Power setting and big tires

[89Vette]

After thinking about this larger tire -- better MPG thing, I have the following ideas/questions....

Too many posters say larger tires cause worse MPG

Smaller tires put down more torque to the road.
Since the axle rotates faster with smaller tires, it's like higher gears, right?
Going to larger tires is like having less gear.

Less gear may make it harder to propel the HEAVY VX from a stop.
Larger tires creates less rpm on the hwy but requires more to launch.

Maybe larger tires raise MPG if primarily driven on the hwy?
Maybe larger tires lower MPG if doing a lot of stop-go driving?

Maybe larger-tire-owners should activate the "POWER" setting so the transmission stays in lower/higher gears longer.
By running in POWER mode, they'd gain back some of the leverage/torque of the smaller wheels.

Just some random thoughts.

[Vendetta]

I can say with near certainty that my larger wheel/tire combination has lowered my fuel "economy," such as it was. The only reason I'm not absolutely certain is that there are a few other factors that may be contributing.

The details: on my first full tank after having the larger wheels and tires, I observed 13.5 MPG. This includes the +10.43% adjustment to the odometer's reading because of the increased overall diameter. During this same period, I had a remote starter installed and would let my car run, on average, for ~5 minutes prior to launch. Furthermore, there was no significant highway driving during this tank - in fact, I would qualify my driving patterns during this time as a "worst case scenario." Additionally, I have a CEL because of O2 sensors, which I understand may impact fuel management. And finally, this is winter gas, which may be less efficient than the summer blend.

I'm going to try and get some highway miles on this tank to achieve a fairer comparison to my average tank. I've not been in Power mode, either. I may wait for the tank after this one, in order to observe one variable at a time.

-V

[SilverBullet75]

One thing to verify from other posters is if they did the calculation for the speedo difference when you change tire/wheel size from stock.

[Vendetta]

I can say with near certainty that my larger wheel/tire combination has lowered my fuel "economy," such as it was. The only reason I'm not absolutely certain is that there are a few other factors that may be contributing.

The details: on my first full tank after having the larger wheels and tires, I observed 13.5 MPG. This includes the +10.43% adjustment to the odometer's reading because of the increased overall diameter. During this same period, I had a remote starter installed and would let my car run, on average, for ~5 minutes prior to launch. Furthermore, there was no significant highway driving during this tank - in fact, I would qualify my driving patterns during this time as a "worst case scenario." Additionally, I have a CEL because of O2 sensors, which I understand may impact fuel management. And finally, this is winter gas, which may be less efficient than the summer blend.

I'm going to try and get some highway miles on this tank to achieve a fairer comparison to my average tank. I've not been in Power mode, either. I may wait for the tank after this one, in order to observe one variable at a time.

-V

Updating with info from tonight's full-tank refill:
15.5 MPG
70% highway driving
Power Mode OFF
CEL still on

Power Mode is now ON for this tank, but I'm not sure I'll be able to replicate the city/highway split.

-V

[89Vette]

Updating with info from tonight's full-tank refill:
15.5 MPG
70% highway driving
Power Mode OFF
CEL still on

Power Mode is now ON for this tank, but I'm not sure I'll be able to replicate the city/highway split.

-V

Yeah,,,that's a toughy. What diameter of tires are you running?

[Vendetta]

Updating with info from tonight's full-tank refill:
15.5 MPG
70% highway driving
Power Mode OFF
CEL still on

Power Mode is now ON for this tank, but I'm not sure I'll be able to replicate the city/highway split.

-V

Nice long highway drive this weekend. What made it nicer was the 370.8 miles I got from the single tank of gas.
18.00 MPG
~85% highway driving
Power mode OFF
CEL still on

-V

[atilla_the_fun]

Larger tires = more weight
Larger tires = greater rolling resistance (larger contact patch)
Larger tires = greater wind resistance
Larger tires = softer tread and design leading to energy loss on deformation (rolling resistance)

[89Vette]

Mostly, I'm talking about tires with similar width to stock. Certainly, no wider than 285mm and treaded for street. I think we can all conceed monster mudders have another purpose.

[Ldub]

Mostly, I'm talking about tires with similar width to stock. Certainly, no wider than 285mm and treaded for street. I think we can all conceed monster mudders have another purpose.

http://www.google.com/url?sa=t&rct=j...DQkWUlEwVSb2fQ

I think the most telling quote from the comments is:

"I agree with the sweet spot. Installing a vacuum gauge is the best method of finding it."

This is why, with the tires, etc that I run, I get better mileage with 4:77's than I did with 4:30's...

And yes, I know all my percentages, & how to properly manipulate the buttons on a calculator.

A little over 2k on the tach is my sweet spot, but the vac load @ whatever speed, with whatever gearing, with whatever tires, will be the tell.

Far too many variables for a definitive answer to a general question.

[89Vette]

Wow. I never drove my Corvette (on the hwy) between 40-60 -- for a LONG stretch. The only speed where I've measured MPG is 70. And, it was in the 30mpg range -- sixth gear.

About the slowest I'd drive in 6th w/o lugging is 60 -- even then steep uphills would require 5th. So, I'm not sure the same approximation applies to the vette. (If so, It must get well over 30mpg in reality.)

That vacuum idea is very good. Not sure how cheap/easy -- but good. OTOH, gut instinct had already told me much over 2k rpms would cause a drop in MPG.

The other issue (and part of the point of this thread), is whether the power option actually improves MPG (vs normal mode) for larger diameter tires -- especially for city driving.

:cheers:

[Ldub]

Wow. I never drove my Corvette (on the hwy) between 40-60 -- for a LONG stretch. The only speed where I've measured MPG is 70. And, it was in the 30mpg range -- sixth gear.

About the slowest I'd drive in 6th w/o lugging is 60 -- even then steep uphills would require 5th. So, I'm not sure the same approximation applies to the vette. (If so, It must get well over 30mpg in reality.)

So comparing apples & oranges shows me...what?

That vacuum idea is very good. Not sure how cheap/easy -- but good. OTOH, gut instinct had already told me much over 2k rpms would cause a drop in MPG.

...And gut instinct is a good substitute for???
Yeh, I'd go with that too...

Re: vac gauge...

Ummmm...yeah, you would have to be able to splice into a vac line, & find a firewall grommet, & mod a universal a pillar pod...not too difficult...

The other issue (and part of the point of this thread), is whether the power option actually improves MPG (vs normal mode) for larger diameter tires -- especially for city driving.

The tc locks @ lower rpm in "normal", @ hwy speed.

Power mode has never been a friend of mileage for me.

:cheers:...

[Vendetta]

I'm on 285/65 R18 Nitto Trail Grapplers. The latest figures are up 2MPG from my last tank, so I'm not disappointed.

Let's see what this Power Mode will do, if anything.

[89Vette]

Whether related to the power setting or not, my last tank did go up (with 32" tires).

The prior tank was 15.1mpg on normal mode. With power mode, it recorded 15.6mpg. So, either it did go up, or it really doesn't matter.

FWIW, I do mostly short trips (5m or less) and most driving is on city streets.

What's more interesting (maybe) is that I went from 14.2 to 15.1 when I replaced my u-joints. I could actually tell the vehicle rolled smoother and definitely was quieter. I'm not surprised it improved, but I was surprised at the amount. I think part of the improvement was due to an increase in outside temps though.

The normal vs power comparison was in the same ambients.

[twalker920]

What's more interesting (maybe) is that I went from 14.2 to 15.1 when I replaced my u-joints. I could actually tell the vehicle rolled smoother and definitely was quieter. I'm not surprised it improved, but I was surprised at the amount. I think part of the improvement was due to an increase in outside temps though.

Anything you fix/replace that makes a noticable difference in how the vehicle rolls should improve your mileage for sure.

But temperature-wise, cold air is better than warm air for performance and efficiency. It's VERY noticeable at 5500 feet here in Denver. I always get worse mileage in the summer (assuming my heavy right foot is the constant) at 90F vs winter at 30F. Hot air = less O2 per cubic foot intake = engine has to work harder to produce the same amount of power. Hence so many "cold air intake" kits.

I suppose that could be offset by warm temps making things roll and lubricate more easily, but that just gets back in to the "too many variables to tell" category.

Basically, we get crap mileage no matter what we do. Plus it's all relative to what you're used to and what you're willing to stomach.

[89Vette]

I agree about the cold vs warm air thing.

The problem with that blanket statement is cold-weather warm-up requires the MOST fuel. So, if your trips aren't long enough (like mine), warm air can prove to be more efficient because the motor has less warming up to do.

Great point(s) though!