Fuel Economy and New Tires

<div class='quotetop'>QUOTE(miniracer @ May 17 2006, 08:07 PM) [snapback]257228[/snapback]</div>

My guess is that there is some other factor(s) involved...I've been running with Goodyear Comfortreds since day 1 and now after 32,000 miles have averaged just under 51 mpg. Do you have the tires inflated to 42/40 psi, or at least 40/38psi? Perhaps also you haven't had your new tires long enough to "break" them in yet, but my best guess would be that your mileage decrease is not the fault of the Comfortreds.

 

If you killed the integrities in only 25k, I'd begin with your
driving style...
.
_H*

 

I replaced the Integrity tires on my 04 at about 20,000 miles with Triple Threads. I sufferred about a 2 MPG hit for around 4,000 miles. Now my mileage seems to be about 3 MPG higher after factoring weather and A/C use in. It seems like there is a decent break-in period with the better tires and probably a good bit of tread wear off needed to get the better mileage.

 

<div class='quotetop'>QUOTE(David @ May 17 2006, 09:35 PM) [snapback]257292[/snapback]</div>

Yes, this is occurring for my 05 Prius as well. I replaced the tires on my 05 Prius at about 26,000 miles, thanks Toyota for the tire warranty, but my recent gas mileage is at 32 MPG...yikes. What's going on?

 

The Comfortreds are a much softer-riding tire, even when you inflate them to 40/38. I'd recommend you go up a little on your tire pressure.

 

<_< Even tires have a "break-in" period... and when rotating tires, there's a "mini break-in" period as well. New tires have moulding "tips" and sharp edges on the tread pattern which wear off after some use.

Evaluate after 1,000 miles... and check the air pressure for balance. Installers can be somewhat casual about specific tire pressures. I purchased a very accurate analog tire gauge from Racer Parts Wholesale and a 12-volt compressor, both stashed in the left rear storage compartment to maintain my "tire pressure of the day or week or month." My OEM Integrities show slight wear af ~19,000 miles - look good for ~30,000 or more.

AND.... SMILE when you pass a Service Station...!!!

 

<div class='quotetop'>QUOTE(miniracer @ May 17 2006, 09:56 PM) [snapback]257345[/snapback]</div>

You probably mean OEM bare-minimum $50 tire. There are other equivalent cost tires that perform at least twice as well.

 

I would believe it. I have an '02 classic and I just changed out the OEM tires with Michelin Hydroedge tires. My gas mileage dropped by 6mpg! That was the only change. I only did this 10 days ago, and today went to the dealer and told him I want to invoke the Michelin 30 day satisfaction guarantee and return them for a set of the original Bridgestone Potenzas. The Hydroedge is a better tire, and I can corner faster with them, but don't want to give away 6mpg. This is not my performance car, so I will deal with a little less performance and better gas mileage.

 

New tires def. have a break in period. On motorcycles you feel this clearly. new tires are very slippery and don't give a smooth traction as the first layer wears down.

I could def. see how new tires can affect it. It was 2-3 years since I was riding on new tires last but I seem to remember my mechanic asking me to take it easy for several hundred miles

 

<div class='quotetop'>QUOTE(miniracer @ May 17 2006, 08:07 PM) [snapback]257228[/snapback]</div>

Only because nobody has already stated this, I'd like to point out that a measured 1 mpg is the expected measurement error when going from worn to new tires. I'll point out that if your MFD and tripmeter calculations said that you you'd lost 1 mpg with the new tires, your mileage actually remained the same. Remember that the car (GPS aside) doesn't actually measure miles traveled. The speedometer, odometer, and MFD measure wheel revolutions. A new Integrity has a 25.4" diameter. That's with 10/32" of tread depth all around. As the tires wear and the tread depth falls to 2/32", the tire diameter falls to 24.9". Similarly, the rolling circumference falls by roughly 2%. Because of this, if a new tire says you've traveled 49 miles over a route, a tire worn down to the wear bars would say you've traveled 50 miles over the same route. If you consumed the same amount of fuel with either tire and the new tire said you obtained 49 mpg, the old tire would would say you obtained 50 mpg. When you replace fully worn tires with new ones of the same design, your measured fuel efficiency will fall by roughly 1 mpg even if your true fuel efficiency remained the same.

If your measured fuel efficiency has fallen by 3 mpg, your actual fuel efficiency has fallen by roughly 2 mpg. I agree that this is due largely to rolling resistance. (As pointed out this can be reduced with higher pressures and without a great ride penalty in a soft rolling tire like the comfortred.) Rolling resistance comes largely from the tire absorbing energy as it flexes or deforms around the contact patch. Part of this is the tread squirm of individual tread blocks deforming as the tire rolls. As the tread blocks wear down this part of the rolling resistance will decrease. Also the unsprung weight or the tire mass slapping the pavement with each rotation (over 800 times a mile) will decrease as the tread thins. This will also conserve energy and increase mpg. It makes perfect sense that measured and actual fuel efficiency would be lower with a new tire, even with the same tire design and after break in.

Can others report what measured drop in mpg they've observed replacing tires with the same design? This would really help those replacing a tire (Integrities) with a new design estimate how much of a mpg decrease is simply due to full tread depth as opposed to a less efficient design.

If you replaced your Integrities with Integrities, how large a drop in fuel efficiency did you measure?

 

<div class='quotetop'>QUOTE(miniracer @ May 17 2006, 07:01 PM) [snapback]257275[/snapback]</div>

I have the ComfortTreads at 40/38 and they do not get harsh. Actually I like them better at the higher pressure. I changed the Integrities at 700 miles, so they hadn't broken in either. No drop in mileage due to the tire change, (but definitely due to weather and short trips + hills.) I'd go ahead and bump up the pressure to about 40/38. Give that a try for a tankful and don't tell anyone that you've raised the pressure for a few weeks. See if they even notice.

FWIW, we're now averaging about 44 mpg, short trips, hills, ambient temps about 55-60. Looking forward to a long trip or two this summer and for the tires to break in fully. Pri is just approaching 2000 miles, tires at about 1300 mi..

 

<div class='quotetop'>QUOTE(theorist @ May 24 2006, 08:41 AM) [snapback]260280[/snapback]</div>

Thanks for this info. Does anyone know if the tire calibration on the NAV system effects how the MPG is calculated by the car's computer? Or is this calibration simply for the NAV system to use when there is a poor GPS signal ?

A second question is how much of an effect does increasing tire pressure have on changing the effective rolling distance of the tire and thus effect the odometer and speedometer. For example, raising the tire pressure from 35/33 to 44/42 would decrease rolling resistance, but would also increase the the rolling circumference of the tire (I don't have any clue by how much) and cause the odometer to read lower say 49 miles when at the lower pressure it might say 50 (if the change was 2 % as in the example above).

 

I am NOT an agrresive driver (anymore) and had to replace my OEM's at 22k. I put Comfortreds on and use 40/38. My mpg came down maybe 2-4 mpg after break in.

 

<div class='quotetop'>QUOTE(maineprius947 @ May 26 2006, 10:16 AM) [snapback]261369[/snapback]</div>

I think it's the latter, only used by the NAV for turns, tight maneuvers, and GPS reception problems.

 

<div class='quotetop'>QUOTE(maineprius947 @ May 26 2006, 01:16 PM) [snapback]261369[/snapback]</div>

What do you mean by "tire calibration on the NAV system"? Can the NAV system, speedometer, odometer or anything else be re-calibrated for new tires?

Also, I don't think raising the tire pressure would increase the rolling circumference of the tire since the tire won't bow in the middle with more air, thus lifting the side tread off the ground. But I'm only guessing.

 

New York,

Remember, the effective radius of your tires is the distance from wheel center to the center of the tire's contact patch. The contact patch presses in farther at lower pressure, so increasing pressure increases effective radius very slightly. Effective radius is expected to be slightly less than calculated radius.
Ernie Rogers
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Driving for Efficiency
May 28, 2006
989 miles /13.3 gallons ---> 74 mpg
B5 5% biodiesel blend
2003 Beetle TDI + wing
Michelin Energy tires
Motul specific 506.01 motor oil
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<div class='quotetop'>QUOTE(nyprius @ May 29 2006, 01:40 AM) [snapback]262297[/snapback]</div>