If I was looking for more fuel economy, and didn't want or couldn't change transmission final drive ratios, could I put bigger (taller) tires on to effectively change the fdr and get better fuel economy? For example if original tire was 185 60R 14 and change to 185 70R 14. Would this gain anything mileage wise? I know that the speedo would be off, and this is not something I care about relating to this question.

Yes it would help.

Slightly.

John Stricker

quote Originally posted by Ken_86gt: If I was looking for more fuel economy, and didn't want or couldn't change transmission final drive ratios, could I put bigger (taller) tires on to effectively change the fdr and get better fuel economy? For example if original tire was 185 60R 14 and change to 185 70R 14. Would this gain anything mileage wise? I know that the speedo would be off, and this is not something I care about relating to this question.

Yes, just make sure you will not be rubbing on your turn radius. Thinner tires also get better gas mileage as well.

also look at what is available in low rolling resistance as well. most tires that have insanely high mileage ratings are very low rolling resistance and get really good gas mileage increases..

But be warned. Low rolling resistance and high wear mileage = low traction. They will not grip as good as sticky soft tires, so stopping distance will be increased on all pavement. I know this as I have done the search for higher gas mileage.

also MPG gains on larger tires diameter will ONLY be at speed. you will lose gas mileage during acceleration due to the higher load. around town driving will probably not see any improvement. A combination of larger tires, lower rolling resistance, thinner profile, and higher tire pressure can add up to 3-4 mpg on a regular car and over 10mpg on a efficient hybrid like the Honda insight. Even more gains can be had by adjusting the alignment to be closer to zero but you will also sacrifice handling significantly. Oh! you also reduce tire life running at higher pressure.

[This message has been edited by timgray (edited 11-01-2007).]

Thanks for the info. I do most of my driving at highway speeds. I may try going up in size (circumference) about 10% when I change my tires next and see if I can get any mpg returns.

[This message has been edited by Ken_86gt (edited 11-01-2007).]

Ken_86gt, your previously mentioned switch from a 185/60R14 to a 185/70R14 should decrease your RPMs at any given speed by right around 6%.

I did this to a S10 blazer. I didnt see enough change to ever do it again. I had rather have the correct speed on my speedometer. Then I was stuck with the wrong size tires for about two years.

I might also suggest that you get two used tires mounted and balanced to test the performance before you buy two good oversized keepers for your car. I get used truck tires for my trailer from time to time very cheaply.

[This message has been edited by josef644 (edited 11-01-2007).]

I had 225/60/16's on my Formula & got an *indicated* 25 MPG on a trip towing a trailer loaded with fastback conversion peices & a set of steel wheels w/4 tires (two of which were NASCAR slicks). I say "indicated" MPG 'cause I never did the math to see how much the odometer was off - the real MPG would be even better.
~ Paul
aka "Tha Driver"

Of all the things I've lost over the years, I miss my mind the most!

exactly, after tire change you need to adjust odometer readings to get true numbers.

quote Originally posted by Ken_86gt: If I was looking for more fuel economy, and didn't want or couldn't change transmission final drive ratios, could I put bigger (taller) tires on to effectively change the fdr and get better fuel economy? For example if original tire was 185 60R 14 and change to 185 70R 14. Would this gain anything mileage wise? I know that the speedo would be off, and this is not something I care about relating to this question.

Just as an FYI, the "performance" tire size for the 14" wheels was 215/60/14s all around.

The standard tire, and also considered the economy size, was 195/70/14.

Although it will upset your speedometer, you should get better fuel economy with the 195 as they are somewhat taller.

Also, a way to improve fuel economy is to increase the tire pressure as well. You don't want to go above the maximum, but keeping the tire on the high-side will improve the life of the tire and your fuel economy. The reason being is that, when the tire is set to a lower pressure, you typically get a wider contact patch (usually front to back of tire) and there is in turn more friction on the tire. This is why drag racers usually lower the pressure in their tires.

In addition to that, the taller the tire, the lower your gear ratio ends up being. This means you can achieve higher top-speeds, or... lower rpms at typical cruising speed.


Your acceleration will be a bit slower though.

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Todd,
2006 Pontiac Solstice
2004 Volkswagen Beetle Convertible (Wife's Car)
2002 Ford Crown Victoria LX
1987 Pontiac Fiero SE / V6 (3.2) 5-Speed
1984 Porsche 944 5-Speed
1981 Pontiac TransAm (Olds 455BB)
1973 Volkswagen Type-2 Transporter

quote Originally posted by Tha Driver: I had 225/60/16's on my Formula & got an *indicated* 25 MPG....I say "indicated" MPG 'cause I never did the math to see how much the odometer was off - the real MPG would be even better.

Relative to the Formula's stock-sized 215/60R15 rear tires, the 225/60R16s you used represent about a 6% increase in your rear tires' diameter. In turn, that translates into your actual MPG being about 26.5, instead of the "indicated" 25 MPG you mentioned.

Best way to do it would be to get a lighter narrower super light rim (trak lite, TE37, etc) and a ultra hard high mileage tire. You wouldn't need to go taller by much if at all. Your car won't handle for crap but it will certainly help your mpg.

If you want to be getting lighter and maybe narrower wheels I suggest checking out Diamond Racing Wheels (http://www.diamondracingwheels.com/). It's a great source of wheels. They sell high perf. steel wheels, either spun or rolled, and they can come at a modest price. I used to think that steel would be heavier, but these wheels are made really light-weight and strong. My brother researched them for his Triumph. He hasn't gotten them yet so all the info I yet have is through research. You can choose every spec: width, height, backspace, boltpattern, offset, etc.

After writing this I realize that if the reason for wanting lower gas mileage is for a lower bill than this is probably quite an ironic thing to do. Unless you're driving millions of miles a year, it'd probably take quite a while to pay off. But, then again, you'll have a very low unsprung weight, which is great for handling.

My two cents,
~Fierology

quote Originally posted by Ken_86gt: If I was looking for more fuel economy, and didn't want or couldn't change transmission final drive ratios, could I put bigger (taller) tires on to effectively change the fdr and get better fuel economy? For example if original tire was 185 60R 14 and change to 185 70R 14. Would this gain anything mileage wise? I know that the speedo would be off, and this is not something I care about relating to this question.

Answer: Yes. But see: http://www.1010tires.com/TireSizeCalculator.asp

The change is size that you are contemplating will result in a 6% (approximately) increase.

As one or more others mentioned keep tires fullly (but not over) inflated. Also be sure to check the allignment.

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FierOmar

Thanks for all of the replies! My idea is to just change the tires and not the rims, as the mileage gains may never offset the cost of new rims. But if it is time for new tires, maybe it is worth trying. I was thinking about going up 10% because then I can easily do the math in my head for the speed conversion. The tire size listed above was just an example, the car I have has the 215/60R14 on it now with 4.11 gears. Based on the calculator it looks like if I go up to 215/75R14 it will give me 10.5% speed difference. Does anyone know if this tire (26.69 diameter) will clear everything on the Fiero?

What makes the final drive ratio is the height (or radius) of the outside of the tire. Nothing in the middle makes any difference. The rule is the taller the tire the more mileage and speed you get. The downside is you lose some pick up power by the same ratio. ie/ 24" total tire height will get better mileage than a 21" tire height. Without doing the math, it will go about 6" farther on each revolution. You will get even better with 78 series tires than 70s over 60s because there generally even taller.

quote Originally posted by rogergarrison: The rule is the taller the tire the more mileage and speed you get.

I agree.

quote Originally posted by rogergarrison: ...24" total tire height will get better mileage than a 21" tire height. Without doing the math, it will go about 6" farther on each revolution.

The increase per revolution actually will be 57% more than what you've mentioned.

With a 24" tire instead of a 21"tire, it'll be 9.4" more per rev, as opposed to 6" more per rev.

quote Originally posted by Ken_86gt: Thanks for all of the replies! My idea is to just change the tires and not the rims, as the mileage gains may never offset the cost of new rims. But if it is time for new tires, maybe it is worth trying. I was thinking about going up 10% because then I can easily do the math in my head for the speed conversion. The tire size listed above was just an example, the car I have has the 215/60R14 on it now with 4.11 gears. Based on the calculator it looks like if I go up to 215/75R14 it will give me 10.5% speed difference. Does anyone know if this tire (26.69 diameter) will clear everything on the Fiero?

You might want to reconsider swapping the 4.10 final drive for the 3.65. That 10.5% difference you realize would be more than 20% even if you stayed with the original tire size.

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FierOmar

[This message has been edited by FierOmar (edited 11-05-2007).]

yes for better milage - taller & narrower tires. the taller for the gear ratio, the narrower for less rolling resistance.
I have the 4.10 gears also, and have been thinking fo going with larger wheels/tires too. but, for the slightly higher top end speed.

check this out:

http://greenseal.org/resour...ollingresistance.pdf

its pretty preachy about recycling etc, but it has rolling resistance coefficients for different tires.

the lowest in their study was the "Bridgestone B381 185/70R14" which has an RRC of 0.0062 (what RRC is measured in, i dont know)

all i know is that the stock wheels on my 84 carried 185/80R13's. roll'n on 13's..... probably why it was so efficient.

JC