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Old 11-02-2020, 04:05 PM   #1
rbourgeo
 
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Default Dogshit MPG - tire difference

Getting absolute terrible MPG. Latest around 11mpg.

1996 V6 4x4 auto. I致e got the bigger 31x10.5x15 tires
I知 aware there is a difference in the odometer due to the larger tires, so I知 actually going further than it says, but still seems to bad on fuel. I知 in Canada so fuel is expensive. I知 not driving much these days due to covid, but I知 frugal, so it just hurts filling up twice per month to only go 700kms.

Has anyone tried quantifying the difference of 235/75/15 tires vs 31x10.5x15?

I致e got a canopy (100-125lbs) on and a nesting boat I leave in my truck (100lbs). These probably hurt the mpg significantly.

New spark plugs, wires, rotor, cap, coil. Fuel filter, air filter, pcv. New timing belt.

Any ideas as to what I should check? Or just a combo of the bigger tires and weight in the back? Fuel injectors? O2 sensor?
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Old 11-02-2020, 04:08 PM   #2
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this truck not matter what engine just gets shit mileage. welcome to the suck.



granted all that stuff impacts you, but so does mileage on the engine (wear), how you drive etc. my 96 4cyl gets 17mpg city. and i drive almost exclusively city with it. hooray.
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Old 11-02-2020, 05:23 PM   #3
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If you use a GPS to find your true MPH, oh sorry KPH, then you can figure what percentage that your speedometer is off. Apply that percentage to your miles/kilometers driven. Use the new total divided by the number of gallons, oops, litres, used between fill ups to calculate your miles per gallon, or kilometers per gallon, or kilometers per litre, or whatever the rest of the world uses with the exception of the U.S. I have an after market rear view mirror camera with GPS that displays my direction and MPH. I do not need to look down at my speedometer, or calculate the 10 or 11% that my oversized tires cause the speedometer to be off. Since I already drive over the speed limit, a true readout of my speed becomes important in avoiding tickets. I once had a SE V6, and had poor mileage too.
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Old 11-02-2020, 05:26 PM   #4
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yeah i guess... would be nice to get like 16 mpg though...

I wouldn't have an issue if we could cheaply insure a second car in this province. Pretty much nil discount on a second vehicle, even though i can only drive 1 at a time. Already costs $1300 per year, with no collision..., and i've never been in an accident. Better to just consume more gas - sorry environment...
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Old 11-02-2020, 05:43 PM   #5
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Yah, insurance sucks down here too. Liability used to cost me about the same as you, before I started using my discounts. I get reduced rates for owning a home, paying 6 months at a time, and going paperless. All tolled, this brings me down to just less than $800 a year. I also tell ask the agent to say "Hey" to Flo each time I call. Since I drive a company vehicle, I put very few miles on my D21 on the weekends, I hear advertisements from Allstate that claim "Pay by the Mile" policys, I am dubious, but maybe I will give them a call in March to see what that looks like.
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Old 11-02-2020, 05:51 PM   #6
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Quote:
Originally Posted by nunya View Post
If you use a GPS to find your true MPH, oh sorry KPH, then you can figure what percentage that your speedometer is off. Apply that percentage to your miles/kilometers driven. Use the new total divided by the number of gallons, oops, litres, used between fill ups to calculate your miles per gallon, or kilometers per gallon, or kilometers per litre, or whatever the rest of the world uses with the exception of the U.S. I have an after market rear view mirror camera with GPS that displays my direction and MPH. I do not need to look down at my speedometer, or calculate the 10 or 11% that my oversized tires cause the speedometer to be off. Since I already drive over the speed limit, a true readout of my speed becomes important in avoiding tickets. I once had a SE V6, and had poor mileage too.

That would give me my accurate current MPG, but i never drove the truck with the smaller recommended tires for a comparison. I'm wondering if anyone actually measured the difference when putting on larger tires? I know there would be a ton of guess work. Driving style, driving area, temperature, tread pattern etc.


Would be interesting if there was a rough formula, but i guess it would vary to much by engine. Something like: the tires are %13 wider, x amount heavier, what would be the net expected decrease in fuel economy.
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Old 11-02-2020, 06:08 PM   #7
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The V6 4wd trucks tend to do better than the 4cyl. More power to move more weight. 11 seems pretty low though. Timing off?



My V6 truck before bigger tires, bigger motor could do 25MPG at 70 all day long. I now see about 18MPG after a TBI VG33 with headers and 31s.
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Old 11-02-2020, 08:14 PM   #8
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Quote:
Originally Posted by rbourgeo View Post
Getting absolute terrible MPG. Latest around 11mpg.

1996 V6 4x4 auto. I’ve got the bigger 31x10.5x15 tires
I’m aware there is a difference in the odometer due to the larger tires, so I’m actually going further than it says, but still seems to bad on fuel. I’m in Canada so fuel is expensive. I’m not driving much these days due to covid, but I’m frugal, so it just hurts filling up twice per month to only go 700kms.

Has anyone tried quantifying the difference of 235/75/15 tires vs 31x10.5x15?

I’ve got a canopy (100-125lbs) on and a nesting boat I leave in my truck (100lbs). These probably hurt the mpg significantly.

New spark plugs, wires, rotor, cap, coil. Fuel filter, air filter, pcv. New timing belt.

Any ideas as to what I should check? Or just a combo of the bigger tires and weight in the back? Fuel injectors? O2 sensor?
If your MPG is off, the MAIN reason is probably those large tires. The engine has to work harder to move them around on the road. They create more drag as well.

Plus you are calculating using a computer that is getting 'abnormal' readings due to the tire size. You have to figure out a way to calculate mileage using real-time numbers on distance actually traveled...not what the odometer is currently telling you.

When I had bigger tires on my truck for a while, the speedometer would read FASTER than I was actually going. I found this out using those radar signs they put up that tell you how fast you are going. My truck would consistently read three miles an hour MORE on the speedometer than what the radar sign said. For example: Sign says you are going 25MPH, but speedo said 28. When I switched to the tires I have now, which were still 15" tires but not as wide, but taller...it dropped to 25 on the radar sign and 26 in the cab. Closer to normal.

Right now, the Teal Terror gets 27.5 per gallon highway, and 25 combined. That's with a camping gear load. Unloaded with the canopy, it can get almost 30 on the highway if I keep my foot off the gas pedal as much as possible and just cruise reasonably at 60MPH. I run the four banger engine. Mileage tests were done on camping trips where I knew the EXACT distance on the freeway between one point, and another point, which was a gas station. I would fill to the very top. I would do that again at the gas station. Instead of using the odometer, I used the known mileage between those two points, aka first gas station and the second one.

It's also possible that your computer isn't helping you get the best mileage because it is getting incorrect readings due to the large tires, but I'm not really sure on that. Supposedly a dealership can 'fix' this with your computer somehow (not sure, but I have heard this) but it costs a lot of money. In any case, automatics will always get lower mileage in these trucks, especially since they aren't the new-fangled transmissions they have today.
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Last edited by XoXSciFiGuy; 11-02-2020 at 08:25 PM.
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Old 11-02-2020, 10:26 PM   #9
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Bigger tires would make the speedometer read LOWER than ACTUAL speed. Simple math, the speedometer reads revolutions of the transmission, thus revolutions of the tire. Correct size tire has a circumference of "X", the larger tire has a circumference of "X plus 1". Each revolution adds "plus 1" to the distance travelled with each revolution of the tire.

Last edited by nunya; 11-02-2020 at 10:28 PM. Reason: poor spellin'
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Old 11-02-2020, 11:55 PM   #10
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A tire calculator tells me the circumstance is %7.3 larger. So multiplying my mileage (kilometreage) by 1.073 will give me a close enough number to what I’m actually getting.

Each forum I look at always ends up in a tire size vs odometer discussion. To me it seems pretty simple. Larger tire means fewer rotation per km= lower displayed speedometer and odometer.

This may be a better question for a physics forum. Increased rolling resistance and what not.


Last I checked timing was 15 btdc. I should recheck. Deerhurst, I believe yours is 33 due to your unique setup. How exactly did you decide on 33? Trial and error, feel, sound?
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