i was trying to figure out my mpg's on my yota. i have a 22re 4cyl 2.4 L. 5 speed. with 4:88 gears and 35 inch tires

 

Borrow a gps with a speed rating. Find a nice flat road where you can keep a constant speed for a while. Record the gps and speedometer speeds.

Divide the smaller number by the larger. Subtract this number from 1. And multiply by 100.
This is the percentage that your speedometer and odometer are off. Use this info to calculate your fuel mileage.

 

My method:

Start with a full gas tank. Reset the trip meter. Fill up again when you reach 1/4 tank. Divide the miles traveled by the amount it takes to refill the tank..

Example: drove 250 miles, refilled the gas tank with 11.5 gallons= 21.7 mpg.

 

Yes, that's the standard way to calculate fuel mileage.
But with non-stock gears and larger tires, he's odometer and speedometer will be way off.
You need to compensate for the changes using my method above.

 

Take a reading with a GPS above 55mph and compare it to the speedometer's indicated speed. For example, speedometer says 55mph, actual speed (GPS) is 65mph.

65/55=1.182 so the speedometer is about 18.2% off. (miles traveled/gallons used) x 1.182 = corrected MPG.

 

Other way around.

Again, backwards.

Using your method, if his speedometer reads 55 while going 65, it would be 15.4% off, which is incorrect. For verification, multiply 55 by 1.154 and you get 63.47. While close, it's incorrect. Multiply 55 by 1.182 and you get 65.

 

If:

m = miles traveled
g = gallons used
a = actual speed
i = indicated speed
e = corrected MPG

Then:

(m/g)*(a/i)=e

Not sure if that simplifies it or not.

 

Would not compensating for larger tires hurt or increase my gas mileage? I tried to figure out the gas mileage on my '94 4Runnner with a 3.0. I filled up, reset trip meter, filled up again and divided it and came out to 12 mpg's with 31" mud terrains, But I didn't factor it in with bigger tires.

 

you guys are making this really difficult. Chances are your gps has a distance traveled function. Fill up with fuel untill the nozzle shuts off. Don't "top it up" Turn on the gps, reset the distance traveled to "0", then turn up the tunes, put on your sunglasses, and drive like you normally would. When you fill up next (just untill the nozzle shuts off like before), check the gps and it will tell you how far you've gone. Then its simply, miles traveled according to the gps divided by gallons pumped into the tank. This is much more accurate then trying to figure out your percentage error. As even with completly factory vehicles, your speedo can be off.

 

It's basic math...how is that difficult. Same thing applies to stock vehicles. If your speedometer is 2% off, MPG can be corrected by determining your actual vs. indicated speed. The faster you are going when you measure actual vs. indicated, the more accurate it will be. Once you know how far off the speedometer is, the calculations are going to be just as, if not more, accurate than using any basic GPS for distance tracking.

 

my apologies, I should have said "more difficult than it has to be" Is it actually hard...no. Did you explain it well, yes. But if he already has the gps in his truck, avoid the extra math equations and just use the number on the gps. No one likes doing extra math

 

Using a GPS for distance tracking would work, but really only for long distance highway driving. Too much stop and go and they lose accuracy. Sit in one spot and watch your GPS...chances are it will bounce around a little. That movement is added to your distance traveled.

GPS distance tracking may be easier, but it has been known to be somewhat inaccurate, especially with consumer grade GPS units. However, they are pretty accurate for measuring speed (to compare actual vs. indicated), and your speedometer measures tire revolutions...a tried and true method. Using the best of both worlds to your advantage results in probably the most accurate way to measure miles/gallon.

Just sayin.