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Old 05-18-2011, 11:05 PM   #21 (permalink)
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Quote:
Originally Posted by arcosine View Post
My car gets better mpg with taller tires and wheels, FACT, not opinion.
How much better, Tony? What change of swap did you make in terms of sizes and weights? -j

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Old 05-19-2011, 08:07 AM   #22 (permalink)
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185/70R13 to 185/75R14, 10% larger OD, 5% mpg improvement, 1987 Honda Accord LX 5-speed

Only one data point on sc1 going from 165/80R15 to 205/70R15, so far not negative.

http://ecomodder.com/forum/em-fuel-l...vehicleid=4394
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Last edited by arcosine; 05-19-2011 at 08:30 AM..
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Old 05-20-2011, 07:44 AM   #23 (permalink)
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Some very interesting posts here, folks!

On my next tyre change I am tempted to get some taller tyres from the 2nd hand place, do some rough tests.

I am also somewhat superficially considering a set of alloys as my well trims (all 3,lol) are chipped, split and tatty (I hate trims!). Again on the next tyre change I'll get on with that - I have seen sets for peanuts with reasonable tyres on em. Viable for sure.

I'm currently charting mpg and usage and can see correlations between driving style, SVO, trip distance etc... And mpg (obviously).

I must say running near straight veg drops the mpg by several mpgs. The offset easily made up by the saving.

I will publish my results and hopefully become more methodical in my approach. I might do half-tank fills as it allows for more data points, it is less weight and I can hopefully derive more meaningful data from this.

Best recorded run so far: 49.2mpg which was 60-40 veg-diesel with 1.25l of petrol.

Best unrecorded run: approx 55mpg on diesel (before my veg days) and speeds to 45mph, changing around 2k.

I don't have high hopes for this run as I thumped a curb and have done about 160 miles with a terrible alignment (new steering arm in yesterday, alignment soon). At one stage I could swear I saw the fuel needle dipping

Cheers for all the replies guys, always interested in unofficial stats and your recorded miles etc... So post away!

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Old 05-22-2011, 04:24 PM   #24 (permalink)
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Regarding the thumped curb; I slid on a downhill corner and hit the otherwise corner curb at 40+ all bent parts replaced
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Old 05-22-2011, 05:03 PM   #25 (permalink)
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Quote:
Originally Posted by arcosine View Post
185/70R13 to 185/75R14, 10% larger OD, 5% mpg improvement, 1987 Honda Accord LX 5-speed

Only one data point on sc1 going from 165/80R15 to 205/70R15, so far not negative.

1998 saturn sc1 Gas Mileage (sc1) - EcoModder.com
I'm gonna just change up from a 19 pound cheapy 185/65-R15 to a 20lb. 205/65-R15 LRR set. Gain nearly 5% in OD and the LRR qualities without adding much weight... sounds like a deal to me.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



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Old 05-23-2011, 12:31 AM   #26 (permalink)
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Quote:
Originally Posted by Big Dave View Post
Increasing tire OD almost always backfires and MPG is reduced.

It might work if you were a long-haul trucker who can maintain the same speed hours on end, but most of us have to deal with stop-and-go situations.

Rotational moment of inertia goes up with the square of diameter. That means those four "flywheels" have to absorb more energy to increase their RPM. Until the tire RPM increases road speed doesn't increase.

Another evergreen myth that deserves a sticky.
A 'MYTH" is something that can be easily disproved.
A 'MYTH" is usually expressed as an absolute.

MYTH: Increasing tire OD almost always backfires and MPG is reduced.

meets the first test.........

It might work if you were a long-haul trucker who can maintain the same speed hours on end, but most of us have to deal with stop-and-go situations.
Meets the second test!


All by the same poster!

please refer to ALL my posts. I run larger tires. I get better mileage.

After all NOT all of use poke around town. Some of us drive 30-35k miles a yr and just achieved 30.4mpg w/epa of 22mpg.
Increased tire size accounted for 2.2mpg of the 30.4.

caveat: you need to spend some time on the freeway or at a cruise speed to gain a noticeable increase.
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Fourth: rear skirts and 30.4mpg on trip!
http://ecomodder.com/forum/showthrea...tml#post247938

Last edited by mcrews; 05-23-2011 at 12:40 AM..
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Old 05-23-2011, 12:50 AM   #27 (permalink)
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I'm considering going one size up in diameter tyre AND compesnating for ride height increase (aero). Where I live I can go up 5% in rolling diameter without having to have low volume certification for the mods.

I suspect this may work to reduce MPGs because I have an automatic, bascially it means the road speed of all shift points are raised.

So thats the question, should an automatic get some gains in a taller final drive outside of TC overdrive lockup?
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Old 05-23-2011, 01:17 AM   #28 (permalink)
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Old-school Tempo 3-sp no locking converter definitely got better mpgs with taller tires- it was revving too much. F150 4-sp a/t w/locking converter got worse mpgs with taller tires- was probably at max tall gearing for efficiency with the stock tire size.
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Old 05-23-2011, 02:47 AM   #29 (permalink)
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Sorry for the cross post, but this probably belongs in this thread. I did some testing of my new 205/50-17 wheels/tires v. the stock 185/60-15 wheels/tires last week. There are too many variables for this to be a realistic comparison, but this testing is better than just guessing what happens if you swap to 6% taller tires and wheels that weigh 12 lbs more, each.

Car with stock tires/wheels

Car with new tires/wheels


Internet tire specifications:
======= ..... new ...... old
diameter ......... 25.07"...23.74"
revs per mile.... 829.6 ....876.1
speedo diff...... -5.605%...0%
diameter diff..... 5.31% ....0%

In their favor, the new shoes are taller geared, look better, and have 51 psi sidewall maximums (v. stock 44 psi). Working against them is their extra weight, worse aerodynamics, and wider tread. On balance the newer, taller tires should get me worse mpg, as it's likely the minuses outweigh the pluses.

New weight 47.3 lbs Old weight 35.3 lbs (tire + wheel weight)

distance calibration (tested against GPS after 40 miles, minimum distance)
new 1.099 old 1.033 (odometer shows 10.0 miles, I've traveled 10.99 miles with the new tires, or 10.33 miles with the old tires)

The drive is a 10 mile loop (cruise control, 32 mph) with minimal traffic, and no stop signs. I do have to turn around at each end to go the other direction. It's at sea level with a few hills - the tallest one perhaps 100' high. Temperature was 65-70 degrees, partly cloudy, winds 0-5 mph from the west. The route runs north and south. Roads were dry. Mpg calculated with Ultragauge.
new1..49.2 mpg.....old1..50.1 mpg
new2..50.4 mpg.....old2..50.0 mpg

Essentially identical performance. It's roughly confirmed by my 900 mile trip last weekend with the new tires. I got 53.7 mpg my first tank and 47.7 mpg topping off the tank in preparation for the drive home. The 53.7 is my best highway tank, and 47.7 is about average for me when I had the old tires. The new tires get similar mpg to the old ones.

Considering the 17" tires/wheels weigh 12 lbs apiece more than the stockers, are less aerodynamic, and are 20 mm. wider, I'm pleased that I didn't lose mpg with the trade.
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Old 06-03-2011, 05:21 PM   #30 (permalink)
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I'm about to do a tire/wheel swap on my '05 XB. It's a 5spd that runs about 3k rpm @ 60 mph in top gear. I'm swapping from 185 60 15 with 15 x 6 steel wheels to 165 80 15 on 15 x 5 steel wheels(echo). Weight should be close to same or less. I'm also going to run the echo struts to keep the ride height about the same as stock. I think the low geared manual xb will benefit from this swap (mpg wise)

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