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This info is probably here, but I didn't find, so I googled around and found these formulas:

Tire Diameter = Rim diameter + 2 x (width(mm) x height(%) / 25.4mm/in)

For 195/55R15 for example, the 195 is tread width in mm, 55 is sidewall height from the rim to the tread as a percent of tread width, and 15 is rim diameter in inches.

So, diameter at the tread is 15 + 2 x 195 x .55 / 25.4 = 23.44"

Circumference = diameter x pi

So, circumference = 23.44 x 3.14159 = 73.64"

There are 5280 x12 = 63360 in per mile, so this tire turns 63360/73.64 = 860.4 revs per mile. At 60 miles per hour, it also turns 860.4 revs per minute.

Turning to gears, we need the product of the gear ratio at the transmission and the rear end ratio (or front end ratio for fwd cars, but you get what I mean). Each ratio is simply a division of the number of teeth on the cogs. One can find the ratios on the web.

The Fit 5-speed, for example, has a 5th gear ratio of .757 and a final of 4.294, for a product of 3.25 which is the engine revs needed for 1 turn of the wheels.

At 60 mph, the Fit tires turn 860.4 revs per minute. Multiply 860.4 by 3.25 = 2796 engine RPM at 60 mph.

The intent of these formulas is to help in the search for low rpm highway cars. It seems that in the US, all the low-cost manual trannys are geared far too low for highway cruising.

If I don't round off along the way, I think the 2008 Fit has 2811 RPM at 60mph.

Gotta run. Will refine later.

... So, circumference = 23.44 x 3.14159 = 73.64"

There are 5280 x12 = 63360 in per mile, so this tire turns 63360/73.64 = 860.4 revs per mile. ...
Gotta run. Will refine later.

Recently I have received the impression that this is not a very accurate way to figure tire revolutions per mile, because the tread and sidewall and almost everything else squishes and squirms around.

Instead, many tire specs list "revs per mile", and it is higher than the figure computed above. This is primarily a measure of the circumference of the steel belt cords underneath the tread, and shouldn't change significantly as the tread wears down.

I'm not a tire expert, so does anyone else have more detail?

Interesting you should post that…

I was also looking at info about tires and gearing.
Taller tires would be just like increasing the gearing.

I am wondering if the bigger (larger diameter) tire will give an MPG increase great enough to cover the cost of new rims.

My car is a 5 speed. IMO it could use a 6th gear or a taller 5th. The car has 16inch rims, Looking at the specks for my car I see it can take 17inch rims. The larger tires would give me an extra 2 inches in circumference.

Here is some tools you might find useful.
I found a nice site to calculate diameter and Circumference for a given tire size.
http://www.car-videos.net/info/tirespecs.asp
A second page at that site you can put in each gear, final drive gear, and tire size. It will then give you your speed for each gear and RPM.
http://www.car-videos.net/tools/speedrpm.asp

my second question, (if it turns out that the bigger tires and rims would pay off)
how to adjust the odometer? The larger tires will cause the speed to show slower than true. The miles driven will also be lower then true miles driven. So I would be getting better mileage but it may not even show due to not recording all miles.

abcdpeterson-

You can increase tire height without changing the size of the rim. A higher aspect ratio (the percent of sidewall height is of width) is what you are looking for. An online site such as tirerack.com offers wheel and tire packages, but note how they shrink the aspect ratio as the rim size increases to maintain the same circumference as OE.

Remember that tire size also affects ride height and aerodynamics, handling, wheel well clearance during turns and under load, startability and gradeability (hills).

Keith

One thing to note is that the calculated tire size is only a general guideline. Actual tire diameters and widths will vary from manufacturer to manufacturer, and sometimes from model to model. The diameter changes don't seem to be very large, but the widths can be.

The listed revs/mile (or even a "rolling diameter", which a few tires will list) is more reliable than the calculated revs/mile, when it is available. When it isn't, you go with what you got.


@abcdpeterson, it is almost universally going to cost you more to buy new wheels and larger tires than you will ever see in fuel savings from having larger diameter tires. The larger wheels and tires will be heavier as a rule as well, and that will hurt your around-town FE. I think the payback time for an "average" set of wheels winds up being 20 years or something like that?

-soD

@abcdpeterson, it is almost universally going to cost you more to buy new wheels and larger tires than you will ever see in fuel savings from having larger diameter tires. The larger wheels and tires will be heavier as a rule as well, and that will hurt your around-town FE. I think the payback time for an "average" set of wheels winds up being 20 years or something like that?

-soD

Thanks
sounds like I will stick with the size I have now.
I will just keep an eye out for low rolling resistance tires when the time comes for new.

I'm not as interested in precision as I am in finding a ride with good gearing for fuel economy. So my deal is comparability. And for that, I need data. It is very easy to find basic tire dimensions. That said, as we have revs per mile data available, I would use it.

Will check the links above and refine the OP when I get some time.

Thanks all. btw, I have no low cost answer yet for a MT car w/ proper gearing. The Fit and the old Scion xA ar worst I've found, followed by Yaris and Scion xD. A little better are the Mazda3 4dr iSport and the Civic. Range so far 2985 to 2451 at 60 mph. Compare my AT Fit at 2171 to the MT Fit at 2811...

I have done some searching and cannot find the gear ratios for my Accord. Although CleanMPG was the first result when entering "1996 accord gear ratio" on Google.

Chevy 2009 Cobalt XFE EPA 25/37 2.2L M5 195/60R15 5th=.7 Final=3.74 gives 2181 RPM at 60 mph. This is easily the best I've found for low price/low rpm manual tranny. Unfortunately, the old model years were geared poorly like other US manual trannys.

The XFE should crush the Fit MT at highway speeds. Should allow 60mpg with FAS, etc.

Chevy 2009 Cobalt XFE EPA 25/37 2.2L M5 195/60R15 5th=.7 Final=3.74 gives 2181 RPM at 60 mph. This is easily the best I've found for low price/low rpm manual tranny. Unfortunately, the old model years were geared poorly like other US manual trannys.

The XFE should crush the Fit MT at highway speeds. Should allow 60mpg with FAS, etc.

For 2009, they are using the automatic's 3.63:1 final drive rather than the 3.74:1. That likely accounts for a good chunk of the boost in highway FE from 35 to 37. With the 3.63:1, you're looking at roughly 2100rpm in top gear. They also removed the spare tire and jack, replacing it with a can of goop-n-air.

I have done some searching and cannot find the gear ratios for my Accord. Although CleanMPG was the first result when entering "1996 accord gear ratio" on Google.

Just wanted to see if I could find anything for a 96 accord.
found this.
http://hondaswap.com/reference-materials/honda-transmission-specs-29132/

Prelude and Accord Tranny Specs
Transmission Name Engine Counter Part 1st Gear 2nd Gear 3rd Gear 4th Gear 5th Gear Reverse Final Drive
P2A5 94-97 Accord DX/LX 3.307 1.809 1.185 0.903 0.685 3.000 4.062
P2U5 94-97 Accord EX 3.307 1.809 1.185 0.933 0.685 3.000 4.062
P2A4 94-97 Accord Lx/EX Wagon 3.307 1.809 1.185 0.903 0.685 3.000 4.062
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My wife's Element is the worst so far, 3000 @ 60.:mad:

Thanks. I got tired of looking after about 45 min or so.