Bigger tires - better gas mileage?

[Number21]

My truck doesn't have a tach so I calculated the highway engine RPMs at about 2400 with no overdrive, 4.11 gears, and 33" tires. I'm always looking to get better gas mileage, and, I've been considering replacing the 33s with something a bit smaller. On the other hand, that raises my engine RPM.

Would I actually get better MPG with bigger, rather than smaller tires? How would 35s affect my gas mileage? Engine RPM would drop to around 2200 on the highway. As I have 16.5" wheels bigger is definately easier to find....

[INSIDIOUS '86]

You can gain highway mileage but keep in mind that city mileage will drop trying to get those stones spinning to speed again from a stop
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[Dunenutt]

With bigger tires, you have more rotating mass. Higher in the air also causes more turbulance and drag. It has been my experience that your mileage will drop significantly.

2200 - 2400 is what I would call perfect cruise RPM. Both lower or higher will suck gas.

[76stepsidechevy]

I just went from 33s to 35s and I saw an increase. My daily commute is pretty much all highway though. I dropped about 300 rpm so with 3.73s and no od I run about 2200 at 65.

[Number21]

Originally Posted by Dunenutt

2200 - 2400 is what I would call perfect cruise RPM. Both lower or higher will suck gas.

Why is that? Wouldn't an overdrive trans have me around 1800 or so?

[TheApocalyptican]

Originally Posted by Number21

Why is that? Wouldn't an overdrive trans have me around 1800 or so?

The idea is that the engine is most efficient in that range and that being lower, the engine makes less power, thus not being as efficient but also causing one to push the pedal in more often to compensate, and higher rpms are self explanatory. Usually switching to overdrive allows the use of lower gears without hurting the highway mileage. It also allows someone with a already lower gear set to drop their rpms to a more acceptable range.

With my Sub, I run under the said rpms at 65, and it doesn't like to run right at 65. Bump the speed up to 70 and it's right below that range, and the truck is much happier there.

[bowtiemanchevyman]

i would change your gears to 3 73. i have them with 33 and i get 16 with a manul 4 speed.

[Number21]

Just out of curiosity, what's the biggest tire I can put on my truck without a lift?

[Your41Plague12]

For a K20 I'd run a 33x11.50 or so, it's equivalent to a 285/75/R16. I know you're running 16.5 wheels though, so you'll have to find a standard size around there. 33x12.50's will rub when you turn your wheels to lock and hit a bump, I wouldn't recommend them. Plus if you're looking for mileage, tall and narrow is your friend. Find a narrow 33 and you should notice a little bit of an increase.

[donut]

No comment on larger tires, don't know. Depending on the actual cruise rpm they may help, at the expense of lower rpm driving.

I wanted the torque down low, so I went with a 245/75 series tire, iirc 31" close to what I think was stock. 65-70 mph was showing 3000-3200 on my tach. Mpg noticeably dropped @ 70. I toyed with the thought of OD, but with the cost, I figured I could buy a lot of gas.

A friend of mine was getting 15 mpg with a 6.0l and OD in a 3/4 ton, so I didn't think 13 mpg with 4.10's and no OD was that bad.
His truck was more comfortable, but I wasn't shelling out hundreds a month for the privilege.

Bottom line, how did you calculate the rpm? Maybe borrow a tach off a buddy for a week?

[N2TRUX]

You need to factor in more components to this equations. What cam is in your motor? At what RPM does it produce optimum hp/tq numbers. Until you calculate that, you won't know if a change in your RPM will improve mpg or not.

In general, a taller tire will also have more section width, more mass, and weight. All of those create more rolling resistance. Those are not conducive to better MPG.

[C-10 simplex]

i've thought about this alot. A smaller diameter tire will raise rpm, but higher rpm does not always mean higher fuel consumption. Remember that some of these c-10's came with 195/75's as standard equipment. A larger tire may weigh more which will probably take more energy to turn. i guess the only way to know is to experiment; i really think you need a tach.




Now, what i would like to possibly experiment with is the use of high-pressure tires such as Michelin XPS rib which takes up to 80psi air pressure----i'm thinking this might equate to lower rolling resistance? But the tire weighs 47 pounds as opposed to the 30lb tires that i have now. 195/75's generally weigh 20lbs.

The tire might do well mpg-wise for strictly highway driving.



P.S. Actually, what i really want to know is what tires do the new silverado xFe come with? This is the extra fuel efficient model that gets 21 mpg. i snooped around at the local chevy dealer to try to find out but i can't tell an xfe from a regular model and felt kind of rushed because i did not want the salesmen to see me because they would have been on me like a bunch of sharks. Also, the other mind-blowing thing is that some of these 1/2 trucks now come with 14 bolt rear axles!

[Firebirdjones]

I take alot of other things into consideration as well.

The weight of the tire, and also the width of the tire. I like wide tires but it increases rolling resistance, not what you want if you are on a gas mileage quest. Tire tread pattern also plays a roll. More aggressive means more rolling resistance. I also consider the weight of the rim. Anything heavier (wheel or tire or both) will increase the amount of power needed to get the truck rolling. Simple physics.

I've also found air pressures to be a HUGE difference in gas mileage with these larger size tires. I'll use my 72 4x4 blazer as an example. I've been on a quest with this truck to make it as efficient as possible because I've used it as a daily driver for several years now.

With a mild 355, small crane cam, rpm intake, 750 holley, headers and 2 1/2" duals with H-pipe. 3.07 gears with 33x10.5 tires and a 350 turbo trans.
I got 13 mpg around town and 16 mpg highway with this setup. It would cruise at 70 mph turning 2400 rpms. Ran this for a few years. Really a decent mpg vehicle for what it was.

Switched to a 6.0 LQ4 and a 4L60E trans, my custom tuning, everything else remained the same. 16 mpg around town and a best of 22 mpg highway.

Later changed to a 3.73 gear, dropped to 15 mpg around town and 20 mpg highway. More gear actually killed around town mpg which is contrary to what most people think. That 6.0 just liked lugging around with that 3.07 gear.

Made some tuning changes in DFCO and some other small tricks along with switching to an SD tune. Now I'm getting over 17 mpg around town and back to 22 mpg highway with same 3.73 gears. Wish I did this with the 3.07 gears, I might have seen the best mpg yet and probably would have kept the 3.07 gears.

My tires of choice are the BFG all terrains for their mild tread pattern and quiet operation. I also chose the 10.5's for less rolling resistance and they also have a higher PSI rating than the same tire in a 12.5 width 50 psi vs 35 psi (go figure, possibly made with a stiffer sidewall??) the 12.5 tires are also several lbs heavier than the 10.5's. Times 4 and you are adding alot of rotational mass.

Anyway, I've found the sweet spot is 40-42 psi for decent tire wear and gas mileage.
If I simply drop the pressures down to 34-36 psi,,,,my gas mileage drops like a rock. Driving around town all week like that netted me 14.7 mpg. Pump the tires back up and 17+ mpg was recorded.

It's all in the total package. If you think it all through front to back you can actually make these old vehicle fairly respectable in the mileage department.

[Number21]

Originally Posted by Your41Plague12

33x12.50's will rub when you turn your wheels to lock and hit a bump, I wouldn't recommend them.

Hmmm...that's what I have now and they seem to fit fine - no rubbing. A narrower 33" would be nice but I'm not sure they exist anymore for 16.5" wheels.

Originally Posted by donut

Bottom line, how did you calculate the rpm?

http://www.csgnetwork.com/multirpmcalc.html

[76k10]

I run 35s on my K10 with no lift and they are fine, but will lightly rub every once in a while when trail riding. Also saw a large increase in mileage versus the 29s it used to have.

[Firebirdjones]

Originally Posted by Number21

Hmmm...that's what I have now and they seem to fit fine - no rubbing. A narrower 33" would be nice but I'm not sure they exist anymore for 16.5" wheels.


http://www.csgnetwork.com/multirpmcalc.html

You'll need to dump the 16.5 wheels if you want more tire choices.

I'm going to have to do the same thing on my 79 1 ton. Getting too difficult to find a 9.5x16.5 tire in a "E" load range now.