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slightly bigger tires=super bad gas mileage?

G

gravedigger472

Guest
so several things have changed at once, and the mileage in my truck has dropped by 4.8MPG. (avg of 15 to 10.2 overnight)

tires switched from 245/75/16 to 265/70/17, calculating a 1.1" increase in diameter. (also the rim/tire combo is heavier, but not sure by how much)

temp dropped from mid 40's to below zero

changed the oil (?)

im thinking the only thing that factored my bad MPG was the tires. have you dropped MPG that much from switching to bigger tires?

also the truck is a 1989 4x4ECSB 5.7, 700r4, 3.73. driving style has not changed, and measurement technique has not changed. earlier this summer i replaced the following

CTS
MAP
02
Plugs
CAI
Fuel Filter

so i suspect those are not the issue. my engine is burning some oil, but has been for a while, and am planning a rebuild soon. i guess this could be a factor for my MPG, but like i said, its been this way for a while. any thoughts??
 
Remember that with bigger tires your speedometer will read slower than you're really going when you switch to bigger tires so you're really getting a few percent more miles per tank than your trip odometer would show.

But going to 1" bigger tires is not going to cut your mileage by 33%, something else is going on.
 
It's not unusual for my mileage to drop to 10mpg in the winter. I usually only worry if my warm weather mileage is off.
 
yeah i am, about 15 mins a day in the morning, 6 days a week. never thought that would be a factor, but you are right im sure that it uses a bit of fuel also.
 
gravedigger472 said:
yeah i am, about 15 mins a day in the morning, 6 days a week. never thought that would be a factor, but you are right im sure that it uses a bit of fuel also.
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Plus the truck uses the most fuel when its idling when cold, the fuel injection makes the mixture way rich so enough fuel vaporizes to make the truck run decently.

You may want to look into a block heater, they're cheap and super easy to install and will probably pay for itself in its first week of use.

And, as an added bonus, your truck will already be warm when you're ready to drive it :)
 
YOU know what, we drive "Bricks", I never concern myself with the mileage I get out of my trucks. My 2000 K3500 gets a whopping 9-10 mpg. (454, 4:10's), I bought it to pull my skid steer and the like. My 82 K30, when done, will probally get 10mpg. I bought a different vehicle to run around in in the meen time that gets a whole 18-20mpg, but when it comes to my trucks, I don't care. Even if I DD them which I plan on doing with the 82 K30 when it's done.
 
The cold weather can make a big difference in the mileage, both simply because the temperature is colder and also possibly that you went from summer fuel to winter fuel that generally gives you a little worse mileage.

Overall you have these things working against you.

- Taller, heavier tires
- Odometer will be slightly off (taller tires will make fuel mileage look worse because the odo shows less miles than actual)
- 40+ degree temperature drop
- possibly change in fuel

This is also assuming that the truck was driven exactly the same between the two tanks of fuel.
 
My first thought was, "What kind of tire did you change to?", as a more aggressive tread will hurt mileage. Not to mention all of the other things that were mentioned.

Just a(nother) thought.

Later,
Buddy
 
BUDDY said:
My first thought was, "What kind of tire did you change to?", as a more aggressive tread will hurt mileage. Not to mention all of the other things that were mentioned.

Just a(nother) thought.

Later,
Buddy
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Nope. Type of tread has nothing to do with mileage. Rubber construction. Yeah. Ever felt drag slicks, they are really sticky. But they also have D.O.T slicks that, if driven on normally, have a good tread life.
 
Those tires are 4 percent larger then your old ones so figure that plus the slightly wider (1 inch) slightly heavier wheel and tire combo your looking at about a 7-10 % hit in fuel mileage without correcting your speedo.

How are you calculating your mileage? The top it off method is unreliable and depending on how much fuel you are putting in your tank can become even more or even less reliable. Depending on the pump, grounding, static electricity etc. can determine how much gas a pump will allow you to put in your tank. Obvious this problem becomes less of a factor the more gas you put in. If you have a 20 gallon tank with a 1.5 to 2 gallon error ratio verse a 40 gallon tank you are talking about a percentage range between 10 percent or 5 percent. or more.

To make a long story short there is too many variables (Was your gas cap on tight, is the vent bad etc. To make a determination off one tank of gas. Average it out over 5-10 tanks and if it continues to be low then start trouble shooting. Keep a log of mileage and gas fill ups.

(I have MPG OCD.... I keep track of how poor all of my vehicles mileage is getting. My 85 Burbalanche Gets 15 (Diesel) My 01 Chevy HD Gasser 6.0 gets 10.5 and my 07 Tahoe gets 15.5 Combined city highway.)
 
yes the tires are alot more aggressive, a cooper STT instead of a mostly bald all season.
i know that the calculations im using are not the most accurate in terms of real MPG, but comparing one tank to the next, not alot has changed, and i lost about 100 miles for a tank, so something is up. and its not getting better, the next tank is almost gone already after 4 days, usually i get 8 depending. i love the tires, so i hope that its not all the fault of them.
 
Merv said:
Nope. Type of tread has nothing to do with mileage. Rubber construction. Yeah. Ever felt drag slicks, they are really sticky. But they also have D.O.T slicks that, if driven on normally, have a good tread life.
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You will get much worse mileage with a set of boggers compared to an equal sized set of BFG AT's. Take a straw poll if you don't believe me.

Later,
Buddy
 
BUDDY said:
You will get much worse mileage with a set of boggers compared to an equal sized set of BFG AT's. Take a straw poll if you don't believe me.

Later,
Buddy
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No. Maybe my pick up is a bad example, because it always gets bad mileage. But i went from 285 BFG to 35 in boggers and there was no change. If the throttle is on, it sucks fuel. But that has nothing to do with the tires.
 
Merv said:
No. Maybe my pick up is a bad example, because it always gets bad mileage. But i went from 285 BFG to 35 in boggers and there was no change. If the throttle is on, it sucks fuel. But that has nothing to do with the tires.
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Did you adjust your speedometer to compensate for the difference in size. A bigger tire will "fool" the speedo into giving you better than expected mileage. I'd expect that your corrected mileage is definitely worse.

Later,
Buddy
 
From http://www.hybridcars.com/gas-mileage-factors/winter-driving-and-gas-mileage.html

What causes the decrease in fuel economy during the winter?
  • Engines take longer to warm up. During this time, they are running heavy at high RPMs. Even after you get going, all the car's systems have to work harder to maintain a normal operating temperature.
  • Nobody likes to step into a cold car. Do you ever start the car up and leave it running for a few minutes while you go inside and slug down the second or third cup of coffee? Is the car running while you’re chipping away at the ice on the windshield? All of this burns gas and reduces your overall fuel economy numbers.
  • ... It’s likely to be windy in the winter. The roads can be covered in snow and ice. This all adds up to more resistance.
  • Tires are stiffer. Until they warm up on the highway, they roll less easily. And the tire pressure is likely to fall below the desired range.
  • In the winter, you use your headlights, defrosters, and heaters more, putting additional load on the alternator and thus engine.
  • Winter-blend gasoline is cut with various additives, such as anti-gel agents. As a result, there’s less tiger in the tank.
Can you do anything to steal back a few mpgs from Old Man Winter? Yes. The same things you should be doing all year round. Combine shorter trips into longer ones. Keep your car maintained and your tire pressure up. Don’t speed. If you can, don’t drive until the road is clear and the day warms up a bit.
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I bet it's the winter mix gasoline.

NOTE: Italics are my edit, and I removed some stuff about Hybrids.
 
The aggressiveness (tread pattern) of a tire definitely can affect fuel mileage. More aggressive tread patterns have more rolling resistance, and are usually heavier, which makes fuel mileage worse.

Letting a truck or car sit at idle while warming up also definitely causes worse fuel mileage. I can't give a number for most light trucks and cars but I do know that a medium to heavy duty diesel trucks (bigger box truck, semi tractor, etc...) use around 1/2 gallon per hour per at idle.

It's also pretty common to see a 10-15% change in MPG in a truck just from a temperature change.....this is in a very controlled test setting.
 
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