Wheels, tires, and economy

[Ever Victorious]

Something said recently got me thinking...

I feel like my gas mileage has gone down since I went from the 15 inch wheels to the 17s and added the Stillen body kit. Whether or not these adversely affect the mileage, I'm not sure.

I've actually been thinking quite a bit recently about my switch to 17" wheels and tires and why I haven't seen an MPG drop that can be attributed to that (I have attributed the drop to fuel blends, it came back up into the 30's a month or so ago).

As I was cruising along on several different spots of the freeway, exiting from the freeway in near zero traffic, I put the Versa into neutral and just coasted. Then a plausible answer came to me.

I noticed that, unless I was going UP a steep hill, my speed was dropping very slowly. VERY slowly... like I could coast down a 1/4 mile flat exit ramp, starting at 60, and still be doing nearly 50 at the end of the ramp. I realized that the Continentals that came on this car could NOT do that.

I repeated the coasting test on various stretches of highway and exits, some flat, some pitched down, some uphill. Every single time, the car bled off a lot less speed than I expected.

Rolling resistance is my theory.

Yes, I went to a bigger wheel and a wider tire, but it seems to me that the tires have an unusually low rolling resistance. That, or the Continentals have an abnormally high resistance. Or a combination of both.

It's a characteristic of tires that's not advertised, I'm not sure if it's even rated, and it's certainly not talked about often. But it exists. I remember reading on several occasions that part of the Prius's formula for fuel economy is to use special low rolling resistance tires. I wonder how many Prius owners got a sudden drop in economy when they changed out the worn tires for X brand special off the shelf at their local tire joint?

Has anyone else experienced this?

[wonderpuff]

jsut passing through this post i thought about what was said. if you're switching to large diameter wheels and tires its pretty much expected that it may take more gas to push the wheels. certain factors apply but mathematically speaking the larger the wheels and tires the harder the engine has to work to make them roll. unless you go with smaller tires and the whole diameter is near or close to stock. its a tricky subject there are other posts on here about the same thing and the wheel/tire specific section helps to answer some questions.

[keanucosmo]

I noticed that, unless I was going UP a steep hill, my speed was dropping very slowly. VERY slowly... like I could coast down a 1/4 mile flat exit ramp, starting at 60, and still be doing nearly 50 at the end of the ramp. I realized that the Continentals that came on this car could NOT do that.

The key is that you put it into neutral.

My exit for work is between 1/4 and 1/2 mile (I'll check for more detail next week). It is a slight incline for half the total distance as I'm exiting, then a slight decline to the intersection. Exiting at 65, my speed won't drop below 50. I'm still on the stock Continentals inflated to 33psi cold.

Also on regular streets that are level or slight decline, if I am at 50 or above I can maintain a good speed for at least a quarter mile when shifting to neutral.

Now to the low Rolling Resistance tires. There is merit to low RR tires increasing mpg.

SAE has had a RR committee as far back as 1975. There was steady state tests J-1269 and J-1379 as far back as 1979. In 1998 a Transient test J-2452 introduced. As general statements, Larger wheels will reduce RR, and tires with less tread depth will reduce RR.

I believe the first Low RR tires showed up in the very early 90's.Back in the late 90's I had an EV-1 (no pun intended EV) It had special Michelin 175/65R14 Proxima RR tires that were inflated to 50psi. I believe most quality tire manufacturers have been working on low RR tires since the late 90's. If you go for the cheapest tires, you probably won't get any low RR benefits. I believe the DOT is soon going to require RR data as part of the UTQG (Uniform Tire Quality Grade) so that you can also include RR as part of your tire buying decision.
Modified by keanucosmo at 11:20 AM 5/23/2008

[Great White Versa]

It may also have to do with the alignment of the tires post install. I just got my tires aligned (after lowering) and I've noticed a huge increase in the 'coastability' of the car.

[Ever Victorious]

It may also have to do with the alignment of the tires post install. I just got my tires aligned (after lowering) and I've noticed a huge increase in the 'coastability' of the car.

Car wasn't aligned after the new wheels/tires were put on. And actually, as it sits right now, the car's probably in the worst alignment state since it left the factory. Never done an alignment on it, and it's had some... well, potholes.

The key is that you put it into neutral.

As I also did with the Continentals. It's just that with the drivetrain disengaged, the difference between the two is so much more dramatic.

Quote »Now to the low Rolling Resistance tires. There is merit to low RR tires increasing mpg.

SAE has had a RR committee as far back as 1975. There was steady state tests J-1269 and J-1379 as far back as 1979. In 1998 a Transient test J-2452 introduced. As general statements, Larger wheels will reduce RR, and tires with less tread depth will reduce RR.[/quote]Hence why I think the lower rolling resistance helps cancel out the added weight of the wheels.

jsut passing through this post i thought about what was said. if you're switching to large diameter wheels and tires its pretty much expected that it may take more gas to push the wheels. certain factors apply but mathematically speaking the larger the wheels and tires the harder the engine has to work to make them roll. unless you go with smaller tires and the whole diameter is near or close to stock. its a tricky subject there are other posts on here about the same thing and the wheel/tire specific section helps to answer some questions.

Yup. I'm running 215/45/17's, which are the absolute closest to stock OD possible with a performance 17" tire. The total difference is something like .4% or .6% or something like that.

If I weren't so busy doing crap on my Rambler, I'd spend the time to weigh a stock wheel with my snows on vs. the new wheels with the performance tires and see exactly how much weight difference there is, too.

[keanucosmo]

The added weight may be helping maintain momentum. Just like a light weight flywheel will accelerate or decelerate faster. It may take a little more power to get the heavier rotating mass to the same speed, but it will maintain that speed better than a lighter rotating mass.

[KimberKenobi]

You had an EV-1?!?

I want to know what you thought of it... if you're not comfortable jacking EV's thread, you can just post it on here...

I personally don't care what EV thinks of me jacking his thread, he can sit on it and spin ;oP

hahahahahaha!!

[Ever Victorious]

You had an EV-1?!?

I want to know what you thought of it... if you're not comfortable jacking EV's thread, you can just post it on here...

I personally don't care what EV thinks of me jacking his thread, he can sit on it and spin ;oP

hahahahahaha!!

There's only one way you're allowed to threadjack me.

You must yell ARRRR! first.

[keanucosmo]

You had an EV-1?!?

I want to know what you thought of it... if you're not comfortable jacking EV's thread, you can just post it on here...

I personally don't care what EV thinks of me jacking his thread, he can sit on it and spin ;oP

hahahahahaha!!

ARRRR

I loved the EV-1. If I could have bought the car I would have, but GM took them all back & crushed them. I drove it for a year and a half as my only transportation. The only expense beside the lease payment ($200 a month) was a few dollars more (<$10) on my monthly power bill. The only maintenance was checking tire pressure & rotating tires. I usually charged it at work or a mall. Occasionally I would charge it at home.

Generally the range was 50-60 miles in cold weather and 70-80 in warm weather. I once made a trip of 103 miles in it and stopped at Chateau Elan for lunch & plugged into their charger. After an hour and a half it was charged enough for the 40 miles left to get home. The only downside was that if it was your only car you would not be able to take long trips.

If you have any more questions, just start a EV-1 thread & I'll be glad to answer them.

[KimberKenobi]

ARRRRR, matey!!

[aleckz]

So what are some good low RR tires? Does the UTQG rating have anything to do with this?

Also with this rolling theory... my Versa is very good at coasting especially compared to my old Civic. By this I mean, I can coast at a constant rate through some good stretches of small ups and downs with only a variance of +-5 mph and also the other day I put the V in neutral in a 65 mph zone on a mildly steep slope and it sped upto 70 mph which would have never been possible in my Civic (I don't remember if there was a tailwind or not)

Also, have you lowered your car? And if so does the V seem better in headwinds with the lower?

I'd appreciate any response.

[Ever Victorious]

As keanucosmo was stating, RR isn't yet incorporated into the UTQG ratings.

The tires that I have are BF Goodrich gForce Sports. They're a kinda spendy sport tire, but I have absolutely no complaints about how they handle, roll, or sound. A little stiff of a ride, but that's to be expected of a plus 2 sport tire.

[aleckz]

Ah, didnt catch that they weren't in it :/, those BFs sound nice though.

[XterraVersa]

So what are some good low RR tires? Does the UTQG rating have anything to do with this?

A general rule of thumb is that high tread wear tires have a lower rolling resistance. The more grippy a tire the more rolling resistance it has.

Another thing is the larger the tire-pavement contact patch, the higher the rolling resistance. Hence why tire pressure can have a huge impact on fuel economy.

For the 15 vs 17 rims, the smaller sidewall flexes less than the larger one, keeping the tire more round. This extra sidewall stiffness may contribute to higher fuel economy. Also the heavier flywheel effect as mentioned above with the 17 inch rim could lead to better fuel economy.

Tread pattern would also have a huge impact on rolling resistance. Groves parallel to the direction of travel have a lower rolling resistance than V groves.

[superskunk]

i mite be a lil late with this but, arent the stock rims and tires almost the same size as aftermarket 17's?

[XterraVersa]

i mite be a lil late with this but, arent the stock rims and tires almost the same size as aftermarket 17's?

Stock wheels are 15"

[Ever Victorious]

i mite be a lil late with this but, arent the stock rims and tires almost the same size as aftermarket 17's?

When you consider total diameter of wheels and tires, that depends on the tire you select to put on the 17's. My 215/45/17's are almost exactly the same diameter as the stock tires.