Downsizing from 205mm & 16" to 195 & 15" wheels/tires worth it?

[oldtamiyaphile]

Originally Posted by oil pan 4

Figure out what the difference between the new tires and old are and add that to the miles per tank.
I have found that most speedometers read high anyways.
Putting on bigger tires makes it read closer to reality.

Worth noting that speedos are calibrated to read higher than actual speed, while odometers are calibrated to be (mostly) accurate.

On my Jeep I had to go to a 2" taller tyre for the speedo to now read accurately (actual speed is 1% higher than displayed - that's kind of how I like it). The odo now reads ~6% less than actual distance traveled.

You can also confirm this via OBD, the (uncorrected) OBD speed may not match what's displayed on your OEM gauges.

[Ecky]

In my Insight, the digital speedometer rounds up, so 49.2 via GPS is 50 on the display, but the odometer is within about a fifth of a percent.

[mcrews]

Originally Posted by MPGee

Hi all,

I'm getting close to needing new tires on my '07 Toyota Matrix. I'm running factory 16" steel wheels with factory size tires. I'm considering going to '07 Corolla 15" steel wheels. I believe my #s for the below are correct:

Existing stock Matrix wheel/tire:
205/55R16, 16x6.5" steel wheel, 5x100 bolt pattern, 39mm offset

Proposed stock Corolla wheel/tire:
195/65R15, 15x6" steel wheel, 5x100 bolt pattern ?? offset

See attached photos for calculator results from rimsntires.com's calculator (I can't post links, b/c I'm a n00b. )

How much gains do you think I can expect from getting a 10mm "thinner" tire and -1" on rim size? The calculator says that the 15" wheel+tire should be ~3 lbs lighter.

I drive, maybe, 40% city, 60% highway. I would be trying to get some Corolla steelies used or from the junkyard.

Thx in advance.

based on the original info, I wouldn't expect any mpg increase.
But here is how to tell.
Remove your spare and 20 more pounds from the car. That would equal the weight saving and the rolling momentum from lower weight wheels.
Do an A-B-A test in a 10 mile section w your scangauge.

Now you have your answer.
the 10mm thinner is nothing on the aero side. Weight only really counts at start and stop. The new tires are effectively the same height so no gearing advantage.

Question: why go to all the trouble for almost no mpg improvement and not go whith a 5% taller tire?
it has been proven to increase mpg in A-B-A tested cars.
with 60% of your driving on the freeway consider lowering the rpms. That's where the rpm improvement comes from.

[mcrews]

Originally Posted by MPGee

I assume you're asking me? I'm not trying to go taller (as in larger overall wheel+tire diameter), because I'd like my speedo & odo to remain accurate... Otherwise, how am I gonna know if I've actually improved my fuel economy?

I laugh......
1. you should own a scangauge that would have the correct speedo.
2. I drove an Infiniti Q45S w taller tires for 170,000 miles and the speedo was off......who cares!!!!! I saved $1000 -$3000 in gas.
Beside, you have the SCANGAUGE which is accurate.


With your logic, then stip the car to bare metal to reduce weight and get better mpg due to light weight.

Tire size is one of the cheapest ways to improve mileage. your usually buying tires anyway. Size up.

[MPGee]

Originally Posted by mcrews

I laugh......
1. you should own a scangauge that would have the correct speedo.
2. I drove an Infiniti Q45S w taller tires for 170,000 miles and the speedo was off......who cares!!!!! I saved $1000 -$3000 in gas.
Beside, you have the SCANGAUGE which is accurate.


With your logic, then stip the car to bare metal to reduce weight and get better mpg due to light weight.

Tire size is one of the cheapest ways to improve mileage. your usually buying tires anyway. Size up.

Call me anal, but I like being able to have a relatively level before/after comparison basis. So, I value maintaining a consistent odometer. That being said, I'm gonna sacrifice that accuracy for the next 20K miles or so...

I found a guy listing OEM 2005 Corolla steel wheels (15x6") for $150 OBO. Also included were Falkien ZX612 (IIRC) 195/60R15. Sooo, the equivalent, that I should be aiming for is 195/65R15. I talked him down to $120, so at $30 a wheel & tire, that's not bad, IMO.

There's still a reasonable amount of tread on them too. (6 to 8/32"). So, at this point if I'm gonna use these tires, not only am I not going larger, but I'm going smaller. Next set will be similar to OEM or larger.

Thx guys for the info on the larger tires. Not sure if I wanna go 1"+, etc, but I will most definitely round up, rather than down. I didn't know about the link between gear ratios and FE.

And, in relation to those who were mentioning that they'd keep the same wheel size, here's one of the big French benefits that I observed on my test drive on the new wheels just now: NVH (noise, vibration, and harshness, for the uninitiated). The ride is noticeably smoother and quieter. That alone, even if this doesn't gain me FE benefits, is a major plus for me.

And I weighed all 8 wheels+tires, and averaged them.

Weights, per tire
18.83 kg (45.51 lbs) 16x6.5" with 205/55R16
16.39 kg (36.13 lbs) 15x6.0" with 195/60R15
2.44 kg (9.38 lbs) Difference

I was surprised by the weight difference just now. Double checked my #s, and those are it. Remember, though, that the 15" tires are smaller. So, if I were to get the correct size, the weight diff should be smaller.

Will post some more pics & datums when I have a chance to review it all...

[MPGee]

Originally Posted by mcrews

I laugh......
1. you should own a scangauge that would have the correct speedo.
2. I drove an Infiniti Q45S w taller tires for 170,000 miles and the speedo was off......who cares!!!!! I saved $1000 -$3000 in gas.
Beside, you have the SCANGAUGE which is accurate.

#1. OMG. I had to look into tire size & speedo, based on our discussions here..and I came across this post from you back in '09 asking about SGII adjustments... I HAD NO IDEA THE SGII had a speed adjustment option. That's friggin' awesome!! Did those Linear-Logic guys have us in mind when they designed the thing? Thx for the tip.

#2. It's not all about the Benjamins ($$$), for me. I think most of the population thinks what we do is nuts (at least, everyone who knows me and finds out about my hobby thinks I am). Most of them accuse me of doing this because I'm cheap. No. Although I am cheap , this MPG thing is a hobby. I've definitely long surpassed the point of diminishing returns.. I'm sure I could get more financial bang for the effort by focusing more time and energy on other parts of my life.. I enjoy this. And I also enjoy having a "consistent" medium (odometer) to measure my progress against.

Originally Posted by mcrews

With your logic, then stip the car to bare metal to reduce weight and get better mpg due to light weight.

I'm guessing, mccrew, you've already heard of this Acura guy who stripped 793.2 pounds off of his TL.. lol. I'm not doing that.

Side note: Based on the rimsntires calculator, the circumference of my 195/60R15s is 2.745% shorter than stock. In order to get my Garmin to show 60 MPH, I have to be going about 63MPH on the built-in speedo. Now that I've learned of the SGII adjustment factor, I will just need to train myself not to look at the factory speedometer and instead at the SGII...

[mcrews]

I probably should have gone into more depth...my bad.
I used my garman and scangauge instead of the speddo needle.
Yeah, my odo was off but I only 'cared' when I was on trips or testing and then I used the scangauge trip odo.
I was pretty obsessive about the mpg on the Q45 so I got over the speedo/odo being off.
There is a pic somewhere of the slim clearance when I mounted the taller tires. it was tight!!!!!

Yeah when you adjust the scangauge switch to meters. it gets closest to the exact distance. THen switch back to miles.

[oil pan 4]

Odometers cam be recalibrated.

[MPGee]

Originally Posted by oil pan 4

Odometers cam be recalibrated.

How do you do that? I'm assuming it takes factory type tools, and/or a visit to the Stealership? I'm guessing it ain't cheap...

[MPGee]

Originally Posted by MPGee

Weights, per tire
18.83 kg (45.51 lbs) 16x6.5" with 205/55R16
16.39 kg (36.13 lbs) 15x6.0" with 195/60R15
2.44 kg (9.38 lbs) Difference

So, as already noted, the ride is noticeably smoother and quieter, which I am enjoying. See attached photo for the before/after. One big bonus about moving to 15" is that there's a lot more wheel covers available, and also, I think properly-sized pizza pans are more available too.

I do not have any immediate plans to do an AB test, as I realized how much time will be involved... I don't have a lift, don't have jack, etc, so I'd be using what Toyota gave me in the trunk. That'll be later when I've got some free time, still weather, and agreeable traffic. Also down the road is weighing what the 15 and 16" steel wheels alone. When it's time for new tires, I'll ask the tire shop to let me weigh the bare wheels before they slap the new rubber on.

Thx to all for the tips.

In summary, I now have tires that are 1 cm narrower, and the total weight savings on wheels & tires is 37.52 lbs, which I hope is good for something...

I'll provide an update in a few tankfuls to see if I (empirically) notice any improvement in fuel economy.