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Discussion Starter · #1 ·
Got a question that I couldn't find in performing a search in here for.

98 Jeep Grand Cherokee.

I just changed the wheels on my Jeep. I had 16" wheels on the Jeep, and I changed them to 15". The reason that I switched is because I already owned a very good set of 15" snow tires, and I live in an area where we get a lot of snow. I also didn't mind buying a 15" set of wheels, so that I could have a set to "beat up" during the winter, and have a set to put on during the summer. I plan on buying a good set of "tear drop" wheels for the summer, and a new set of tires for them.

Here is my question.

Since I performed the switch to 15" wheels, it appears that my fuel milage has gone down by 2mpg (according to the overhead in car computer). The night that I switched the wheels, I also put in STP fuel injection cleaner, changed the oil, and the overnight temp was in the teens. When I got up in the morning, I had to drive over 200 miles. On the first 100 mile leg, my fuel milage (by the computer) read 15mpg, and when the engine kicked down into a passing gear, the check engine light would blink. When the passing gear switched to a "non passing gear" the check engine light went back off. On the second leg of the 100 miles, the check engine light never came on (even when in passing gear). The only difference was that the outside temp went up about 20 degrees (23 in the morning, 43 in the afternoon).

So, does the size of the tire change my fuel milage, or is it something else? If I decide to get tires for the summer, am I better off going with the 16", or the 15"?

Thanks, sorry about the long note!
Pittfan
 

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Discussion Starter · #4 ·
Well, I wonder if I need to drive down the interstate sometime, and see how acurate my odometer is (by checking it versus the mile post markers). Maybe the size of the wheel has thrown that off, and thus has caused my computer to read the fuel milage differently. All in all, I keep track of when I purchase gas in a gas book (a little notebook), and I can refer to that and see what has actually happend to my fuel milage and not rely just on the computer to do it.

This all comes about, but it doesn't say anything in the owner's manual regarding this. Also, I know that 15" wheels must be okay for it, because when I went looking for a set of wheels for this, the overall response was 16" wheels, but 15" were available. It was like I had a choice of 4 different styles in the 16" size (stock wheels), and 1 choice in the 15" size.
 

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Discussion Starter · #6 ·
So, there is really no way to calculate the actual mpg unless the speedometer is true. Or, as long as the speedometer stays the same (between the wheel size change), then my calculations and the Jeep's computer should stay the same. The speedometer wasn't true when I tested it for 60 seconds for 1 mile using the interstate mile marker posts. It turned out that when the speedometer read 60mph, I was atually doing 57mph. I haven't tested it since the change to 15" wheels, and I guess this gives me a good reason to test it now.
 

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Chech waht size the actual tire is, like 235 75 15, and compare the 15 vs 16 to get the actual difference in size. You will then be able to tell how far off your odometer and speedo is.
 

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So your old tires overall height is about: (225*.70*2)/25.4+16= 28.4"
and the new ones: (225*.75*2)/25.4+15= 28.3"
Thats a pretty similar tire size.
 

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Discussion Starter · #10 ·
If you say so. I didn't know that formula
, so I'll take your word for it. So, it takes me back to my original questions.

1. Does it really matter what size wheel is on the vehicle, or is it just the appearence? I would prefer to buy a set of 15" "teardrop" wheels, but not if it is going to affect my gas mileage. Not with the price of gas now.

2. What would have affected my mpg, or have turned my check engine light on? It happend this past sunday, and hasn't happend since.
 
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Well, I know when my sister put snow tires on her car, all the same size as the summer tires, her mileage went to crap. Studs and softer tread compound can both contribute to decreased mileage.
 

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Discussion Starter · #12 ·
Well, I guess that could have something to do with it. I mean, the tires that I took off of it (with the 16" wheels) were worthless, and that was part of the reason that I needed to change the tires. One of the rear tires was so bad that it was about to start showing the steel belts. The other 3 weren't as bad, but probably would not be able to pass a PA state inspection. The snow tires that I did put on are about 12/32 tread. Tough stuff as well.

So, just having these tires on in the winter, and then returning to a summer set of tires in the same size should make the difference in my mpg?

I'm surprised that there hasn't been a study on this subject. Like: The difference in mpg using a different size tire/wheel. Or, the differnce in mpg comparing the all seasonal tires to snow tires. Would make for some interesting reading. Especially with the
gas prices.
 

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Pitfan,
Do some searches. There is plenty of info. on tires and mileage. I too am frugal; a nice way to say "tight wad." Maybe your older tires got better mileage since they were close to bald offering less rolling resistance? If you really want to be precise with measuring your speed on the road use the airplane markings on the highway. I do not know if they do it in PA, but in OK the Highway Patrol uses airplanes to monitor speed. Hence, there are marks on the road now and then on rural two lanes (they look like two boxes on top of each other with the one on top half the soze of the one below) which are exactly one mile.
 

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Discussion Starter · #14 ·
Thanks. I'll do some research on the topic.

As for the speedometer. I have a handheld gps unit, and it is VERY accurate in displaying how fast you are traveling. I did a little testing on the interstate tonight. It reads that I am traveling at 57.3mph, when the speedometer reads 60mph. Also, using the mile marker posts (Pa does not use the airplane markings on all of the interstate roads), the Jeep reads that I have gone 1.1 mile, for every mile that the mile markers indicate.

Is there a simple problem that Jeeps tend to have with the speedometer? The reason that I ask is because my Dad has several Jeeps, and at least 2 of them also read high (60 when actually going 57).

Thanks.
 

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Not to sound like a nerd, but I would have some concern over using GPS if you want high accuracy. As you know, the Department of Defense intetiionally degrades teh signal non-military recievers use. As far as distance, they say it is accurate within 100 meters. For time, I beleive the clock signal is off by 30 or 300 (not sure which one) nanoseconds. Might not sound like much unless you are a maniacally possessed with specificity like me.
 

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If the thing reads that you go 1.1 miles per each mile marker, than when the speedo says 60, you will actually be going faster. I usually go 5 or 10 miles to check my odometer, bacause sometimes a marker is off a little if an exit is there.
If the odometer is right, then I will go 60 for 5 miles, and time it to make sure it was exctly 5 minutes.
 

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Discussion Starter · #17 ·
So, over the weekend I put about 400 miles on my Jeep. And, I checked the mile marker posts (by what the last poster said), by checking it over 5 miles or so. Not just 1 mile. And, it seems to be right. 5 miles, seems to be right about 5 miles.

I measured the height of the wheels/tires. The 16" wheels that I took off were 27 1/2". The 15" wheels that I put on are 28". The 16" wheels had about 2/32 tread left, where the newer tires have about 12/32 snow tires.

I have a feeling that the gps deserves a little more credit then what one of the previous posters mentioned. Putting the gps unit in a different vehicle, it reads dead on what the other vehicle's speedo says. And, while driving my vehicle, anytime that I pass one of those interstate construction zone signs that displays your speed, my gps is always on with it.

So, I guess what I am considering now is. Is there something that might get worn out in a Jeep that would change the speed?
 

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Just the tire size or the speedometer itself. Speedo gets it's info after the clutch and or torque converter so those items cannot affect it.
 
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