I'd like to figure out what kind of gas mileage I'm getting, but I have a problem: My speedometer is off. Is there ANY way to figure it out? The speedo is pretty close until past 25 mph. 45-50 is actually closer to 55-60, if that info helps at all. I'm guessing I'm getting around 12mpg, but I'd like to be sure. It takes almost a qaurter tank to go about 40 miles but my fuel gauge isn't all that accurate either. Am I looking for a needle in a haystack or is there a way to figure it out??

 

Are you sure the odometer's off too? Sometimes speedos can wear out and read high, but the odometer is not affected. If that's not the case, you may need a different gear on the end of the cable that goes in the transmission. Are you running a different tire size than stock?

 

take a vehicle you know has a correct speedo/odometer go 100 miles and turn around go back fill the truck with gas make the exact same trip fill the truck again divide the gallons used by 200 and now you know your gas milage

 

Yeah, I'm running 31inch tires. Not positive if the odometer's off but I just kind of figured it would be if the speedo was. I like your idea plowpusher, maybe I'll try that this weekend. Thanks for the help.

 

This one's gonna throw you for a loop. I had a similar problem and was also curious as to what my mileage was. I found that using a gps unit will give you a way to verify your speedo and odometer reading. All one has to do is get or borrow a gps unit and reset the trip meter to zero or record on a piece of paper what mileage is already on the trip meter. Then, leaving the unit on while driving, go for a ride (a passenger to monitor the gps for speed is best) and check your mileage when you return.

p.s. the gps does not care what size tires are on your truck!

 

Bigger tires cause the speedo and odo to read slow. Get out on the highway where they have the mile markers and go at least 10 miles. Times the number of miles on your odo by 10, then subtract from 100 and that's the percentage that your speedo and odo are off.

For my truck my odo says I've gone 8.9 miles over the 10 mile section. 8.9x10 = 89, 100 - 89 = 11. My speedo and odo read 11% slow. Now when you look at your mileage between fillups, add 11% and you have the correct mileage. So if it was 150 miles on the odo, add 16.5 (or 11%) and you really went 166.5 miles.

The longer you measure your miles for, the more accurate you can get. Also if you check the error from time to time you can make sure it's just the tire size that's causing it and not a mechanical problem.

As for the gas mileage equation, fill the truck up to the top, record the miles, then drive it how you want to for the gas mileage that you want. For example, if you want to know the highway miles, only drive on the highway. You probably just want to know your everyday mileage, so just drive normally. When the tank is empty, fill it up to the top again. Now you have the total gas used, and the miles. Divide the two and you have your mileage.

If you went 142 miles on 10.2 gals of gas, your mileage is 142/10.2 or about 13.92mpg. Remember to adjust your mileage for your speedo error.

Other then that, you'd have to clock a known distance and drive it and see how much gas you use. Take a car that has a good speedo/odo and, starting from a gas station, drive a set course and record the miles. Fill up the truck, drive the course and fill up at the end. Divide the two and you have your mileage.