Tire Change To Improve Mileage

[dhalltoyo]

I have a 2008 Express 3500 Cutaway chassis with dual rear wheels.

 

The only differential available is the 4:10 with the 6.0L.

 

It is obviously harder on fuel than my standard 2008 cargo van with a 3:73 rear end.

 

The factory tires are 225/75R/16's.

 

I found a 235/85R/16 and I was wondering if the higher profile (increased diameter) would improve my fuel mileage without confusing the ECM and torque converter. I can live with the odometer reading differently because I typically drive by GPS which provides an accurate MPH reading, but I don't want to negatively impact the complex workings of the drivetrain.

 

A GM Technician's input would be most helpful.

 

Thanks!

[Zembonez]

I have a 2008 Express 3500 Cutaway chassis with dual rear wheels.

 

The only differential available is the 4:10 with the 6.0L.

 

It is obviously harder on fuel than my standard 2008 cargo van with a 3:73 rear end.

 

The factory tires are 225/75R/16's.

 

I found a 235/85R/16 and I was wondering if the higher profile (increased diameter) would improve my fuel mileage without confusing the ECM and torque converter. I can live with the odometer reading differently because I typically drive by GPS which provides an accurate MPH reading, but I don't want to negatively impact the complex workings of the drivetrain.

 

A GM Technician's input would be most helpful.

 

Thanks!

Larger overall diameter will actually decrease mileage unless you are doing all highway. Around town the added load (and weight) will tax the drivetrain.

[dhalltoyo]

I have a 2008 Express 3500 Cutaway chassis with dual rear wheels.

 

The only differential available is the 4:10 with the 6.0L.

 

It is obviously harder on fuel than my standard 2008 cargo van with a 3:73 rear end.

 

The factory tires are 225/75R/16's.

 

I found a 235/85R/16 and I was wondering if the higher profile (increased diameter) would improve my fuel mileage without confusing the ECM and torque converter. I can live with the odometer reading differently because I typically drive by GPS which provides an accurate MPH reading, but I don't want to negatively impact the complex workings of the drivetrain.

 

A GM Technician's input would be most helpful.

 

Thanks!

Larger overall diameter will actually decrease mileage unless you are doing all highway. Around town the added load (and weight) will tax the drivetrain.

 

[dhalltoyo]

I am an expediter.

 

99% of driving is at Interstate speeds.

[dewfpo]

Narrower tires will provide better mpg, taller may or may not as others have said. On the highway, sometimes they do, in the city, or if you haul heavy loads, or tow, they likely will not. You want to keep the engine at the rpm where it's at it's peak volumetric efficiency while you are driving down the highway at your typical speed. Too low or too high and you will loose mpg.

 

Speed is a huge factor in mpg gains. Air resistance is the square of the speed, it's not linear. You can do a simple test. Run down the highway at 75 mpg and test the mpg. Again at 65 and test the mpg, and again at 55 and you will be very surprised.

 

DEWFPO

[dhalltoyo]

Narrower tires will provide better mpg, taller may or may not as others have said. On the highway, sometimes they do, in the city, or if you haul heavy loads, or tow, they likely will not. You want to keep the engine at the rpm where it's at it's peak volumetric efficiency while you are driving down the highway at your typical speed. Too low or too high and you will loose mpg.

 

Speed is a huge factor in mpg gains. Air resistance is the square of the speed, it's not linear. You can do a simple test. Run down the highway at 75 mpg and test the mpg. Again at 65 and test the mpg, and again at 55 and you will be very surprised.

 

DEWFPO

 

 

Yes, I am extremely aware of the resistance factor; hence, I utilize an aerodynamic cargo box to reduce wind resistance.

 

I always drive by the tachometer to obtain PVE.

 

It seems that a larger diameter tire would cover a greater distance at the same RPM's and over the course of several hundred miles (my average trip) there would be an increase in fuel economy.

[95Sierra2500]

You're correct that in steady-state (highway speeds) you'd see a better MPG, especially by only going up from 75 to 85. I do NOT advise going WIDER, however, as that will actually leave you about where you are, if it doesn't just DECREASE your mileage. Remember that by widening the contact patch you create more rolling resistance. You also don't want those duals to touch at the bottom under load, as that will increase wear on the sidewalls and cause them to blow up/separate. You may also want to consider going to a harder compound tire, if possible, as this will reduce your rolling resistance. However, it could also give you traction issues, so experimentation is required.

 

Some other suggestions for you - What speed do you travel at? Knocking 5 mph off can increase your mileage by a point or two. Since you're running pretty well steady-state, opening up the intake and exhaust on the truck will help you out, though I think these work trucks generally have that covered. Getting lighter rims has been proven to help, but since you regularly carry weight that's a factor to consider.

 

Basically, anything you can do to reduce weight (especially unsprung weight) will help the most.

[dhalltoyo]

You're correct that in steady-state (highway speeds) you'd see a better MPG, especially by only going up from 75 to 85. I do NOT advise going WIDER, however, as that will actually leave you about where you are, if it doesn't just DECREASE your mileage. Remember that by widening the contact patch you create more rolling resistance. You also don't want those duals to touch at the bottom under load, as that will increase wear on the sidewalls and cause them to blow up/separate. You may also want to consider going to a harder compound tire, if possible, as this will reduce your rolling resistance. However, it could also give you traction issues, so experimentation is required.

 

Some other suggestions for you - What speed do you travel at? Knocking 5 mph off can increase your mileage by a point or two. Since you're running pretty well steady-state, opening up the intake and exhaust on the truck will help you out, though I think these work trucks generally have that covered. Getting lighter rims has been proven to help, but since you regularly carry weight that's a factor to consider.

 

Basically, anything you can do to reduce weight (especially unsprung weight) will help the most.

 

 

Usually, I drive at 2100 RPM's which puts me at 60 mph. If I get too much lower....the truck wants to downshift on slight grades when in cruise. PVE seems to be at 2100 rpm.

 

I couldn't find any 16" tires in an 85R profile with a 225 width.

 

I didn't think the 235's would touch on the duals, because my loads are not heavy. I don't exceed the 10,000 GVWR.

 

If anybody out there runs across some 225's in a 85R profile I would appreciate the info.

[dewfpo]

Tire tread is a huge factor as well. Depending on your driving conditions, if you can, highway tires will give you far better mpg's than AT's or aggressively treaded tires.

 

DEWFPO

[Darkanion]

PVE seems to be at 2100 rpm.

PVE is a new term to me. How do you determine that?

[svensk]

Just run the stock diameter, but upgrade the POS that came stock to some Michelins or another higher grade tire.

[dewfpo]

PVE seems to be at 2100 rpm.

PVE is a new term to me. How do you determine that?

 

 

PVE used to be considered the rpm at which peak torque is made, but there are more factors involved than that. It's the rpm at which the engine is 'breathing' most efficiently. Folks add exhausts and intakes and that can change the rpm at which PVE happens. For example, if a given diesel engine's torque curve starts to flatten out at 1,600 rpm (it may continue to go up slowly for another 1,500 rpm), then you want your normal highway cruising rpm to be at least 1,600 rpm (depending on load, towing, trailering, etc..).

 

DEWFPO