Newbie advise appreciated

Chris41586 · May 2, 2020

7 hours ago, JimCost2014 said:

Kind of sounds like they knew what they were doing. If you are not getting the "flares" of RMP's going up and down, thinking you are ok.

Have you checked to see where your speedometer is? Did they do a relearn, or are you off because of the tire/wheel size?

I just thought of something (sorry to ask so many questions lol): are you saying it sounds like the previous owner "did" do some sort of tuning or adjusting to make up for the lift and tires?

JimCost2014 · May 2, 2020

45 minutes ago, Chris41586 said:

I just thought of something (sorry to ask so many questions lol): are you saying it sounds like the previous owner "did" do some sort of tuning or adjusting to make up for the lift and tires?

Ask away, if you are off on the speedo, I would say that they did not have it reflashed to recognize the larger diameter tires.

Pretty sure the dealer can do it for a fee, or you can buy a programmer and set it yourself along with different shift points, but you may also void your warranty doing so.

JimCost2014 · May 2, 2020

1 hour ago, Chris41586 said:

I just thought of something (sorry to ask so many questions lol): are you saying it sounds like the previous owner "did" do some sort of tuning or adjusting to make up for the lift and tires?

You might want to get involved with this thread:

Speedometer calibrators

Ravenkeeper · May 2, 2020

On 4/30/2020 at 1:58 PM, Chris41586 said:

Hi everyone! I found this forum, in hopes that I could get some advise from experienced and knowledgeable people about trucks, performance, ECT... So a few months back, I purchased a 2017 Sierra 1500, with the 6.2 motor. It came with a 6" lift and 35" tires. I love the truck, but noticed immediately, that I'm only averaging about 12mpg. Recently, I came across a guy with basically the same truck, but a Silverado, and he insisted that I need to buy both a superchip F5, as well as a pedal commander, to both help with gas mileage, and says that my transmission will go out if I don't do this, because it's lifted, and it'll keep shifting too low of rpms. Does anyone know if I actually "need" to do this, and how I can tell if it's already been adjusted by the original owner? It seems to be running just fine, but I can't find anything that tells me the correct shifting rpms. I want to give my truck what it needs, but also can't spend a bunch of money on things that aren't necessary at the moment. I would greatly appreciate any advice on that, as well as why my mpg are so damn low. Thanks!

Are you going off of the truck's computer to see that you're getting 12MPG? The onboard computer tends to be wrong, you're most likely getting less than that.

Chris41586 · May 2, 2020

14 hours ago, Ravenkeeper said:

Are you going off of the truck's computer to see that you're getting 12MPG? The onboard computer tends to be wrong, you're most likely getting less than that.

Geez,I sure as hell hope I'm not getting any less than that lol. I've hand calculated it a few times, and it's been pretty close to the computer. I suppose that also depends if the pump is stopping at the same exact time every time I fill up as well. Here in Oregon, they pump our gas for us.

Pearl2017 · May 3, 2020

First thing you need to do is quantify the speedometer error from the tall tires. That error will also be the MPG error so it is nice to know. so for a 10% speedo error your actual mpg would be 12 + 10%,1.2 = 13.2mpg

Use a GPS or the Waze app on your phone to get the true speed on highway and note the dash speed difference. I believe that correcting the speedo differential will also help the trans some dealing with the larger heavier tires. The trans functions on torque loads and rotational speeds to calculate all sorts of things. I have the same truck with max tow and get in 13's pulling 10k trailer at moderate speed with large diameter road tires.

Edited May 3, 2020 by Pearl2017

Chris41586 · May 3, 2020

1 hour ago, Pearl2017 said:

First thing you need to do is quantify the speedometer error from the tall tires. That error will also be the MPG error so it is nice to know. so for a 10% speedo error your actual mpg would be 12 + 10%,1.2 = 13.2mpg

Use a GPS or the Waze app on your phone to get the true speed on highway and note the dash speed difference. I believe that correcting the speedo differential will also help the trans some dealing with the larger heavier tires. The trans functions on torque loads and rotational speeds to calculate all sorts of things. I have the same truck with max tow and get in 13's pulling 10k trailer at moderate speed with large diameter road tires.

Holy crap, it'll make that much of a difference to change the speedo? What's the easiest way to do that, take it to a mechanic? Are you getting this numbers with a similar lift and tires?

Pearl2017 · May 3, 2020

Speedo correction wont make more mpg. It will read correctly on dash and I believe my trans TC shudder issues cleared up after correcting using hptuners. Rear is actually lowered and front raised - leveled with bilstein shocks. I have Michelin defenders LT275/65R20 crammed into the wheel wells which are ~10% larger than stock and 53 lbs each. There are several ways to get the speedo corrected either diy or take it to get tuned by someone reputable. If you do a lot of stop and go traffic a lower gear ratio may help your mileage a bit also. tall heavy tires on tall gear you must use a lot more gas pedal to get things moving.

Edited May 3, 2020 by Pearl2017

Chris41586 · May 3, 2020

7 hours ago, Pearl2017 said:

Speedo correction wont make more mpg. It will read correctly on dash and I believe my trans TC shudder issues cleared up after correcting using hptuners. Rear is actually lowered and front raised - leveled with bilstein shocks. I have Michelin defenders LT275/65R20 crammed into the wheel wells which are ~10% larger than stock and 53 lbs each. There are several ways to get the speedo corrected either diy or take it to get tuned by someone reputable. If you do a lot of stop and go traffic a lower gear ratio may help your mileage a bit also. tall heavy tires on tall gear you must use a lot more gas pedal to get things moving.

So basically at the end of the day, 12mpg is just the life of having a big truck huh? I guess I'm just surprised that some people are still getting 18-20 with a similar setup. I was trying to read other conversations on different topics in these forums, but when I go into one, the conversations are like back in 2013. Are these discussions still going on from back then, and I have to skip forward to the last pages? Im brand new to this forum, and I figured out how to post a topic, but not find other new ones.