tire size vs. mpg - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 13 (permalink) Old 10-13-2009, 01:18 PM Thread Starter
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tire size vs. mpg

my E350 has 245/75/16's now and I am wondering if I switch to the 265/75/16's what I could expect in mpg gains? thanks

1997 f250 4x4 sclb, auto, 3.55's, 191k, diy intake w 6637, ccv mod, pyro, boost, trans gauges, 3" dp to 4" single exh. super chips for now. scangauge
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post #2 of 13 (permalink) Old 10-13-2009, 09:55 PM
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Re: tire size vs. mpg

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my E350 has 245/75/16's now and I am wondering if I switch to the 265/75/16's what I could expect in mpg gains? thanks
You could just as easily lose mileage. There really isn't any good way of predicting what your mileage will be.

With tire size, there's always a sweet spot. Go any smaller than the sweet spot, and you lose mileage. Go any larger, and you lose mileage. Here's just one quick and dirty way of looking at it:

Some of it has to do with your driving style and what your vehicle primarily does most of the time. For instance, those who mostly drive highway might see a slight gain with larger tires because you're running lower average RPM's, but will suffer mileage loss when driving around town. In contrast, if you strictly drive around town (constant start/stop), then you would probably gain with smaller tires, since there is less rolling resistance to overcome when accelerating from a stop.

In short, just because you're going to a taller tire, doesn't mean you'll gain mileage. The only way to know for sure is to try it. If you lose mileage, then go back down in size.

Of course, there are plenty of other factors too, such as wind resistance (increases with vehicle height), tread design (highway tires get better mileage than all-terrains/mud tires), current gearing, etc.

Curtis
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post #3 of 13 (permalink) Old 10-13-2009, 11:15 PM
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Re: tire size vs. mpg

Going from 245s to 265s, I'd expect at least a 0.5 MPG loss unless you drive 500-milke legs on the Interstate and nothing else. If you drive a lot in town, look for a bigger loss.

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post #4 of 13 (permalink) Old 10-14-2009, 01:37 AM
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Re: tire size vs. mpg

I went from 255/85x16 LT Cooper Discoverer to 285/75x16 LT Cooper Discoverer ATR (within one tenth of an inch on height, the 285 is a couple inches wider) and lost 1.5 MPG due to tire width. Put 235/85x16 rib tread on the front only (slightly shorter, much narrower) and will be putting them on the rear when current drive tires wear out. On my F250 the 255/85 filled the wheel wells nicely, the 235/85 is noticeably smaller but still looks OK.

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post #5 of 13 (permalink) Old 10-14-2009, 03:00 AM Thread Starter
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Re: tire size vs. mpg

I think the diameter diff is 1.12" or so. I drive primarily highway and at higher speeds, mostly 70 to 78, when I weighed it, it was at 8400lbs- typical I think for my van. I was thinking the taller tire would lower rpms= better mpg? but I do know that going to a "large" tire like some 4x4's do will lower mpg's. I tend to slow down and get decent mileage as fuel prices increase
I don't feel like I'm doing to bad on fuel economy, just want to get what I can. I am getting 15-17mpg at the higher speeds and have hit 19mpg when slowing to 65 and limiting idle time. Btw, for those of you really trying to get good fuel economy- how long do you idle in summer and winter?? thanks for the input

1997 f250 4x4 sclb, auto, 3.55's, 191k, diy intake w 6637, ccv mod, pyro, boost, trans gauges, 3" dp to 4" single exh. super chips for now. scangauge
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Besides there is absolutely no way to idiot proof something, since they are always coming out with better idiots by the day
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post #6 of 13 (permalink) Old 10-15-2009, 12:54 AM
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Re: tire size vs. mpg

Every time you accelerate, those bigger tires exact a penalty. Wheels/tires are in essence big flywheels, so in addition to accelerating your 8400 lb truck, you have to pour energy into the "flywheels" to accelerate them first.

The moment of inertia of a wheel goes up with the square of the outside diameter. You will be increasing your moment of inertia by about 8%.

I have heard from hundreds of truckers and their story is always the same: Bigger wheels = reduced MPG.

Super-high MPG subcompacts (Honda Insight, Toyota Prius, Geo Metro) all use tiny wheels. Think there might be a reason.

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post #7 of 13 (permalink) Old 10-15-2009, 03:22 AM Thread Starter
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Re: tire size vs. mpg

makes sense, thanks for your answer! what about idle habits?

1997 f250 4x4 sclb, auto, 3.55's, 191k, diy intake w 6637, ccv mod, pyro, boost, trans gauges, 3" dp to 4" single exh. super chips for now. scangauge
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Besides there is absolutely no way to idiot proof something, since they are always coming out with better idiots by the day
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post #8 of 13 (permalink) Old 10-15-2009, 05:22 PM
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Re: tire size vs. mpg

Thanks for your input Dave. You always have a ton of info when it comes to mileage.

Curtis
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post #9 of 13 (permalink) Old 10-15-2009, 08:36 PM
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Re: tire size vs. mpg

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makes sense, thanks for your answer! what about idle habits?
i work out of my truck....summer here in Texas is HOT...
some days will idle up to 4 hours at a time

winter, not as much, but still some, always try to keep it to a limit, but its not worth sitting in a truck not running when its 100+ outside

mines a 99 7.3, and there is a chart somewhere online, but if i recall right its burns diesel at idle at around 1 gallon every 3 hrs?
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post #10 of 13 (permalink) Old 10-15-2009, 09:17 PM
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Re: tire size vs. mpg

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i work out of my truck....summer here in Texas is HOT...
some days will idle up to 4 hours at a time

winter, not as much, but still some, always try to keep it to a limit, but its not worth sitting in a truck not running when its 100+ outside

mines a 99 7.3, and there is a chart somewhere online, but if i recall right its burns diesel at idle at around 1 gallon every 3 hrs?
Bout time you joined jeff

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