Smaller Wheels = Back in the 55mpg Club!

[Denton T. Young]

I just completed a 55+ mpg tank.

845.5 miles/15.13 gallons = 55.88 mpg

Car: 2000 Golf TDI, 5 speed. Me plus two small passengers, the 10yr old is 65 lb and the little one is 45 lb.

I had been running 16" wheels with 205/55/16 tires but I switched to a set of 15" Avus wheels with the stock Michelin MXV Plus 195/65/15s (inflated to 40psi). My best tank with the 16's was 52 mpg and I averaged around 48 mpg.

Of the 845 miles, 830 was on the highway. For most of the trip, the speed limit was 65mph so I tried to stay between 65-70mph. There were some elevation changes....I started at 5000 ft.....climbed to 7300 ft.....down to sea level....back up to 7300 ft.....and down to 5000.

 

[Peter Cheuk]

I had a feeling that wheel size would affect the mileage numbers.

Hey, you coming to the Dyno GTG?

 

[dlai]

Nice numbers. I'm with Peter, I'm not surprised your mileage went up...So, are you in for the dyno day?

 

[Norman]

Hey, Cy, if you'd go north-south (like 395), you'd probably get up around 57 mpg due to the prevailing winds...

Yep, Avus wheels...I got 'em.... NIIIIIICE! /images/graemlins/laugh.gif /images/graemlins/wink.gif


Hey, so are you coming to dyno day?

 

[RogueTDI]

Less than 1% increase in tire rolling diameter - I dont think that was the cause of the mileage increase.

Im sure the smaller wheels might have contributed, but I dont see why would give several (3-7mpg) better.

Can anyone offer a reason?

And I know all pretty much all of the obvious technical differences - rotational inertia, wheel weight (possible increased damping losses).

Other than that, I would say this is a result of a stiffer tire, that is better(less) rolling resistance, NOT smaller wheel size. I guess tire profile, being partly defined by wheel diameter, might have had an influence on rolling resistance (for otherwise identical tires).

What tires were you running before? Big change in tire pressure?

 

[Denton T. Young]

I can't make the dyno day.....I have guests coming in. Guns and explosives have priority over cars.

[ QUOTE ]
What tires were you running before? Big change in tire pressure?

[/ QUOTE ]

My 205/55/16s were Michelin MXV Pluses at 40psi.

I normally don't drive with mileage in mind. I was trying for a 800+ mile tank but at halfway, I thought I had a chance for 900 miles so I tried to stay at 65mph. My low fuel light came on at 806 miles. I chickened out and filled up at 845 miles.


I believe the lighter wheels and thinner (and lighter) tires are giving me 2+ mpg. I'm not sure how much lower rolling resistance a 195 tire will have versus a 205 but the results speak for themselves.

First 44K miles with 195/65/15 Goodyear Eagle LS: 50mpg
Next 60K miles with 205/55/16 Michelin MXV Plus: 48mpg

 

[peter pyce]

It has very little to do with wheel weight. You can put Energy 195 on heavy steelies and still get better mileage than a 16" super light wheel but with 205 Energy on it! Tire width and rolling resistance have almost everything to do with your result. 14 lb. 16" wheel with 205 Energy (total about 34 lb now) sets me back by about 2 mpg compare to stock Avus 15" with 195 same tire model. And the Avus combo is about 36 lb. I can give you a set of Avus and Yokohama ES-100 and you will see how you will lose at least 3 mpg..... it really is not the wheel weight. It may incide slightly if stop and go traffic is the main use, and helps some in acceleration and braking, but it has almost nothing to do with economy.

 

[OilSmith]

On my 96 Saab I had 15 inchers on it with Michelin Energy tires (winter tires). I got 33.5 mpg on a trip to Florida loaded with 3 children and 2 adults a 22 cubic foot trunk packed solid. I made the same trip the following year with 16 inch tires (optional wheels summer) same diameter i lost about 2.5 mpg and the a/c was running less. The tires make the difference. I plan on buying a new Passat TDI I like the optional wheels 16 inch but no if i take a big hit on mpg. The old Saab is a 2.3 auto non turbo the trany has a lockup converter and it is a 4 speed. Im sure the Passat figures are lower than real world mileage my Saab was. My old MK II golf has 300,000 miles on it with the orginal starter in it cannot wait to get a new dub. /images/graemlins/grin.gif

 

[abctdi]

Did anyone notice that as tire diameter gets smaller, the odometer spins faster? Even with everything else being equal, just compare the ratio of diameters. But then every other thing mentioned above also changes.

 

[RiceEater]

I think the loading, climbing to ~7k ft., dropping ~7k ft at 65-70 mph has something to do with it. Also what would be the confidence interval for a population sample size of ...one tank?

 

[peter pyce]

Ha! How did I miss this:

[ QUOTE ]
..... I normally don't drive with mileage in mind.....

[/ QUOTE ]

Riiiiiight! LOL!

 

[Denton T. Young]

FYI...my worst tank ever was when I was in a caravan with Peter. (Capistrano to the bay area) /images/graemlins/tongue.gif

More on tire width....
The difference in tread surface area of a 195 vs a 205 tire is roughly 30 cubic inches. How much more rolling resistance 30cu inches adds.....I don't know....but it can't be much. Also, the wind resistance difference between the 195 and 205 is probably insignificant but I can't prove that.

Can any brianiac come up with the formulas?

 

[zeratul]

going from stock to 235/40r18 (i manually correct odometer reading due to larger diameter) i found that my mileage went down by about 2mpg (was at around 48 average). i dont have exact wieghts but the new wheels are very heavy. this will significantly cut into urban mileage but despite much wider tires i am impressed with highway results. about 75% of my driving is on the highway so the 3% i gained in circumference counteracts width and wieght to some extent.
note:i keep tires at 40 psi and speed is usually right at 75

 

[peter pyce]

[ QUOTE ]
FYI...my worst tank ever was when I was in a caravan with Peter. (Capistrano to the bay area) /images/graemlins/tongue.gif


[/ QUOTE ]

Yeah, that one was my BEST tank ever, LOL! /images/graemlins/laugh.gif

[ QUOTE ]
More on tire width....
The difference in tread surface area of a 195 vs a 205 tire is roughly 30 cubic inches. How much more rolling resistance 30cu inches adds.....I don't know....but it can't be much. Also, the wind resistance difference between the 195 and 205 is probably insignificant but I can't prove that.

Can any brianiac come up with the formulas?

[/ QUOTE ]

How can a "surface" difference be expressed in "cubic" units? I am confused.... Awaiting for the experts in calculations to jump in, but meanwhile can offer you to try the "heavy" steelies I have in the garage and see for yourself that weight is not really that relevant for the economy you got. Also, when comparing the 205 to the 195, take in consideration that there are four tires, so the "small" difference that one tire gives, becomes four times bigger when applied to the whole picture.

 

[Denton T. Young]

I meant square inches.

 

[Denton T. Young]

Second tank.....

746.9 miles/14.002 gallons = 53.34 mpg!!!