Oops, I did it again! Bought another cheap Mirage "beater".

[MetroMPG]

I had these 15" Civic wheels in my junk pile. Was thinking they might be good to put some Prius tires on.

EDIT: tires currently on those wheels are 185/65R15

Upsides: lower rolling resistance (possibly/probably); slightly taller effective gearing; more funner.

I'll have to check out a gearing calculator to figure the RPM drop. I think it's around a hundred RPM at highway speeds? (EDIT: Nope, it's better than that. See a calcs, a couple of posts down...)

Downsides: heavier; slightly higher ride height (worse aero); wider (worse aero - somewhat mitigated by front air dam, and I'll add rear tire deflectors also)

[Fummins]

What size tires are those? They sure fill the wheel wells!

[MetroMPG]

They're 185/65 15's.

But the pic is a little deceptive - the wheels aren't actually ON the car - I just leaned them against the winter wheels and scooched down to get the right angle for the picture.

[MetroMPG]

I plugged the tire numbers into this tool: https://www.blocklayer.com/tire-size-calculator.aspx

And it tells me that change would reduce engine RPM (in all gears) by ~8%.

At 80 km/h (~50 mph), the engine is turning around 2250 RPM on the stock tires. Those Civic wheels & tires would drop it to ~2070 rippems!

The final drive swap I did to my Firefly dropped cruising RPM by 20% and increased steady state cruising fuel economy by about 5%.

So all else being equal (it's not), this might net me around 2% better MPG's.

Assuming I remember to slow down a bit because now the speedometer will be off (reading lower).

AND! It'd be way funner to drive because all that extra rubber would ground it to the ground!

[Fummins]

Had me fooled, that'd explain the invisible lug nuts.

[Top_Fuel]

I've been running 185-60-15 tires since the late Fall. I don't have enough mileage data yet, but they dropped my freeway RPMs by about 200 and seem to be worth at least 1mpg. I will know for sure once we are into solid warm weather and the end of winter blended fuel. Running these tires is like having a car with a 5.5 speed transmission.

These tires don't rub...but...there are times where I can't even get a finger between the tire and the front fender liner when turning the wheels.

I don't have a picture of my car with these tires, but here's a Mirage with 185-60-15s...




I say bolt those things on and see what happens!

[MetroMPG]

Thanks, Top_Fuel!

+1 MPG makes sense to me based on the gearing change.

The unknowns are the aerodynamic & rolling resistance changes. That's always a bit of a crapshoot changing wheels/tires.

I'm not sure how the offset on these Civic wheels compares to stock, but I'm probably going to throw them on today to check it out.

[Mark]

Thanks, Top_Fuel!

+1 MPG makes sense to me based on the gearing change.

The unknowns are the aerodynamic & rolling resistance changes. That's always a bit of a crapshoot changing wheels/tires.

I'm not sure how the offset on these Civic wheels compares to stock, but I'm probably going to throw them on today to check it out.

Top_Fuel quote -
"I've been running 185-60-15 tires since the late Fall.
These tires don't rub...but...there are times where I can't even get a finger between the tire and the front fender liner when turning the wheels."

175/55r15 (22.6") - 15" factory size
185/55r15 (23") - common alternative (+1.8% variance)
185/60r15 (23.7") - (+4.9% variance)
185/65r15 (24.5") - (+8.4% variance)

Just stating the differences for anyone reading this thread.

If Top Fuel's 185/60r15 tires are that tight when turning the wheels, I would test one 185/65r15 tire for clearance before mounting all four on the car.

I doubt 185/65r15 tires will improve your economy over stock size tires, and it will surely make your speedometer/odometer readings be off. The possibility of damage under the car when hitting bumps would be my biggest concern, however. It doesn't take much to bottom out a Mirage, & I would rather it bottom out without the wheels damaging the underside of my car. I know that I am stating the obvious, but that's a big tire for a small Mirage.

Some of what I stated is just a hypothesis on my part that someone else can test out. You'll have to let us know how it goes!

[MetroMPG]

I doubt 185/65r15 tires will improve your economy over stock size tires

Why wouldn't reducing cruising RPM improve fuel economy?

[Mark]

Why wouldn't reducing cruising RPM improve fuel economy?

Larger, heavier, wider tires may take their toll? Depends on what sort of driving you do I guess. If you do long trips often, maybe it will help? Can't imagine it would help city traffic (lots of stops/starts) driving?

It's all a hypothesis on my part, & I am stating that up front. A hypothesis is meant to be tested/proven wrong or right. Either way you come away with new knowledge.

If large tires give you an increased in mpg rating, it may be worth the downfalls of speedometer/odometer being off & possible rubbing issues.

Why aren't CVT Mirages (running at lower RPMs @ highway speeds) not blowing away the manuals models in the mpg department? I've been following this forum for 4 years, & I have never once seen anything that shows a CVT is superior in economy. Yet, in theory a CVT is stated to as being better. Better fuel economy is the justification companies use to put these things in cars.

I know lots of factors go into this. Tires, fuel blends, wind/weather, driving style, load in car, etc.... are all factors. Comparing apples to apples is nearly impossible.

Can a person take a long trip with one set of tires, & replicate that same trip with a different set of tires? Most likely not, because weather/wind alone would most likely be different (plus traffic & anything else).

I am just speculating tires that large & heavy may not improve your economy. I will not be hurt if you prove me wrong at all. I've been wrong before on stuff!