Tall tire test (taller gearing advantage offset by wider, heavier wheels/tires?)

[SentraSE-R]

I did some testing of my new 205/50-17 wheels/tires v. the stock 185/60-15 wheels/tires. 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.

*Edited 5/19/11 to correct incorrect original tire diameter

[California98Civic]

Originally Posted by SentraSE-R

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.

Well, the new wheels look cool. And these numbers could mean that your MPG has actually improved, as you noted, the test itself is usefully suggestive but not conclusive. You might learn more as you drive around more. The extra weight will of course matter more in city driving, while the taller gearing won't help as much, no?

At minimum you know your new wheels are not costing anything too significant in mpg. And they look sharp.

[SentraSE-R]

Aside from the looks, the main reason I wanted to go taller was to get the engine buzzing less at freeway speeds. It used to buzz at 3200 rpm @ 60 mph. Now it's "only" 3000 rpm @ 60 mph.

[ngrimm]

Looks really nice and they didn't seem to hurt your impressive mileage. If you don't mind me asking, have you lightened your Scion from the factory curb weight? I noticed you have it listed at 2400 lbs with no mods whereas wikipedia states 3000 which is what I would have expected. Not doubting you at all but just curious.

[SentraSE-R]

The second generation (2008-2011) xBs weigh 3000 lbs. First generation (2004-2006) xBs are substantially smaller and lighter at roughly 2400 lbs. I haven't stripped any weight off of mine.

[Bill in Houston]

Same mileage, fewer revs, looks better, probably handles and stops better. All around a good move, I think. Thanks for testing and posting.

[phunky.buddha]

Very nice! I didn't lose any efficiency going to a taller wheel combo compared to stock (215/50-16 on 16x7 +40 wheels) on my wife's xB either- didn't do an A/B test like you did though.

[mcrews]

why do i feel like we already had this discussion......

12x4=48lbs
4 tires =192lb additional moving mass
almost 8%

[Bill in Houston]

Originally Posted by mcrews

why do i feel like we already had this discussion......

12x4=48lbs
4 tires =192lb additional moving mass
almost 8%

Soooo, yeah, just think if the wheels weren't heavier, there could be a noticeable improvement in mpg, maybe. Also, it probably means that this change would not help "city" mileage.

[SentraSE-R]

That's a possible option - to put taller tires on the stock wheels. 195/65-15s would be 5% taller than stock while only weighing 37 lbs. I'll keep my eyes and ears open for used tires in that size, for a future test.

I don't follow Mark's math. Unsprung weight affects inertia and handling, but I don't see how it equals an 8% increase in the effective weight of my vehicle, nor how that necessarily affects fuel economy. I pulled 48 mpg with 600 lbs of passengers on a 150 mile drive last Fall. The added momentum on downhills pretty much made up for the extra work getting the weight up to speed.