My son's 2020 Fit gets considerably better mileage than my 2015. I work at it; he doesn't. Driving into town for new tires, it showed over 44 mpg, including stopping at red lights.

One thing I like better on my 2015 is the instrument cluster. The right cluster on the 2020 shows nothing but mileage and (I forget what). Mine shows time, temp, current mileage, and something else. On the 2020, I'm not allowed to switch odometer readings or check the oil change situation. I can do that on mine. The displays on mine are larger.

I've never liked the Fit speedometer. Mine has numbers for every 20 mph, even numbers, only. When I see police radar up ahead, I'd like to be able to tell my speed with a quick glance. I can't. Were they saving money on paint by not adding the odd numbers? At least it has a line indicating the 5 mph increments. The 2020 doesn't even have that.

Resetting the TPMS warning light is so simple on my 2015: hold the button for ten seconds. On the 2020, it's hit or miss. I have to use that switch on the steering wheel. Sometimes it works, and sometimes, it doesn't.

It's like Honda decided to make the 2020 different from the 2015. "Make it different. I don't care how."

I wish Honda had continued importing the Fit because the HR-V is larger, heavier, uses more gas, and costs more. Good for Honda, bad for customers.

 

Are the efficiency figures in your signature up-to-date? That's not a very big difference, and could be easily explained by (off the top of my head):
1) Different payload (driver/passengers/cargo) in each car.
2) Different tires and tire pressure or even slightly different wheel alignment.
3) Different driving routes.
4) Different driving speed or acceleration/braking rates.
5) Different transmission.
6) Different maintenance schedules (air filters, valve adjustment, etc).
7) Slightly dragging brakes in one car.
And on and on.

The real test would be to the swap cars for a few weeks or a month as the driver is usually the biggest difference.

My wife consistently gets 2-3 mpg less than me when driving the Fit and less range in the Leaf EV than I can squeeze out. I bought my Fit from my brother after he had it for eight years. He and his wife averaged nearly four mpg lower than I have gotten since 2020 (even with my wife dragging me down) despite their driving happening on generally flatter Midwestern roads.

 

No, they aren't. That doesn't seem to be keeping up here online, and it's more difficult keeping track of my son's car's gas. I don't know what's going on with that mileage display.

As for those differences you list, I drive better, yet my mileage is a bit worse. When I drive my son's car, I always get better mileage than I do with my car. It's the design of the two cars that makes the difference.

 

What is different in the design of a 2015 vs 2020? Engine? Transmission? Wheel/tire diameter and width? Weight? Aero improvements?

I have never driven a third-gen (only my 2010), or even looked at one all that closely. It would be a coin flip if I could pick out a 2015 vs 2020. If you're saying they're different I'm curious what is different?

 

All I can think of is electronics of the CVT and fuel injection.

 

Try switching to 15 inch wheels.
Your miles per gallon should increase.

I'm thinking about switching to 15 inch wheels due to the rising cost of gas prices recently.

 

Telling someone to put on a different set of wheels to get better efficiency is not very useful, especially if you have no idea what wheels + tires they're running and you don't recommend a specific wheel + tire combo with documented weights so that they can compare against what they currently have on their car.

Switching to smaller wheels and taller tires should increase efficiency if your new wheels + tires are lighter than your old wheels + tires. If they're heavier, you will almost certainly lose efficiency. The weight distribution of the wheels + tires also plays a role (e.g., a lightweight wheel with a heavy tire vs a heavy wheel + lighter tire).

 

If memory serves, the GK5 midcycle refresh was 2018.

Slightly different body design, different dash design.

Door, sunroof and hatch leaks all became worse with the midcycle refresh.

Fuel pump recall on '18 and later models.

Still problems with DI carbon fouling, start/stop switch, weak spring in the VTC actuator. Those did not get solved in the midcycle refresh.

I believe that Honda convinced itself the GK5 was too problematic to be the loss leader, so they stopped selling them and made the HR-V the loss leader.

Not the best move. I drove an HR-V and was not impressed. Severely underpowered, for one thing. The GK5 was a rocket in comparison.

 

Just swapped 2018 engine (l15b1) into 2015 . It’s not plug and play swap. Took me a week or so to get it running. From my swap experience, major difference would be as follows:
1. Exhaust camshaft
2. Vvt sprocket.
3. HP fuel pump.
4. ECU( likely fuel trims/ whatever transmission controls).
5. ??? I’ve heard, but can’t confirm at the moment steering ( Ecu controls steering and or rack and pinion/ steering components are upgraded in 2018).

tranny/cvt is a exact match from my research.

Took me a fortune to get it running on the budget. But it drives/handles flawless now. I don’t have any data on MPG difference as this one is my wife’s DD and I don’t keep a track of it.
Fit was purchased in 2021 with shot tranny. I did replace tranny back than and engine recently without dropping subframe/ assy. I think it’s easier this way. I do have an access to the lift, but if I’d to replace either again I’d go hoist and jack.

 

I'm surprised there were so many changes.

 

The first Trump administration rolled back CAFE fuel consumption standards in 2017. It's entirely possible Honda de-tuned 2018-2020 Fits to take advantage of the fuel efficiency head-room that gave them. As irrational as that sounds, it may be a use-it-or-lose-it mentality, expecting a future administration to demand x-percent improvement in the future. The more fuel their fleet uses now, the easier it will be to clear that future requirement. They may also be using that fuel to improve other elements of tailpipe emissions. Instead of lean-burning their way to better fuel economy (and higher NOx emissions), they run closer to stoichiometric (a chemically ideal air-fuel mixture, no "unburned" air or fuel left over). Worse fuel economy, reduced raw emissions, possibly cheaper catalytic converter.

Actually, I think I'll go check if the catalyst PN changed.
Looks like 2018-2020 CVT-equipped Fits got a unique catalyst, ending in L00 (L is the California-emissions region code) while the rest end in A00 (A being US Federal emissions). The Cali converter is more than twice the price of the Federal version. So much for that theory.