Plus one tire sizing effect on mpg?
 

Looking at upgrading from my 175/65/14 tires on 14x5.5 steel wheels to 185/55/15 on 15x5.5 aluminum rims with the same offset.

Overall tire diameter is very close and should make no difference.

I have not been able to compare wheel weights, but I'm hoping they will be the same or slightly lighter for the aluminum wheel.

How will the slightly wider tire affect me? I'd like to hear first person testimony from those who have done such an upgrade.

A wider tire will increase your frontal area, so you will see a slight uptick in drag. Otherwise, I'm not sure.

93Cobra#2771 -

Here's my assessment :

http://ecomodder.com/forum/showthrea...ires-2642.html

I didn't go for wider tires, so I can't comment on that. I think it would be detrimental.

Hope this helps,

CarloSW2

cfg83 - you did go for a wider tire. A 195/65/15 is 10 mm wider than the stock 185/65/15 and slightly taller as you found.

What have you found in your mileage comparisons so far?

I'm wondering if the decreased deflection/ heat generation will cancel out any loss from the slightly wider tire? 10mm is less than 1/2" from sidewall to sidewall.

93Cobra#2771 -

Thanks for catching that! I will post that on the thread as a negative. I will calculate the added frontal area. I am thinking it will be in the range of :

2 (tires) * 0.39(10mm / 25.4 = inches) * 6 (to 8 inches of exposed tire)

There will also be a horizontal increase in exposed tire area because the tire is slightly taller.

My main conclusion is based on the GPS/odometer correction. In my fuel logs I document a +2% distance correction from what my odometer is reading. I would suggest that whatever tire you get, you do a GPS before/after comparison. That will give you conclusive evidence as to what the gain is,

Caveat: I didn't think to do a GPS before test, so there is the possibility that my odometer was already -2% before the new tires.

CarloSW2

The pariticular upsize I have chosen is within .2% of the oem size tire. That is, if my memory is correct. I'm posting from my phone and too lazy to look at the miata tire size calculator.

I purposely want to keep diameter the same as I don't want to induce error into my odometer reading.

It won't be too much longer before I get the new tires, as my current ones are about shot. However I wonder if I can get definitive results as my mpg keeps increasing on each tank! Check my fuel logs and see my progress, mainly from refined driving techniques (and possible an über secret trick that I will share if it shakes out).

93Cobra#2771 -

I actually don't mind this because no matter what the odo reads, I *know* it's (just a little bit) better, :) .

Is it possible for you to do the before/after GPS test? That would (hopefully) add a little more data to your tire mod. Even if you go with the same size tires, I would think that the odometer would read different going from old/shallow to new/deep tread depth.

CarloSW2

As a general rule, wider tires are going to cause more fuel consumption - everything else being equal. The rolling resistance of a tire is mostly controlled by the amount of deflection, the amount of tread rubber, and the type of tread rubber used in making the tire. In your case, the amount of deflection is the same - the overall diameter is the same, the load is the same, and if you use the same pressure, that would result in the same deflection.

However, careful selection of a tire can change this. Just be aware that a tire with good RR generally does that by sacrificing the treadwear / and or traction. (the tread rubber compound part of the equation).

Capri - wouldn't deflection me less due to the shorter and stiffer sidewall?

I actually made the exact same tire-size switch, and my new tires aren't exactly long-life or LRR tires, but my MPG has improved (however, I think it's mostly a function of my driving). At worst, the slightly wider tires aren't hurting to any extent that I can detect.

I also get a warm, fuzzy feeling from multiplying my calculated mpg by 1.02. It's like a bonus!