft -1.56
Ltft 5.47
Maf 4.76g/s

 

Are you absolutely certain about your mpg calc?

 

Filled the tank last week and set the trip meter. Today I thought I would be making a trip out of town and decided to fill up before I left. When I arrived at the pump the trip showed that I had traveled 120.2 miles since last fill up. I put 11.79 gal in the tank. 120/11.79=10.17. I will be the first to admit I am terrible at math but I think that is correct. But I have been wrong before…

 

Your estimate should be accurate for a rough estimate. Start using an app to track mileage over a handful of fill ups to get a better gauge but certainly 10mpg ain’t right if that’s actually accurate. Does the engine appear to run and idle fine? Maybe do a test on the coil packs and wires to see if they need replacing?

 

What test can I do?
Never tested wire or coil packs before. Those generally throw a code when they are bad? Also added some fuel trim data to the orginal post

 

What size are your tires compared to what's on the door sticker?

 

Not sure about how to test a wire. This is how to test the coil pack…

 

My single cab 99 with 50k miles and 4cyl got 17-20 max. If the truck is paid off, live with it.

my 05 gets 13-16.

 

Run a GPS speed test and see how far off your speedo is. My guess is those tires are 'under reporting' both speedo and odo.

Take the tire calculator and compare stock to what you have, use that % to up your miles driven, and you'll get a better number.

 

Per the door. 225/75 r15

 

https://www.tacomaworld.com/tirecalc?tires=225-75r15-265-70r17

11.7%

 

I am not convinced what the door sticker states is always accurate. FWIW, mine says the same 225/75r15, but it is a TRD model that came with 265/70r16s with 4.10 gears.

Some of it is going to be the larger than stock tires. Do you have a lift on it? Weight will cause a sharp decrease in MPGs too.

 

Truth be told, I'm not either, but it's a start

 

Mine is the TRD off road also, hard to imagine such small tires/rims being stock.

No lift

 

at 11.7% puts me at 134miles/11.79=11.36mpg
that is better I guess

 

Is that city or hwy mileage?

Is the torque converter locking up when it should?


How are the vacuum hoses?

 

City mileage.
Umm, I don't know. How can I tell if the torque converter isn't locking up as it should?
I have replaced a lot of the vac lines. It is a work in progress...

I am actually looking at the FSM for the Vac line diagram, but i cant find it. Anyone know what section that is in?

 

I recently replaced my O2 sensors with denso. The previous owner used an unknown brand. I went from 23.3 to 25.01mpg. I got 23.3 for 38,000 miles. Never less or more my commute is very consistent. I was shocked the last two tanks. I normally have to fill up every Thursday. I can make it all week now.

Reg cab, 5 lug 2.4L.

Make sure the coolant temps are correct. My fan was locked up for about 1000 miles and the mpg dropped to 22ish

 

Get up to about 50 mph and hold the speed. Without taking your foot off the accelerator tap the brake pedal. The engine rpms should just jump slightly.

 

Will try this