Drop in mpg

InertiaFaze · December 11, 2016

I have had my 2015 Silverado 1500 crew 5.3 for the past year. One of the many things that I loved about my truck was the fuel mileage. I always averaged 19 to 21 mpg. I am usually driving about 65 mph. When I got my service done at 15000 miles at the dealership I started to see a drop in mpg. I have kept my eye on it and I have taken the truck in for another service and now 6000 miles later I am still getting anywhere between 15 to 16 mpg. Nothing I do changes it. Does anybody have any of an idea on why it is doing this. The dealership doesn't have an idea. Thanks

You've had a lot of great feedback to your post, but how about a simple test? Trade truckswith your brother for 1 tank or 1 week or 500 miles, enough normal driving to determine if it's the truck or the driver.

A Red X study? In a Red X study you swap things between a BOB and a WOW and look for differences in the results. If you were to swap with your brother you'd be swapping not only drivers, but locations/routes, payload(?), and maybe fuel sources.

A few more ideas. Combined with ideas in previous posts

Swap Drivers, etc
Keep an eye on steady state V8 FE @ 40/50/60/70 mph and compare with brother or others on GMT.com
Keep an eye on AFM/V4 operation. Any noticeable difficulty in achieving AFM?
Keep an eye on instantaneous FE while coasting. It should read 99 mpg after a short amount of coasting. This is due to DFCO operation (no fuel consumption). This is a big one.
Keep an eye on 25/50/400 mile FE average. Do you get poor FE even on a 25 mile trip with mostly 55 mph cruising?
Any Remote start idle time?
Tire pressures ok?
Excess weight in vehicle? Tools, etc?
Try premium (even if winter fuel)
Check oil level, oil type?
Check air filter
Check average speed to see if low (more idle time/low speed-city driving than others)
Any 4WD/Auto 4WD usage?
Verify DIC Calculation by calculating manually

Boardmember · December 11, 2016

I calculate mine manually every tank and it has been within +\- .1 of the DIC.

You are an anomaly.

Sean Neilan · December 11, 2016

You are an anomaly.

Make sure you turn your truck off then turn it back on before you compare. It seems like the mpg average will correct itself on a reboot. I consider the mpg shown while continuously running to be an estimate and after the restart to be the actual number.

You are an anomaly.

Make sure you turn your truck off then turn it back on before you compare. It seems like the mpg average will correct itself on a reboot. I consider the mpg shown while continuously running to be an estimate and after the restart to be the actual number.

MICH 5.3 2016 SILVERADO · March 7, 2019

My last 25miles show an average of 11.7 mpg. I had no idea my mileage could get this low. Michigan winter driving

Z45 · March 7, 2019

There are many variables that affect mileage such as: tire inflation, the type of fuel, weather, elevation, the speed at which you drive, the gear in which you drive, the speed with which you accelerate, engine maintenance, excessive idling, cruise control, the grade of motor oil you use, and of course etc. In short, mileage is complicated.

Gagliano7 · March 8, 2019

H

On 3/7/2019 at 10:24 AM, MICH 5.3 2016 SILVERADO said:

My last 25miles show an average of 11.7 mpg. I had no idea my mileage could get this low. Michigan winter driving

How many miles on your truck?

jbibler · October 2, 2019

I recently noticed that my truck performance was considerably worse than usual. My truck is 2015 5.3L and has right at 30k miles. I bought the truck new and from the beginning averaged about 16-18 mpg around town. I would get 20-23 mpg on the highway. The last month I've noticed I'm getting about 12-14 mpg around town and only slightly better on the highway.

Interestingly enough, I got a call from my dealer that OnStar/GMC has notified them of "poor performance" on my truck and I need to bring it in.

Well, like many others I'm sure I avoid the dealership at all costs. The whole process is an inconvenience and their prices for everything are ridiculous.

I'm going to check air filter, oil level and tire pressure. I don't have any lights on or notifications from the truck itself, everything appears to be fine.

I'll report back when I find something out, but taking to the dealer as a last resort.

Josh B.

Grumpy Bear · October 2, 2019

:lurk:

Grumpy Bear · October 4, 2019

It's not winter gas. It's WINTER period. Here is 100K of fuel records. Tank by tank. Winter gas has 1.5% less BTU content than summer gas meaning winter gas has a maximum impact of 1.5% or 0.3 mpg on a 20 mpg truck. As you can see WINTER has a much more profound effect. Winter meaning COLD. You would have to be driving a vehicle capable of 66.667 mpg to effect a 1 mpg difference in mileage based on the fuels composition between summer and winter.

So basically the majority of swing from summer to winter is 'viscous drag' of cold lubricants.

jbibler · October 4, 2019

Grumpy Bear - Good info & I'm sure that may be the case for some folks. In my instance, I'm in Florida (Tampa Bay region) and it's still 90+ degrees out.

Josh B.

Grumpy Bear · October 4, 2019

1 hour ago, jbibler said:

Grumpy Bear - Good info & I'm sure that may be the case for some folks. In my instance, I'm in Florida (Tampa Bay region) and it's still 90+ degrees out.

Josh B.

You do have some seasonal shift, yes? So your swings would be much more muted.

I have this sign at the edge of town where I come in at 45 mpg on cruse control and exactly at that sign hit the cancel button and then observe how far I coast to reach the city limit of 30 mph. There is a hundred yards difference between a 95 F mid day glide down and a 2 AM 65 F repeat. Most of that drag is in the wheel bearing grease in the front hubs. Just an example of viscous drag. Distance is almost half on a 30F winter day. Anyway just given some things to think about and dispense with the idea that 'winter fuel' has anything to do with anything the average guy would ever notice while doing his 'once in awhile' hand calculation checks. :thumbs:

mookdoc6 · October 4, 2019

1 hour ago, Grumpy Bear said:

You do have some seasonal shift, yes? So your swings would be much more muted.

I have this sign at the edge of town where I come in at 45 mpg on cruse control and exactly at that sign hit the cancel button and then observe how far I coast to reach the city limit of 30 mph. There is a hundred yards difference between a 95 F mid day glide down and a 2 AM 65 F repeat. Most of that drag is in the wheel bearing grease in the front hubs. Just an example of viscous drag. Distance is almost half on a 30F winter day. Anyway just given some things to think about and dispense with the idea that 'winter fuel' has anything to do with anything the average guy would ever notice while doing his 'once in awhile' hand calculation checks.

Hey Grumps? I noticed you did put Amsoil into your Rear Differential @ 100K. I do believe I lost couple tenths on MPG's when I converted mine over but I did up the capacity by 2qts so maybe that dinged me too? The gear oil is different grade than OEM but is 100% better imo I am super impressed with AMS gear oils!

Grumpy Bear · October 4, 2019

6 hours ago, mookdoc6 said:

Hey Grumps? I noticed you did put Amsoil into your Rear Differential @ 100K. I do believe I lost couple tenths on MPG's when I converted mine over but I did up the capacity by 2qts so maybe that dinged me too? The gear oil is different grade than OEM but is 100% better imo I am super impressed with AMS gear oils!

I used it because....I needed it like yesterday and a local dealer had some on hand. I could not wait for my regular shipment from Hoer as I hit it on a weekend. Still, I'm not disappointed and it did not seem to have any impact on MPG. Cool! Then again I was already using Red Line 75W90 instead of the OEM 75W85. There is a point were protection is more important to me than fuel efficiency. Did I say that out loud? :crackup:

I bought one more quart than I actually needed. That hurt $$$$$.

Stephen Mastrianni · February 28, 2021

My 2016 5.3 liter ranges 11-12 and I keep it in two wheel drive unless I'm on trails... I thought it was my goodyear duratrac Tires I put on but not sure, no engine or idiot lights on.

Stephen Mastrianni · February 28, 2021

My 2016 5.3 liter ranges 11-12 and I keep it in two wheel drive unless I'm on trails... I thought it was my goodyear duratrac Tires I put on but not sure, no engine or idiot lights on.