Mpg down then up?

I’ve got a ‘21 SR5 double cab std bed. It’s got just under 13k miles.
Right around 12k miles I added a transmission cooler, changed engine oil to AMSOIL 0w20, and both diffs to AMSOIL SG75w90. It had the factory 75w85 in the diffs. I’m also still running the ATF WS Toyota trans fluid.

Immediately my MPG dropped literally by 5 mpg’s. I tested my mileage using the trucks mpg display along with the tank to tank calculation. The difference between the two was 2/10 of a mile. The result average was 11 to 12.1 city and 13.5 to 16.1 highway. It’s been this way for about 800 miles now. I spoke to AMSOIL and they were as as baffled as I was.

Today I go and fill up the gas tank and run another mileage test. One running the a/c and the other without. I drove identical routes and speeds. The route was 18 miles. 9 miles up a gradual grade and 9 miles down same grade with the first and last 3 mile segments in the city.

To my astonishment the mpg without a/c was 20.6 and with a/c was 19.8! This is right where it used to be and just a little better.

What am I missing here?

 

I don’t have an explanation, but I have never seen 20 miles per gallon in my truck, ever

 

The 20mpg can be attributed to the 9 miles downhill part I’m thinking. Two days ago I did the same downhill but extended it to about 18 miles downhill and only got 16.1mpg.
I’m stumped.

 

So you compared your display mpg and hand calculations and they were both down by 5mpg? I ask because if it was only display settings, and your battery was disconnected, it could be reading odd until things smoothed out. Sounds very strange.

On a side note though, I've sniffed 20mpg but it was a similar setting as you, just a 40 mile round trip that was perfectly flat to go fishing and back home after a fill up. Then normal driving happened and I came back to my normal 15-16.

 

Eventually you will figure out that mpg don't matter and that there is no need to worry about it. You will have to get gas when empty no matter what. Same concept as paying no matter what the gas prices are.

 

I'm solving my MPG issues via the Harrop protocol.

scheduled for next week.

 

For sure! I think he was just concerned with such a huge margin of change.

 

The trucks mileage computer is always grossly off, if you hadn’t been doing actual tank to tank calcs before, you won’t know if your numbers now are askew or not. Start trying to calculate smiles per gallon instead; much easier and beneficial to do.

 

agreed. never let your total tank MPG exceed your avg air fuel ratio.

 

Tank MPG will be all over the place , and short hopping isn't going to give you any useful info, it's a V8 ! Now if you go extended distances it'll make a difference. I just did a 230 mile, mostly highway trip, never over 70 and going easy on the throttle , I hand calculated 19.8 mpg. I snapped a pic of my distance to empty after filling up.

 

Based upon my short ownership, IMO any reading the the dash is giving me is a big "so what" as it is pretty much meaningless big picture. Hand calculations on when you fill up your tank after driving hundreds of miles is the way to do it. And to have accurate comparisons tank tank, you should really try to use the exact same gas pump as there can be differences between pumps on when they shut off, meaning one pump might give you an extra quart or two which obviously changes your mpg, especially on short fill. Just my 0.02.

 

Short tripping in these trucks and driving over 70 mph kills the gas mileage in these things. I average 17 mpg’s but I avoid short trips when possible. After a year of ownership I have have many trips where I average 18ish but they were all highway. The sweet spot was when I was on back country roads doing 55 to 65…I got just about 20. It’s not fun driving on the conservative side but you CAN get respectable (for a 5.7) mpg’s out of the Tundra. And I never idle…

 

I appreciate all the input.

This is my 4th tundra. Of all of them this one has averaged the best in mpg. I should have mentioned the tank to tank calculation vs the computer readout was done a week ago driving over 120 miles on mostly country roads. 2/10 of a mile difference. Pretty darn close if you ask me.

The battery was never disconnected but I’m glad y’all brought it up anyway. Something to remember next time the battery is disconnected.

Just another unsolved mystery.

 

10 to 11 MPG crew reporting in.

 

Is yours a Flex Fuel truck? Sometimes the computer gets confused (I've read, never experienced) after a tank of E85 and takes a few tanks to fix itself.

11mpg member here!!

 

No flex fuel.
10-11 mpg? Sheeeesh!! Sounds like a ‘78, 3/4 ton Dodge Power Wagon I once had. Good ole birdbath hood, too. Towed and hauled like a tank.

 

I have a 21 CM and there is only 1 situation i get 22 mpg, per the computer. Going down a 9,000 ft elevation mountain road for 45 miles, then going 70mph for 45 miles on a flat, open highway. All other times i average 15 mpg.

 

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$39 75w-90 GL-5 Limited Slip, Ultra Long Drain 500k Miles, Manual Transmission, Gear/Differential Lubricant (1 Gallon)

75w-90 in the diffs will result in less fuel economy, can't beat the physics on that. That's the cost of the extra fluid strength. If you drive lots of highway where the diff oil gets hot, the difference is probably going to be hard to measure. When it's that first cold trip down the road, oh yeah you'll see it. Especially if it's slow city driving or cool weather.

 

My commute is 12miles each way (takes 25-30 minutes) and I grab a bite to eat in town for lunch. Avg 36 miles a day, mostly City streets or congestion. If the highway is empty I drive 80mph. 36 miles driving per day….

 

I appreciate your input as it confirms what I kinda suspected. Heavier gear oil equals better protection but less mpg. 5 mpg less got my attention, though.

Subject change: I ask an AMSOIL rep if there are any particular differences doing a AMSOIL transmission flush compared to the ATF WS flush since Toyota has multiple fluid temperature requirements when working with WS. His response was “that’s why alcohol was invented after having to deal with all the quirkie transmission fluid temperature requirements Toyota came up with!”
He also said to do exactly what Toyota requires as if using WS.
Getting a flush done next week with the AMSOIL ATL.

 

Ummmm NO.... How is the required fluid killing MPG?

 

Viscosity is by definition the resistance to fluid flow. A higher viscosity fluid takes more energy to make it move. This means the carrier and ring gear will have more resistance as it moves through a thicker fluid than a thinner one. If you ever see a truck with a clear diff cover you'd be amazed just how violently the fluid is churned around in there. It also means it takes more forces between the gear surfaces before there's enough to squeeze enough out to make metal contact; this is the more load and shock protection quality.

Fuel mileage is why Toyota switched to 75w-85 instead of the prior 90w spec; why car companies went to thinner 0w-20, 0w-16, even upcoming 0w-12 engine oils because it's less drag on the oil pump to make it move.

If you went more extreme and ran 85w-140 you'd see a few mpg loss. On my 4 cyl truck I tried it once and it definitely affected it. With 75w-85 it routinely got 22-24 mpg, once 28 mpg. With 85w-140 my best hypermiling efforts couldn't ever break 19 mpg.

 

I think the difference between 75w90 and 75w85 is negligible.

 

That's what I thought and why I just used mobil 1

$39 75w-90 GL-5 Limited Slip, Ultra Long Drain 500k Miles, Manual Transmission, Gear/Differential Lubricant (1 Gallon)

75w-90 when I drained the factory fill and break in shavings. Then the truck almost never got up to 15 mpg over three years, unmodified suspension plus a tonneau cover, 255 width stock size P-rated tires, and not so much as a passenger for added weight. For kicks I switched back with the redline 75w-85. The difference is noticeable if you're watching numbers real-time. Plus the aggressive engine braking I was so used to is almost completely gone because it wasn't really engine braking at all, it was oil drag.

Most people wouldn't notice. Most people haven't gotten over 15 mpg with E85 on a 5.7L either.