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I just did some highway driving this weekend. I filled up soon after my highway driving had begun and again after my return. Only 20 miles of nearly 300 were city driving, so my tank was over 90% highway.

Now, I've reported here earlier that when I bought my car new, my speedometer would read 78mph at 3000rpm. It now reads 73mph at 3000rpm.

Being quite anal about my car, I wanted to calculate my highway mileage. However, since I was traveling a known distance, I calculated it two ways.

My odometer informed me that I had driven roughly 280 miles between fill-ups. I calculated the distance I had driven to be roughly 300 miles.

Interestingly, I multiplied 78/73 (my difference in speedometer readings) times 280 (miles my odometer read); which results in 299- about what I calculated my actual distance to be.

My conclusion? My speedometer did indeed fall out of wack.

Now, should I get this fixed? This means I'll eclipse 100,000 miles, yet have an odometer reading 93,000 miles. It'll deceivingly help resale value.

I got 10.3 gallons of gas. That means according to my odometer I got 27mpg and according to my physical calculations I got 29mpg. Either way, that's pretty poor for the 4cyl engine.

However, 2/3 of that was with the defroster accidentally on, and hence the A/C compressor. Furthermore, since I had the cruise set for 77mph, I was actually going nearly 83mph. Since I occasionally sped up to 85mph, that means I was actually going over 90mph. Not good for mileage.

That's a lot of statistics. Your thoughts?
 

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Reading Topic: Highway mileage

I think you drive too fast, and if you know about the problem, a cop isn't going to give you any slack.
 

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Reading Topic: Highway mileage

In other news, I thought "hey, that's pretty good mileage" until I saw that you have the 4...
 

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Replying to Topic 'Highway mileage'

i believe tire wear... ie tread wear has an effect on speedo readings.. that may explain why at new it read different then later.... then again what the hell do i know :D :p

then again i think everything that wears and breaks in, such as the parts etc play a role,,, plus without a control group or larger sample then just your observations.. you can,t make any conclusions... just my two pennies...
 

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Reading Topic: Highway mileage

I would definetly get that looked at. Why knowlingly have something wrong with your car, especially if it has something wrong with it? I guess it's just me but I would get it fixed.
 

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Reading Topic: Highway mileage

I don't know that anything is wrong. You'll drive yourself nuts, literally, and stop enjoying the car if you pay this close attention to everything.

Had the motor been totally broken in when you recorded the 78 at 3k? That has to have had some effect. Tire wear definitely has an effect - but it's doubtful, at least in my mind you'd see it this early.

Things are bound to move around, and speedos are notoriously inaccurate to start with. The faster you go, the greater the error.

As for reading 93,000 at 100,000 - don't look a gift horse in the mouth. :) I'd leave well enough alone.
 

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Reading Topic: Highway mileage

Hmm, so your speedo and odometer read about 6-7% too low. It wouldn't hurt to get it looked at.. otherwise, you just have to keep that in mind when you're following directions to the mile or trying to calculate mpg. (Or interval between oil change and other regular service?)
 

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Reading Topic: Highway mileage

First thing to look is if you got the good tire size for your car, second ensure you got the right tire pressure. After that it depend on speedometer/odometer (both work together) calibration or accuracy.

On my Mazda Protege 92, I had 4% error. When the speedometer was showing 100 km/h, I was actually going 96 km/h.
 

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Discussion Starter #9
Replying to Topic 'Highway mileage'

Thanks for the responses. I should clarify though.

My speedometer has changed in relation to my tachometer. In the same gear, it no long reads the same speed at a given RPM. It did this well before my new tires, but even so, this measurement has nothing to do with the tires since it is a measure of axle rotation (independent of tire size); not actual travel speed.

Differently sized tires (or tread wear) would throw my speedometer off though, you're right, and I did recently go from 16's to 17's. However these tires have not made any change from my 16" tires. With both, I still read 73mph at 3000rpm, whereas when the car was new I would read 78mph. This change happened long before the new tires.

I used radar to confirm that 78mph at 3000rpm is a legit figure- it's accurate. Since my clutch is not slipping, either my tach or my speedo must no be reading correctly.

What I didn't expect was that my calculations show that the difference the speedometer is reading matched up EXACTLY with the difference in distance I calculated that I had driven.

78/73 * 280 = 299, whereas I estimated I actually travelled 300 miles on a tank, not 280.

So, again, my speedo is out of wack. I probably was driving over 90mph, not knowing it. My mileage sucked as a result. At least, I hope that's why. No wonder I made good time!

Anyway, I hope I get better than 29mpg in the future. And, if somehow I'm wrong about this and my speedo is correct (and my calculated distance wrong); then I got an even worse 27mpg.

This is with the 4cyl engine.

I'm anal. When I first got the car, I was gentle on the engine during break-in. Consequently, I averaged over 30mpg in city traffic! I actually got 540 miles on one tank. After Mazda's recommended 600 mile break-in, I opened it up a few times and had some fun.

I've since not been able to break 30mpg.

Not that I should be getting over 30mpg in the kind of congested beltway traffic I drive in, but I had been, and I want to again.

Since I figure I'm getting about 24mpg city and 29mpg highway, there's nothing wrong with my car. It's in line with what the EPA says it is, but my first few fill-ups showed it capable of much more.

How does this relate to the speedometer being out of wack? I don't know how far I've driven! It's hard to be certain if my mileage is problematic!

Consequently, I love to hear about what kind of mileage others are getting. Three things seemed to happen after break-in: my mileage dropped, but my low-end torque clearly went up, and my speedometer went out of whack. All at the same time, and yet none really constitutes a real "problem" per say, as the car still sounds and feels great.
 

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Reading Topic: Highway mileage

"Since I figure I'm getting about 24mpg city and 29mpg highway, there's nothing wrong with my car."

The EPA lists highway mileage for the 6i/stick at 32 mpg. Getting 29 isn't bad given:

The AC compressor was on.
The speed the car was traveling.
The odd oxigenated fuel we get in the winter in North America.
 

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Replying to Topic 'Highway mileage'

The dealer should have an electronic device he can hook up to the speed sensor input in order to check speedometer accuracy. I'm assuming this since I have had to get my speedo replaced on my Harley under warranty, the Harley dealer hooked up a gizmo that allowed him to dial in preset speeds and then observe the readout on the speedo. No reason why Mazda should not have something similar.
 
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