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Discussion Starter · #1 ·
So my wife and I just made a drive from Texas to Florida, and we each took our own vehicle (Tundra for me, Range Rover for her). I filled both trucks up to full at our first stop and recorded the information with the Fuelly app. So, off we go driving together for a few more hours before we stop and fill up with gas again. I was curious to see who would be more fuel-efficient since the driving conditions were identical and we both had refueled at the exact same time.

According to the numbers both vehicles got 18.7 mpg for that leg of the trip, but the kicker is the vehicles recorded different mileages. Her RR odometer only went up 327 miles while my Tundra went up 340.

Only thing I can equate this to is the difference in our speedometers. I downloaded a GPS app that shows your actual speed, and my Tundra speedo is fast by 2-3 mph. The wife’s RR’s speedo is 1-2 slower than actual speed. I know it’s only a difference of 13 miles, but that would make a significant difference over the life of the vehicle, especially for warranty service. I wonder how wide that mileage gap would be even from Tundra to Tundra!


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Are the tires the same size on both vehicles?
 
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I don't think that the odometer and the speedometer are actually linked, so measuring speed and then figuring distance probably isn't relevant in this case. Somebody actually in the know on that could come along and correct me.

Using that GPS to mark the exact start and stop point or measuring actual distance traveled independently of any reading from the vehicles would probably be an interesting exercise. GPS is probably the best baseline measurement most of us mere mortals can easily use.

As for tire size, as long as both vehicles have the spec'd tire size for the specific vehicle, then that source of variation is probably eliminated.

Metrology (instrumentation and measuring) can be a fairly complex undertaking. If you only have two sources of measurement, and no baseline reference, how do you know either one is actually correct?
 

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I suspect that speedometers and odometers aren't all that accurate on any vehicle so a 3.8% difference isn't too surprising. It would be interesting to know what the actual mileage really is for that trip.
 

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I use to run marathons and would ware a Garmin GPS watch while running. The courses were usually officially sanction marathon, but sometime I would run more, sometimes I run less distances. Why, because of the turns and what lane I chose to take. Inches turns into feet, feet into miles on a long halls.
 

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Discussion Starter · #7 ·
Yes both vehicles have spec tire sizes, so differences from that should not be making an impact.

I do understand the marathon reference, but considering we were driving in line with each other the whole time I doubt that it would have accrued a 13 mile disparity. But who knows? That’s why I’m writing about it!

Also I always thought the odo and speedo were linked together. I’ve looked it up in the past and could not find any definitive information. I wonder if it marked my location when I was at the gas stations...I’ll check later today and see if I can use google maps to see what it says for that distance


2011 Tundra DC 4.6L V8 2WD 137k miles

 

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Yes both vehicles have spec tire sizes, so differences from that should not be making an impact.

I do understand the marathon reference, but considering we were driving in line with each other the whole time I doubt that it would have accrued a 13 mile disparity. But who knows? That’s why I’m writing about it!

Also I always thought the odo and speedo were linked together. I’ve looked it up in the past and could not find any definitive information. I wonder if it marked my location when I was at the gas stations...I’ll check later today and see if I can use google maps to see what it says for that distance


2011 Tundra DC 4.6L V8 2WD 137k miles

I would also check to see if you have standard or metric air in your tires.
 

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In the days of mechanical speedometers and odometers they absolutely were linked. The odometer and speedometer were physically geared to each other.

Now with digital speedometers they aren't mechanically linked. But more than likely they are still linked in the sense that they share in input: the speed sensor. It doesn't make sense from a cost or any other perspective to use separate speed sensors for speedometer and odometer.

A margin of error of 2-5% is quite normal on any speedo/odo. This is why many states put five mile "odometer check" sections on highways. Zero your trip meter when the section starts, and when you hit the 5 mile marker you should be able to calculate the error for your speedometer.
 

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My speedometers have always been off, no matter the make or model - usually they read fast. The worst was an Accord we owned which was off by nearly 7%! If the speedo read 80 we were actually doing 75. Hand calculated with the freeway markers as well as GPS verified with several devices. I think that's how it got such good mileage :D

We had an explorer that was close to accurate once I changed the tires to LT tires which, even though they were the stock spec'd size, were actually about an inch taller than the P rated counterparts. My tundra is only 1-2% fast with stock p rated tires. With LT tires it is nearly spot on.

Another thing to consider is tire wear. Worn tires are shorter than new tires, giving them a smaller circumference as well. Say you upsized your tires to LT275/70R18 tires that started with 17/32 of tread depth. Now they are pretty worn and measure 5/32 - a bit too worn for me but still usable summer tread I guess. You've lost 12/32 off of each side of the tire, so 24/32's or 3/4" total. That would make these almost the same size as the stock 275/65R18 tires. Per google (and even though there's something wonky with the math), revs per mile for the two different tire sizes are 627 and 648, respectively. The difference is 21 revs times a circumference of 100.4" is 2,108" or 175.7 feet per mile. At 340 miles on the tank, the difference between the two tires would be 59,738 feet or 11.3 miles. Someone check my math...

I suspect that the real reason most cars read faster than they are really traveling is liability - if somebody was cited for speeding and could prove that their speedometer read slow by 7% (remember, my accord legitimately read 7% fast), the car mfg could be on the hook for selling "faulty" equipment that led to the citation or, worse, an accident. It's all about the darn bloodsuckers...
 

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.... Her RR odometer only went up 327 miles while my Tundra went up 340.

Only thing I can equate this to is the difference in our speedometers. I downloaded a GPS app that shows your actual speed, and my Tundra speedo is fast by 2-3 mph. The wife’s RR’s speedo is 1-2 slower than actual speed. I know it’s only a difference of 13 miles...
I think you answered your own question. A 327-340 mile trip probably took about 5 hours. If the tundra's electronics read about 2-3 MPH high (call it 2) and the RR reads 1-2 MPH low (call it 1), then they will have a difference of 3 miles per hour, so over a 5-ish hour drive they'll accumulate a difference of 3 MPH * 5 Hr = 15 Miles. Since the difference was only 13 miles then the real difference between the two speedometers is probably more like 2.6 MPH instead of the 3 that I assumed for the calculation.

As others have said, they are both likely within the expected tolerance.
 

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I think you answered your own question. A 327-340 mile trip probably took about 5 hours. If the tundra's electronics read about 2-3 MPH high (call it 2) and the RR reads 1-2 MPH low (call it 1), then they will have a difference of 3 miles per hour, so over a 5-ish hour drive they'll accumulate a difference of 3 MPH * 5 Hr = 15 Miles. Since the difference was only 13 miles then the real difference between the two speedometers is probably more like 2.6 MPH instead of the 3 that I assumed for the calculation.

As others have said, they are both likely within the expected tolerance.
You had to bring math into it didn't you......
 
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Discussion Starter · #15 ·
Thanks for the good insight. Alas, I was not able to find location data for where I stopped. Must be because I refuse to log my location in a third party app for privacy reasons...


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