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Old 06-02-2012, 06:30 AM   #1
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This can NOT be possible!

For those of you that have been following my MPG progress, I just learned
something fantastic!

I got pulled over tonight for doing 67 in a 60 zone, but my speedometer was on 60... I did the math, it's roughly 12%. To test it further, I reset the trip meter before I left Aberdeen, SD and headed home to Watertown.
It's 96 miles from from my old house to my new house, but tonight my odometer only registered 85 miles traveled. Again, this is about 12% error.

My freakin speedometer is 12% off due to the new wheels and taller tires I bought from AM.... My odometer registers fewer miles than I have actually traveled

So.. this means that my numbers that I posted about have new meaning!

24.8 MPG city (odometer) was actually 27.7 City (actually traveled)
32.8 MPG hwy (odometer) was actually 36.7 Highway (actually traveled)

I have been making a LOT of highway trips recently since I broke my collarbone and have been off work for the past 3 weeks. I've gone to Fargo, Minneapolis, Sioux Falls, Aberdeen etc. I have been keeping close track of my mileage, but I always based it off of my odometer.

This recalculated fuel economy seems just too good to be true. I know the math is accurate, but it just seems impossible?

Tomorrow I'm going to use my tuner to see if I can correct my speedo, then I'll know for sure. I still have 3 more weeks off work and a lot of places I need to go so I'll have plenty of time to do further testing.

I'm beginning to think that my UDP is making more of a difference than I had originally thought...
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Old 06-02-2012, 08:14 AM   #2
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what size of wheels did u upgrade too?
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Old 06-02-2012, 09:06 AM   #3
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Before I got my new FR500s my dad told me to make sure it wouldn't change my speedometer because of the new wheel size and I found this calculator on some website where you put in the old size, in my case was 225/20 R-16 (rough guess), and your new size, which was 275/40 R-17, and it said that when I was going 60 mph I would be goind 59.9 mph so bigger wheels made it go down for me instead of up in your case
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Old 06-02-2012, 10:38 AM   #4
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Re: This can NOT be possible!

Quote:
Originally Posted by Horseplay View Post
what size of wheels did u upgrade too?
I don't know what size tires were on my old 15 inch wheels, and I don't have them anymore so I can't check. The new ones are 245/45-17. I remember standing them next to each other and these are about 7/8 of an inch taller than the old 15's I had on.

I completely forgot about tire size and for the past 2 months I have been simply driving the car without regard to speedometer error. I'm amazed it took me so long to get pulled over by a trooper, considering how many miles I have driven since installing these new tires.
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Old 06-02-2012, 10:59 AM   #5
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Quote:
Originally Posted by Just_John

I don't know what size tires were on my old 15 inch wheels, and I don't have them anymore so I can't check. The new ones are 245/45-17. I remember standing them next to each other and these are about 7/8 of an inch taller than the old 15's I had on.

I completely forgot about tire size and for the past 2 months I have been simply driving the car without regard to speedometer error. I'm amazed it took me so long to get pulled over by a trooper, considering how many miles I have driven since installing these new tires.
I went from 15s to 17s with the same exact tire size and my speedo is still right on..
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Old 06-02-2012, 12:06 PM   #6
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Quote:
Originally Posted by Just_John

I don't know what size tires were on my old 15 inch wheels, and I don't have them anymore so I can't check. The new ones are 245/45-17. I remember standing them next to each other and these are about 7/8 of an inch taller than the old 15's I had on.

I completely forgot about tire size and for the past 2 months I have been simply driving the car without regard to speedometer error. I'm amazed it took me so long to get pulled over by a trooper, considering how many miles I have driven since installing these new tires.
thats weird. i had the same old 15" stocks on mine before i got my new wheels. i got 275/17 up front and 315/ 17 in the rear and i havent had any problems
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Old 06-02-2012, 12:12 PM   #7
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Re: This can NOT be possible!

I called my friend that now has my old tires. They are 205/65-15.

The calculator at Tire Dimensions Made Simple - Discount Tire shows less than 1/4" height difference. However, when I was installing the new tires I put them side by side and there was almost a full inch difference in height.

So...what the **** is going on? Tire sizes are made to an international standard. The height, diameter, circumference, width of any given tire size are all supposed to be the same no matter who manufactured them... so, how can there be a 7/8 (. 875) inch difference in height from the old tires to the new ones, when the calculator says that there is only .19 inch?

Until I can figure out where the discrepancy is, I'm not adjusting anything...
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Old 06-02-2012, 12:50 PM   #8
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Quote:
Originally Posted by Just_John
I called my friend that now has my old tires. They are 205/65-15.

The calculator at Tire Dimensions Made Simple - Discount Tire shows less than 1/4" height difference. However, when I was installing the new tires I put them side by side and there was almost a full inch difference in height.

So...what the **** is going on? Tire sizes are made to an international standard. The height, diameter, circumference, width of any given tire size are all supposed to be the same no matter who manufactured them... so, how can there be a 7/8 (. 875) inch difference in height from the old tires to the new ones, when the calculator says that there is only .19 inch?

Until I can figure out where the discrepancy is, I'm not adjusting anything...
There are slight differences between tire manufacturers. Your first number on the tire is the width of the tread in mm. sexond is the ratio of the diameter to the height of the tires sidewall. So a 250/50/xx tire has a 250mm width and a 125mm tall sidewall. My stocks are 245/45/17 and my aftermarkets are 275/40/17. If you increase width you have to decrease sidewall ratio. The actual sidewall height should not change to keep your speedo in spec.
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Old 06-02-2012, 02:05 PM   #9
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Re: This can NOT be possible!

you know you can use your tuner to change the speedometer right?

nice job on the mileage though, i am not anywhere near that....
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Old 06-02-2012, 04:05 PM   #10
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Maybe tire wear played a part in your 3/4 inch discrepancy
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Old 06-04-2012, 10:38 PM   #11
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Something to also consider.. The officer's radar is not 100% accurate.
Get a GPS, that's really the most accurate speed reader.
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