Wheels

[denguy]

I failed to take into account the tire diameters when posting the mileage differences between the 12 and 13s. How about this though? The 13" wheel with tire weighed 30#, the 12" w+t weighed 20#. Don't you think that should be factored in?
Also you have to be losing some mechanical advantage in each gear when using the taller tire requiring more foot in the gas to accelerate at the rate you were used to.

[Safety Guy]

Tire weight will affect your mpgs mostly in city driving, with lots of stop and go traffic. That's when the extra weight (and extra height, if applicable) will ding you.

Once up to a constant speed, extra weight and height (as in less mechanical advantage) won't be as important, since weighted mass at speed tends to remain at that speed (like a flywheel).

Your tire diameter, however, will affect how fast the speedo gear turns, which directly influences what the speed records. Taller tires will make it turn slower, thus showing fewer miles travelled, as well as slower speed.

Switching from a tall tire back to your original height 12s will result in the odometer showing more miles travelled per tank, thus making you think you gained mpgs. Some or even most of that gain will be false, due to the tire size difference. The lighter wheel/tire combo may account for only a fraction of the change.

When I figure my mileage on Aqua, with 175/70-13s, I take what the odometer shows for my tank full and add four percent to reflect the larger tires:

250 miles odometer reading
times four percent = 10 more miles which gives me an actual calculated mileage of:
260 miles

Then I divide my gallons used into that figure.

Why do I use four percent? Because that's what my GPS shows is the difference between what my speedo shows and what a buncha satellites up above say.

Karl

[Pu241]

Karl, I agree with your assessment of the weight.
But, the extra height will take it's total more so at a constant highway speed.
I have gotten some of my best MPG fully laden, lowering the body to the point that the top of the rear tires were not visible. The extra height from going to a taller tire is more than off set by the resulting taller "gear" obtained, provided you are not cruising at high speed.

[Clay]

Yep, 1025 is correct. I found those markings a long time ago when I was pioneering the 140MPH protege speedo swap.

Is it possible to take the odo out and change it and put it back together and have it work properly?

htchbck, I want to advance that one I got from you in order to keep the odo consistant.

[htchbck]

Never tried it before, so I don't know... I have a spare protege speedo with a broken gear in the basement. I suppose I could try it, if I remember to