185/75 - MPG improvement?

[gone.a]

New member here, just picked up a new/used Mirage, currently at about 30K miles and looking into first set of new tires.

I do a lot of highway driving at about 70-75 mph and would like to see lower rpms and better mpg (I could go slower, I know). Was wondering if larger tires (and their effective re-gearing) would help? For sake of discussion lets not worry about the spedo or mileage indicator being off.

For example a 185/75 tire would do about 78mph when the stocks would be doing 70 mph and reduce rpm by about 400.

Question is
1- Would it even fit?
2- Is there any mpg advantage under steady state highway driving in switching to a larger diameter or does the increased width negate this?

[cyclopathic]

We use 185/60 and I don't think there will be any issues with 75 profile fit. I would recommend against it for 2 reasons: 1) speedo error, 2) it will render spare unusable.

We have a separate thread on tires, but one we have on now and like alot is H rated General RT-43 in 185/60R14. Inexpensive, top 5 rated at TireRack, wildly available, even Walmart has them. They are non-lrr but one of the better rolling non-LRR tire. Max pressure 51psi and from handling point of view 48/42 f/r seems to work the best. The only negative took forever to break in, but it will likely last way past 60k warranty.

If you travel alot at 75mph+ I would look into aero mods. Ecomoddler has information on aerodrag reduction, and MetroMPG posted on his mods take a look.

Edit: Mirage engine seems to have very flexible torque, so I don't believe there is much to be found by taller gearing. At least CVT and 5MT do not differ significantly MPG-wise.

[gone.a]

We use 185/60 and I don't think there will be any issues with 75 profile fit. I would recommend against it for 2 reasons: 1) speedo error, 2) it will render spare unusable.

We have a separate thread on tires, but one we have on now and like alot is H rated General RT-43 in 185/60R14. Inexpensive, top 5 rated at TireRack, wildly available, even Walmart has them. They are non-lrr but one of the better rolling non-LRR tire. Max pressure 51psi and from handling point of view 48/42 f/r seems to work the best. The only negative took forever to break in, but it will likely last way past 60k warranty.

If you travel alot at 75mph+ I would look into aero mods. Ecomoddler has information on aerodrag reduction, and MetroMPG posted on his mods take a look.

Edit: Mirage engine seems to have very flexible torque, so I don't believe there is much to be found by taller gearing. At least CVT and 5MT do not differ significantly MPG-wise.

Thanks for the great info, will definitely keep those in mind. Any noticeable mpg hit with the 185 sidewall or is it negated with the higher pressure?

[Eggman]

it will render spare unusable.

I have to challenge this notion, as it will not render the spare tire unusable. The spare tire is already designed as a temporary solution, meant to get the car to a repair facility and have the tire repaired/replaced as soon as possible.

[Eggman]

Also, wouldn't a taller profile have a negative impact on fuel economy?

[gone.a]

A taller profile would effectively reduce rpm by 400 per my initial example, as the tire size is essentially a final gear. Reduced rpm will in theory help mpg. However, the slight rpm reduction might be negated by a heavier tire, taller tire might be less aero efficient, and a wider tire has more contact patch which creates more rolling resistance. So I am starting to think a taller/wider tire might not help much.

Also, if one has very tall tires, you can still use the spare, just make sure its on the back so as not to stress the differential too much. But even if it was on the front, I doubt the differential couldn't handle a few hours of extra use, its basically in use all the time anyway. It will set the tpms light off but who cares.

[Eggman]

TPMS light on with use of spare, or with a underinflated (flat!) tire is unavoidable. We can set those worries aside now.

Taller tires, wider tires: it still takes a certain amount of power to move an object of any given size & shape at 65 mph. That aspect cannot be negated through tires alone!

[cyclopathic]

I have to challenge this notion, as it will not render the spare tire unusable. The spare tire is already designed as a temporary solution, meant to get the car to a repair facility and have the tire repaired/replaced as soon as possible.

To be honest it is a speculation. I have not tried to see if it will trigger ESP. You definitely don't wanna have donut and taller tire on driving axle, it will mess up diff, but having them on rear should be safe from that. Except it must trigger ESP. I suppose you could turn it off to get home, and absolutely NOT mount spare upfront?

[cyclopathic]

Thanks for the great info, will definitely keep those in mind. Any noticeable mpg hit with the 185 sidewall or is it negated with the higher pressure?

Anything besides OEM Enasaves will take MPG hit.. They are really lousy handling tire, but perhaps the best LRR tires on market. Rt43 are not bad, Consumer Reports rated them better than avg on rolling resistance test. Despite that we saw initial 10 MPG drop and it took forever to break in. Now MPG is up with high pressure (48/42psi) we can get 48+ MPG but with Enasaves it was in low 50s.

If you read reviews Mirage managed 0.71-0.73 on skid pad. Partly because of soft suspension, but mostly due to tires. They are no compromise LRR tires, the rumor is Toyota put them on Gen4 Prius ECO to get it certified at 56 MPG. Base Prius carries 52 rating.

[Eggman]

To be honest it is a speculation. I have not tried to see if it will trigger ESP. You definitely don't wanna have donut and taller tire on driving axle, it will mess up diff, but having them on rear should be safe from that. Except it must trigger ESP. I suppose you could turn it off to get home, and absolutely NOT mount spare upfront?

If the differential is either locking or limited slip, different tire sizes would then be a problem. Otherwise, this should be bathed in oil and be able to take a few miles of action. There are interesting videos on YouTube on planetary gear sets - one that sticks out is transmissions for tanks.

Not sure what you mean by ESP. What do you mean by saying 'turn it off?'