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Old 09-25-2017, 10:55 AM   #1 (permalink)
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Question What am I doing wrong.

On Saturday I went for a good long highway run. 40 miles out, then a couple of hours later, 64 miles back. The weather was fine and dry, no appreciable wind, kept my speed in hand, drove to the best of my hypermiling ability, and recorded 55.6 mpg UK.
This morning, in the dark and pouring rain, I made a 3.8 mile town drive (roads fairly quiet) where the coolant temperature never got up to operating temperature.
Then 4 hours later I did the same trip, this time in busy traffic and the highest the coolant temperature reached was 76C. Average was 50.3 mpg UK for both trips.
(both figures from ScanGauge)
Surely the highway miles per gallon should be much better than the town driving, or the town driving much worse! A 10% improvement seems very poor. Is it me? Is the car not ideal for highway driving? Any suggestions would be most appreciated.

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Old 09-25-2017, 11:08 AM   #2 (permalink)
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My suggestion is to get more data points before getting too riled up.
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Old 09-25-2017, 11:10 AM   #3 (permalink)
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I never paid much attention to trip mpgs, especially short in-town trips versus long highway runs. There are just too many variables involved to get a good comparison.
To me a better judge is tank-by-tank, but even then there are a lot of variables which can affect a tank, even in my old Insights which could go 700 miles between fill-ups
You're probably doing nothing wrong, just keep at it and drive consistently and you'll see the numbers creeping up (weather permitting).
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Old 09-25-2017, 11:15 AM   #4 (permalink)
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The Fit/Jazz has the aerodynamics of a brick.

My wife's car's mileage drops off precipitously with speed.
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Old 09-25-2017, 11:17 AM   #5 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
My suggestion is to get more data points before getting too riled up.
I have several month's worth of data. And what I am seeing is my longer highway FE is only marginally better than my short town hops.
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Old 09-25-2017, 11:18 AM   #6 (permalink)
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It's not that simple.

Your highway mileage is mostly the result of air drag, followed by driving technique (DWL and DWB). Good mileage comes from driving at a steady, economical speed and not using the brakes.

Your city mileage is mostly the result of road conditions and driving technique. Good mileage comes from keeping moving without using the brakes. If you can match pulses and glides to the inevitable speed variations, then it is possible to get better mileage in the city than on the highway.

If my only driving was back and forth into town (11 miles round trip), I would be averaging about 45 MPG in the summer. That trip has a top speed of 40 to 45 MPH and two stop signs.
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Old 09-25-2017, 12:03 PM   #7 (permalink)
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I agree with JRMichler about the city mileage. My best in town trip was 24.11 mpg, or 172% EPA for city. That night it was completely dead. There was never any cars in front of or behind me. I just had the opportunity to use all of my pulse and glide in the correct spots.

I only have one decent trip on record for my highway, it was also about 24 mpg, my only technique was constant throttle at 60mph. You might be looking at it wrong. Your city mileage is only 10% from your highway. Thats how I think about it anyway.

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Old 09-25-2017, 12:11 PM   #8 (permalink)
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Your Honda Jazz is light, not particularly aerodynamic, and has a tiny 1.2L 75hp engine. Each of these characteristics will give it better city mileage than highway mileage. Most cars get significantly better mileage on the highway because they have a higher weight-to-drag ratio and engines that are really oversized for city driving.

I wouldn't be surprised if your car got peak fuel mileage at a cruising speed as low as 25mph. If you have good hypermiling techniques there's no reason your city mileage shouldn't be close to your highway mileage. Hypermiling techniques are also much more effective in the city than on the highway. On the highway there's not much more a person can do than reduce speed and be mindful of how you handle hills.
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Old 09-25-2017, 12:22 PM   #9 (permalink)
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Quote:
Originally Posted by twj347 View Post
Your Honda Jazz is light, not particularly aerodynamic, and has a tiny 1.2L 75hp engine.
Yes, on the highway I have to keep around 55 mph or the least little incline claws my speed right down. I have tried Pulse and Glide, on the highway, but as you say, the car is so light and not all that aerodynamic, that if I push the clutch in at 60, I'm down to about 40 in a couple of hundred yards.
All my highway driving is done when the roads are really busy. I cannot be holding up other drivers, particularly on the two lane.
This is typical of highway driving in Scotland.
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Old 09-25-2017, 12:42 PM   #10 (permalink)
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What about its gearing? IIRC it was optimized for city traffic.

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