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Old 04-10-2009, 01:16 PM   #1
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Hello, and now a question!

Greetings to all the regulars. I am interested in trying to improve my driving technique and also have a tire question.

I have a stock 2001 Ford Focus ZX3 with tires that are near requiring replacement. I am hoping to improve my fuel mileage a bit when I get new ones. The stock tire size (currently on the vehicle) is 205/50R16. I'm running them at 50 PSI and driving carefully. I usually get between 31-33 MPG for my commute.

My car has the 5 speed manual and turns around 3000 RPM at 70 MPH. It has a high power to weight ratio (2.0L 16 valve Zetec) and I think could use some gear ratio adjustment, with a tire change being the easiest method possible.

I had considered swapping wheels to get a skinnier tire, and also considered just going up on my tire diameter by going with a 205/55, 205/60, or even 205/65R16 size tire. Calculations via this site:

http://www.bigcustomwheels.com/rt_specs.jsp

would indicate that going to a 205/65 would drop RPMs by 9.8% for a given speed, which I would expect to help my fuel economy significantly. I do mostly highway driving, but increasing the moment of inertia with larger tires has me a bit concerned about acceleration losses.

What would be the best move in terms of increasing efficiency by a tire change?

a) upsize the tire modestly (to a 205/55 or 205/60)
b) upsize the maximum (205/65R16, which I think is the largest I could fit in my wheel wells)
c) change to a different wheel--get a skinnier tire on a 14"x5.5" old steel wheel, for example, which would (I think) fit the car

I've read the other threads I could find regarding rolling resistance, and would also appreciate any help I could get in identifying the best tires to get for options A-C above, subject to frugality!

Thanks for any guidance--

Harry
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Old 04-10-2009, 01:28 PM   #2
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from theholycow's signature...

http://miata.net/garage/tirecalc.html
http://www.gassavers.org/showthread.php?t=7713
http://www.gassavers.org/showthread.php?p=110803

in short...

stay wide, go taller, and buy LRR tires.
don't know if these are LRR, but a good price...
http://www.tirerack.com/tires/tires....peed_rating=(Y)

or just go to www.tirerack.com and follow the prompts.
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Old 04-10-2009, 02:05 PM   #3
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Thanks for the link--the tires you specced on tirerack.com are the same as my current ones (size-wise).

I am considering something like the Falken ZE-912, which I found on savontires.com when I put in "205 65 16" as my search term, and cleverly looked for the cheapest tires... Those would give me the 9.8% lower RPMs at a given speed, at a cost of $76 each with free shipping.

Is there any public info on rolling resistance? Seems like a lot of guesswork when we should be able to look it up somehow.
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Old 04-10-2009, 03:55 PM   #4
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There's little info on rolling resistance. Even when you find info and do the math, it actually makes more sense (or saves more cents) to buy tires with longer treadwear than to spend more on LRR tires. The difference in RR from one tire model/size to the next is probably very small, even when looking at LRR tires.

Changing wheels is also something that usually won't pay for itself.

If you're looking specifically to get high MPG numbers then those things could be worth it, but if you're looking to actually save money, not so much. Try the 205/65R16 tires and beware that they may rub. Be sure to calculate all the variables correctly after changing size. If they have a 9.8% difference, you'll have to multiply your odometer readings by 109.8% to get accurate distances (like when computing your fuel economy), and you'll be driving 109.8% of the speed that your speedometer reads (60mph will read 54mph, or when it reads 60 you'll be going 66).
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Old 04-10-2009, 07:31 PM   #5
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Some of the trends I found are that a tire with a lower speed rating will usually have a softer sidewall, which reduces RR and sometimes reduces weight. Softer sidewalls also improves comfort, so you can bump up the pressure without feeling like a masocist. Tires with low speed ratings commonly come with high treadwear rating. With mostly freeway driving, aero is more important than RR, and weight is less important. Narrower tires will improve aero drag and reduce weight a little. Maybe look into 195/60 or 195/55 with an H rating and greater than 400 treadwear rating on stock wheels?
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Old 04-10-2009, 08:19 PM   #6
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would it help more to drop your speed from 70 mph to say 60-65 that alone may help tons on the milage. I would also stick with OE size tires but a high pressure version. I run 205 65 15 Khumo ASX they are rated at 51psi. Add a scangague and start reaping the benefits.
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Old 04-10-2009, 10:58 PM   #7
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The Uniroyal TigerPaw AWP-II are the same thing as a Michelin Energy. X-Green stamped, 65,000 mile tires (the UniRoyal doesn't have the stamp, but it is the same tire). Roughly the same price as the tires you mentioned. Just cant get them through Tire Rack.
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Old 04-13-2009, 07:30 AM   #8
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Thanks for all the additional advice.

I have made my driving changes over the last couple of months to enhance efficiency (slower overall, more coasting downhill, DWL uphill [when not in heavy traffic]) and am currently running my tires at about 50 PSI without problems. Those tires will need a swap soon, so I'm just hoping to enhance my MPG rating a bit with any changes.

I hope to get access to the Ford WDS tool, since I have a new keyless entry module to install in the car, and while I reprogram that to work I will also reprogram the computer to account for the new tire size. I'll let you all know how things turn out after a few tanks with the new rubber!
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Old 04-26-2009, 05:00 PM   #9
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i just joined here, but i am a mechanic and often work in the tire shop at work and can say beyond the shadow of a doubt do not overinflate your tires more than 10psi over the cold rating on the sidewall in the winter, or 5psi in summer (in climates that routinely exceed mid 80s). i know its less rolling resistance, but this can cause the sidewall and tread area to seperate and cause a sudden and uncontrollable blow out.
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Old 04-28-2009, 03:36 AM   #10
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Quote:
Originally Posted by hi tech hillbilly View Post
i just joined here, but i am a mechanic and often work in the tire shop at work and can say beyond the shadow of a doubt do not overinflate your tires more than 10psi over the cold rating on the sidewall in the winter, or 5psi in summer (in climates that routinely exceed mid 80s). i know its less rolling resistance, but this can cause the sidewall and tread area to seperate and cause a sudden and uncontrollable blow out.
Good advice and welcome to the forums. Tell us if you have ever seen any problems with lower profile tires?
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