[landtortise89]

So I am running a 89 bronc with a 4" lift and 33's, with a 4.9L and a C6. All of this massive power (yeah right) is transfer to the pavement through stock 3:55 gears. I know my speedo is about 10mph slow. At anything above 40mph. My question is how do I get an acurate figure on my gas mileage? I don't want to change the speedo gear because I am going to 4:10's and lockers by the end of April. Anybody have a formula? I searched around and could find what I was looking for.

Thanks

 

[WasACop3436]

I use a GPS for accurate distance traveled.

 

[mickaila]

road trip a known distance. and IF you think your speedo is is 10mph slow, might your odometer be 10% low?

 

[TheDogofWar]

you could use the route planner on mapquest, IME it's been damn accurate on distances. a 180mile trip I frequent is accurate to within 1 mile.

You could use that 10% as suggested but it's dependent on the speed you travel, filling up near and traveling on the interstate would make this easier and more accurate.

 

[landtortise89]

these are great solutions. I will try a couple anymore? I was thinking about clocking a weeks worth of driving via my trip. and then adding 10% to the milage traveled and figure out what I get form there. right now it is saying I am getting like 11-12 and I know it should a little better than that.

 

[AbandonedBronco]

Use this formula:

Look at your door sticker and find out what your stock tire size is. It'll probably be in metric so convert it to inches and find the diameter of your tire.

If they're 235 75 R15s, take (235 / 25.4) = 9.25" (tires are 9.25 inches wide)
Then 9.25 * .75 = 6.94" (tires sidewalls are 6.94")
The diameter would be 6.94 * 2 (since there are two sidewalls) + 15 for the rim.

Diameter = 28.88"

Now take a ruler and measure your current tire, don't believe what's written on it. My 30" x 9.5" tires are only 28" in diameter.

Take whatever that is, lets say it's 31.5"

Your speedometer is calibrated for 28.88"
If it says 70mph, you're really going 70 * (31.5 / 28.88) or ~76mph.

If your tripometer says you just traveled 30 miles, you really traveled 30 * (31.5 / 28.88) or ~33 miles.

If you went 400 miles on a tank with 32 gallons and you calculate 12.5 mpg, recalculate it with the above info. That means you really went 436 miles and got 13.6mpg.





Also, if you changed your rear end, factor that into the equation too the same way.

Like if you had a 3.5 stock and change it to a 4.1, just add that in.

400 miles on the tripometer on a tank of gas.

400 * (31.5 / 28.88) * (3.5 / 4.1) = 372.4 actual miles on a tank of gas, which would mean you'd be getting 11.6mpg when it reads 12.5.

 

[landtortise89]

Use this formula:

Look at your door sticker and find out what your stock tire size is. It'll probably be in metric so convert it to inches and find the diameter of your tire.

If they're 235 75 R15s, take (235 / 25.4) = 9.25" (tires are 9.25 inches wide)
Then 9.25 * .75 = 6.94" (tires sidewalls are 6.94")
The diameter would be 6.94 * 2 (since there are two sidewalls) + 15 for the rim.

Diameter = 28.88"

Now take a ruler and measure your current tire, don't believe what's written on it. My 30" x 9.5" tires are only 28" in diameter.

Take whatever that is, lets say it's 31.5"

Your speedometer is calibrated for 28.88"
If it says 70mph, you're really going 70 * (31.5 / 28.88) or ~76mph.

If your tripometer says you just traveled 30 miles, you really traveled 30 * (31.5 / 28.88) or ~33 miles.

If you went 400 miles on a tank with 32 gallons and you calculate 12.5 mpg, recalculate it with the above info. That means you really went 436 miles and got 13.6mpg.





Also, if you changed your rear end, factor that into the equation too the same way.

Like if you had a 3.5 stock and change it to a 4.1, just add that in.

400 miles on the tripometer on a tank of gas.

400 * (31.5 / 28.88) * (3.5 / 4.1) = 372.4 actual miles on a tank of gas, which would mean you'd be getting 11.6mpg when it reads 12.5.

Great info. I just printed this to keep for my records.

 

[AbandonedBronco]

If you want to be really accurate, drive over a piece of string, rope, or flexible tape measure and wrap it around your tire. Then divide that by 3.14159 (pi). More accurate than just measuring across, which leaves a lot of room for error.

 

[onestep]

You want your SLR to do your calcs. Measure from the ground to the center of the hub and double that for your dia.

 

[landtortise89]

You want your SLR to do your calcs. Measure from the ground to the center of the hub and double that for your dia.

That will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.

 

[Herkdriver007]

That will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.

perhaps, but it is easier to measure from the center of the hub to the ground and multiply by 2 :thumbup

 

[landtortise89]

perhaps, but it is easier to measure from the center of the hub to the ground and multiply by 2 :thumbup

Yeah totally. Easier is always better.

 

[onestep]

That will probably be just as acurate. however measureing the circumfrence then dividing by pi seems even better.

Sorry but no.

A mounted tire on a vehicle is not round. If you look close it is quite flat on the bottom where it is in contact with the ground. The Static Load Radius of the tire especially with larger low pressure balloon style tires can be quite different than the marked tire diameter.

Check them both and see.

 

[landtortise89]

will do I am going to try some of these and figure what I am actually getting per gallon.

 

[AbandonedBronco]

That may be the case, but the diameter really has nothing to do with it in reality. What the calculation is based on is how far you travel every time your tires turn. The above equation just removed pi since it was on both sides of the equation.

So, if the speedometer is calculated specifically for 28.88", it's probably basing it on a perfect circle, which would mean that every time the tire makes one revolution, the speedometer figures the vehicle travels 90.73 inches.
What you're saying is valid to a point except for what we're trying to achieve with this equation, which is the difference of distance traveled. For that reason, you're going to want to measure how far around the tire it is, which is why the string to measure the circumference is better in this case.

If, for example, you measure straight across and it measures 31.5", you calculate 98.96" around, but when you measure the circumference, it's only 98.4" or something, since it's not perfectly round. Since you're calculating the difference of distance traveled per tire revolution, you're going to want to go with the 98.4" and not the 98.96" Not that big of deal, but it is more accurate.

 

[Joe'Bronco]

Wow, i just had a flash back of my high school geometry class...

 

[landtortise89]

Yeah no joke huh?

 

[AbandonedBronco]

And you probably always wondered where in life you'd ever use that stuff, huh?

 

[landtortise89]

yeah I always thought it was a waste of time. who'd a thunk it?

 

[cdeal28078]

:cry My head hurts just reading this post. What in the world do ya'll do for a living that you can makes these calculations? Great work by the way even though I was lost after the first *.
How about driving down the interstate at 60 mph, pass a mileage marker and time your trip to the next marker. Should take 60 seconds and be 1 mile. lol
clint