Increasing Gas Mileage With Taller Tires? Options

[joyworks]

One of the few things that I dislike about my '99 Celica Convertible is that the gas mileage isn't as good as it could be. The 1997-2001 Camry, which had the same engine, got 3 mpg better on the highway with a 5-speed than my Celica does with a 5-speed, and the Camry weighed about 300 lbs more! I can only attribute this to the high revs of my Celica's 5th gear. So, my question is, can I achieve better fuel economy with a larger diameter wheel/tire combination? I know it would throw my speedometer off a bit, but to save fuel, it would be worth it to me. On that same note, what would would the tallest tire that I could put on a 16" rim and not suffer from any fender rubbing? What about a 17" rim? Thanks for your help!

[playr158]

you'd probably negate any savings by adding the rotating mass.

try doing some general maintenance (ie tune up) on your car first.

[samir0189]

Ill have to agree with playr, tune it up. The 5sfe is easily capable of 32+mpg hwy. Plugs, wires, correct timing, proper tire inflation, and a light foot with conservative shifting points (2500 rpms) will get you great mpg.

[joyworks]

Actually, I already do all of those things. I am currently averaging 32-33 mpg in 85% highway driving. I am just wondering if I can do even better with a taller tire, as I am planning on getting rid of my old stock rims for 16" or 17" new ones.

[samir0189]

I'm not a physics major, but I'm thinking that the added weight of the tire will probably negate any gains that you may get from it. Especially if you intend to increase the wheel size, i think its important that you follow the "plus" sizing for both rim and tire where if you increase the wheel size you must decrease the tire profile, otherwise you'll increase the ride height of the vehicle, and the higher you are the more wind resistance you will encounter.

[joyworks]

You really think that lifting the car a half inch or so is going to negate the effects of lowering the engine revs per mile?

[dgbldr]

No, no and no.

1. No, you can't attribute the difference between your Celica and Camry to shorter gearing only. You can also attribute it to:
a. "Measurement" error. Unless you have a chassis dyno and can run a precisely controlled driving cycle on both cars, your "measured" fuel economy is just a rough guess.
b. As noted, state of tune, tire type and inflation, etc.
c. Convertibles are generally not as aerodynamic as coupes or even sedans, but that only comes in play with extended high speed driving, like hours on the interstate.
All in all, #a trumps everything else.

2. No, taller tires (within reason) do not add significant weight nor "rotating mass". We're not talking Indy cars here guys. Take a couple of tires, one 205/55 and one 205/65 and weigh them. Less than the difference between a skinny driver and an average one.

3. No, taller tires (again within reason, like one inch larger diameter) will not make a 3 mpg difference. If the taller tire is also wider, the extra drag trumps the extra diameter so you end up worse. What you want is skinnier, higher inflation pressure and low rolling resistance rubber compound.

4. No, increasing the ride height by an inch will not make a significant difference. Again, aerodynamics play a minor role unless you do most of your driving on the Interstate, at speeds significantly over 60.

This post has been edited by dgbldr: Oct 23, 2008 - 4:02 PM

[D-Man]

Actually, I already do all of those things. I am currently averaging 32-33 mpg in 85% highway driving. I am just wondering if I can do even better with a taller tire, as I am planning on getting rid of my old stock rims for 16" or 17" new ones.

DON'T BE A HYPERMILER!!

As for bigger tires, remember that this will put more stress on the engine, causing it to work harder. Guess what? more work = more fuel consumption. Now quit hypermilin' or buy a SMART

Just play'n

This post has been edited by D-Man: Oct 23, 2008 - 4:32 PM

[DEATH]

No, no and no.

1. No, you can't attribute the difference between your Celica and Camry to shorter gearing only. You can also attribute it to:
a. "Measurement" error. Unless you have a chassis dyno and can run a precisely controlled driving cycle on both cars, your "measured" fuel economy is just a rough guess.
b. As noted, state of tune, tire type and inflation, etc.
c. Convertibles are generally not as aerodynamic as coupes or even sedans, but that only comes in play with extended high speed driving, like hours on the interstate.
All in all, #a trumps everything else.

2. No, taller tires (within reason) do not add significant weight nor "rotating mass". We're not talking Indy cars here guys. Take a couple of tires, one 205/55 and one 205/65 and weigh them. Less than the difference between a skinny driver and an average one.

3. No, taller tires (again within reason, like one inch larger diameter) will not make a 3 mpg difference. If the taller tire is also wider, the extra drag trumps the extra diameter so you end up worse. What you want is skinnier, higher inflation pressure and low rolling resistance rubber compound.

4. No, increasing the ride height by an inch will not make a significant difference. Again, aerodynamics play a minor role unless you do most of your driving on the Interstate, at speeds significantly over 60.

For the most part - very good points.
You too D-man [retard]

[playr158]

2. No, taller tires (within reason) do not add significant weight nor "rotating mass". We're not talking Indy cars here guys. Take a couple of tires, one 205/55 and one 205/65 and weigh them. Less than the difference between a skinny driver and an average one.

:read: OP said larger wheel and tire combination...larger wheels = heavier and yes that will make a difference. its not just tires.

[samir0189]

I really don't think that the rpm difference is one you are going to be able to see on your gauge cluster. It will probably be even more insignificant since we are talking about 0.5" of height increase. As far as the drag coefficient goes, isn't it something along the lines of .01 per inch of height increase, so, .005 for half of an inch? Also insignificant. If you do end up doing this, I wouldn't expect even a noticeable difference in your fuel mileage, better or worse.

And d-man makes a valid point, this is unsprung weight, your engine will have to work harder to move those wheels.

This post has been edited by samir0189: Oct 23, 2008 - 5:26 PM

[GriffGirl]

Larger tires/increased height actually can decrease mileage, not increase mileage. On our cars it would probably be negligible, but I know from experience when I drove a big-ass lifted '87 Pathfinder that when you go up, the mileage goes down.

[RickJamesBish]

It always pisses me off when my car is rated 27/34 and I get 25 in the city.

[playr158]

I really don't think that the rpm difference is one you are going to be able to see on your gauge cluster. It will probably be even more insignificant since we are talking about 0.5" of height increase. As far as the drag coefficient goes, isn't it something along the lines of .01 per inch of height increase, so, .005 for half of an inch? Also insignificant. If you do end up doing this, I wouldn't expect even a noticeable difference in your fuel mileage, better or worse.

And d-man makes a valid point, this is unsprung weight, your engine will have to work harder to move those wheels.

wheels are not considered unsprung weight....you consider them rotational weight.
unsprung weight would only be the spring around your shock at a stand still

This post has been edited by playr158: Oct 23, 2008 - 7:08 PM

[samir0189]

Lol. i did say i was not a physics major. I understood unsprung weight as all suspension/wheels/tires, the stuff that's not being supported by the suspension. Oh well. Me=-1

[playr158]

well you are correct still i suppose.

"In a ground vehicle with a suspension, the unsprung weight (or, more properly, the unsprung mass) is the mass of the suspension, wheels or tracks (as applicable), and other components directly connected to them, rather than supported by the suspension. (The mass of the body and other components supported by the suspension is the sprung mass.) Unsprung weight includes the mass of components such as the wheel axles, wheel bearings, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is also part of the unsprung weight."

i guess we can say a wheel is/should be both unsprung and rotational mass.

we should be more specific and state that during rotation weight is the same but mass increases as a function of speed.

rotational mass effects the car during acceleration and the effects of the work required to accelerate a heavier object.
heavier wheels result in slower acceleration, where as lighter wheels result in faster acceleration.
this gets into a whole event of rotational dynamics, moment of interia and such.

but easy way to say is....dyno on 18s and dyno on 16s...and the 18s will dyno less power.
it takes more work and force (ie more gas) to move larger and/or heavier wheels.

end story is removing weight from things that spin(wheel) can be more beneficial than the same weight of something that doesnt (sub box)

This post has been edited by playr158: Oct 23, 2008 - 7:20 PM

[dgbldr]

wheels are not considered unsprung weight....you consider them rotational weight.
unsprung weight would only be the spring around your shock at a stand still

Incorrect. Read this: http://en.wikipedia.org/wiki/Unsprung_weight

[samir0189]

righttttt. So then, in theory the added weight of the tires/wheels will cause the OP's mpg to actually decrease because of the engine working harder. Case closed?

[joyworks]

Thank you all for your comments so far. But, let's take the weight of the wheels out of the equation for now, as I will probably buy 16" rims that weigh about the same as the 15" stock rims anyway. If I were to consider the following two sizes from a low rolling resistance tire:

A) 225/50R-16 22 lbs. 831 revs/mile
B) 205/60R-16 20 lbs. 811 revs/mile

Do you believe that tire B would yield significant fuel economy improvement (1 mpg or more) over tire A? If not, then I guess I would go with tire A, as it would most likely handle better.

[blu94gt]

Thank you all for your comments so far. But, let's take the weight of the wheels out of the equation for now, as I will probably buy 16" rims that weigh about the same as the 15" stock rims anyway. If I were to consider the following two sizes from a low rolling resistance tire:

A) 225/50R-16 22 lbs. 831 revs/mile
B) 205/60R-16 20 lbs. 811 revs/mile

Do you believe that tire B would yield significant fuel economy improvement (1 mpg or more) over tire A? If not, then I guess I would go with tire A, as it would most likely handle better.

No, it will not yield significant fuel economy improvements over tire A. Even if it does yield you 1 more mpg, 1 more mpg on top of 32-33 isn't that big of deal. If we were driving Hummers and could find a way to get 1 more mpg, that would be great.