slightly bigger tires=super bad gas mileage?

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??

How much heavier are the wheels? I know swaping from 15s to 22s made that much difference for me, easy.

uhh not sure how much exactly. rims went from 16x7 to 17x9. a few pounds per corner?

Rotating mass creates a bunch more load than you would expect. So a few extra pounds on each corner adds up and is a significant difference.

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Originally Posted by bangbangskeetskeet

Rotating mass creates a bunch more load than you would expect. So a few extra pounds on each corner adds up and is a significant difference.

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Going from 16s to 17s "SHOULDN'T" cause 4 mpg difference though. Depending on the wheels, you're talking about 25 pounds added at the MOST. Then bigger tires, maybe 50 pounds total. Not like the 200 pounds I increased.

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Originally Posted by pl4yboy

Going from 16s to 17s "SHOULDN'T" cause 4 mpg difference though. Depending on the wheels, you're talking about 25 pounds added at the MOST. Then bigger tires, maybe 50 pounds total. Not like the 200 pounds I increased.

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True story. It could happen though......

did you adjust the computer? Bigger tires actually go further then smaller ones...so if your odometer says 200, you may be going 240...which would increase gas mileage


Weight makes a big difference. I went from stock 245/70/16 (about a 28") to a 36" tire, that is double the weight....my gas mileage went from about 24 on average to about 13 on average. (that is with speedometer adjusted)

10 lbs rotating mass is the same as 100 lbs total mass

I did something similar. I went from steel 235/75/16 to 2009 Tahoe 265/70/17's on my 2004 Silvy. I'm pretty sure mine decreased a little, but not really sure.

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Originally Posted by gravedigger472

temp dropped from mid 40's to below zero

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This will cause a noticeable decrease in mpg, especially if your mileage is based on around-town driving and not highway miles. Colder air requires richer fuel mixtures, and it takes longer before the engine achieves closed-loop mode, where the O2 sensor controls the air/fuel ratio. Depending on your type of driving , 4+ mpg decrease is not unreasonable.

I go from 24 average to 18 average switching to my winter tires and rims, 18 pounds difference at each corner. I dunno bout you but here the winter gas has more ethanol in it too, so thats a loss.

Like many have said, it's not just the issue of larger tires.

It is likely a cause of many different factors. Here are some possible contributing factors:

-Decrease in temperature. The colder it gets, the longer it takes for the engine to warm up causing more fuel to be burned
-Increase in tire size and rotating mass. Likely caused about a 1 mpg loss
-Bad gas. Did you fill up at walmart or other cheap gas station?
-driving habits. You may drive the same roads to work or school, but if there is traffic, or you were in a hurry, it could have a big affect on gas mileage.

How long did you log your gas mileage? If it was less than 2-300 miles then I wouldn't say that it it's a valid decrease just yet.

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Originally Posted by Madd Maxx

This will cause a noticeable decrease in mpg, especially if your mileage is based on around-town driving and not highway miles. Colder air requires richer fuel mixtures, and it takes longer before the engine achieves closed-loop mode, where the O2 sensor controls the air/fuel ratio. Depending on your type of driving , 4+ mpg decrease is not unreasonable.

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Well said Sir. :D

When I switched over from factory sized tires (cant remember what size exactly) to some really hevy BFG Mud Terrains 287/75/16 and they weigh a ton my mileage went from 19 to about 15 on the highway so I feel ya man... especially since premium is hoovering around 4 bucks

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Originally Posted by tatersalad25

When I switched over from factory sized tires (cant remember what size exactly) to some really hevy BFG Mud Terrains 287/75/16 and they weigh a ton my mileage went from 19 to about 15 on the highway so I feel ya man... especially since premium is hoovering around 4 bucks

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I would venture to say this guy got it figured out over a year and a half ago.