No, it should be correct. If you have an oversized tire, and your odometer says you traveled 1 mile, you actually only traveled something less, let's say .75 mile. If you say you used 10 gallons to go 100 miles, based on the mileage your odometer spits out, then you would think you were getting 10MPG. In reality, you would have only traveled 75 miles, and thus your actual MPG would be less, in this case 7.5MPG.
Likewise, if you have an undersized tire, it may cause the odometer to show you traveled 1 mile, when you may have actually traveled 1.5 miles. If you calculate your MPG based on using 10 gallons in 100 miles from your odometer, you think you're getting 10 miles to the gallon, when in actuality, you would have gotten 15MPG.

The odometer spins slower with oversized tires, and faster with undersized tires.

Of course I realize these #s do not apply directly to this case, and I;m not saying anyone gets 10MPG or 7.5MPG, or for that matter 15MPG. I was only using those numbers to make it easy to calculate for demonstration purposes.

Wrong information, the above post



is right.

Im pretty comfortble in my assessment. I have compared my odo to the hoghway markers on a handful of times over decent stretches of ~30-50 miles. I get a multiplier for 33s on mine of 1.12 times the odo.

Triple is right about showing less miles, so you need to mark the odo reading up to compensate before you do your MPG calc.

OH NO ! somebody pull up the airplane on a big treadmill post to solve this mystery! <img src="/forums/images/graemlins/smile.gif" alt="" />

http://www.4x4wire.com/forums/showf...&o=&fpart=1&vc=1

The circumference of a circle is 2* PI * R or PI times the diameter. So a 33 inch wheel has a circumference of 33 * 3.14 = 103.62 inches. A stock tire at say 28 inches = 87.92 inches.

Divide 87.92 by 103.62 and you find a 33 size tire travels about 15% more distance per revolution.

The Odo does not know this so it calculates you went 87.92 inches each revolution but you really went 103.62 inches.
Therefore the ODO under reports distance. It says you went a mile but you went 1 mile plus 15% with 33's

I'm no Kevin C or FastEddy but I got a A in math <img src="/forums/images/graemlins/lol.gif" alt="" />

I agree with the math as stated. I also know that when I comp'd to the mile markers it was a consistent 12+%. Of course if you play with the numbers, you can change sizes by just 1/2" and get a different markup. [Actual results may vary...]

This got my curiousity going...

If I did my math right, having 5.29 gears (which are actually 37 RG teeth / 7 pinion teeth = 5.2857142857142857142857142857143-to-1 ratio) and 33s, I should be traveling about 2.4% further than indicated on the odometer. This is based on 28.2 inches (P225/75R-15) being a stock-sized tire for a 4-cylinder Gen I.

Does that sound about right?

My 1987 2.6, at 75,000 post rebuild, about 255,000 miles total, with a semi-stock Mukini, stock tires, and no emissions stuff gets anywhere between 20 and 22 mpg in my running around. I drive a combination of twisty country road, interstate, and city each way to work and back -- I get about 20.4 doing that -- when everything is working right. When the bowl vent valve went out I got about 8-10mpg, and had similar poor mileage when the aux acceleration valve went out.

Lee

is right. [/quote]

Correct. My with my 31s, I travel 1 mile and my odo shows .9.

Sounds about right, the 5.29's bring you back toward normal. Check it against a GPS to be sure.

My stock 88 is averaging 20 mpg around town. Stock tires stock motor. I am thinking of going to a weber carb or swapping for a diesel.