Hillbilly MPG calculator!

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Hillbilly MPG calculator!

Postby squatch » Tue Jan 28, 2014 1:48 pm

Mods please sticky.

When you change your tires to larger ones for off roading or whatever you need to compensate for this. Assuming you are running stock gears and don't have the coveted 4.77's. You can change the driven gear on the speedo cable at the transfer case. This is the best method. Or just make note of how far off the speedo is by using a gps or another vehicle beside you at 60mph indicated.

I'll help. Assuming a stock 1st gen Trooper with 4:56 gears and 235 75 r15 tires is accurate. Then when you swicth to BFG TA at 31x10.50x15 tires you will be traveking at about 66 mph for and indicated 60. Same tire in 32x11.50 will be about 67mph.

So you can do the math and figure the difference in diameters as a %. But that is often off because the measurement that matters is "static radius" This is the distance from the ground to the center of the wheel. That accounts for the deflection (flat area) caused the weight of the vehicle compressing the tire. It's usually a good bit less than 1/2 the diameter.

For figuring MPG and total mileage I use this backyard method. it is usually more accurate than the % calculations many use based on supposed diameter.

Say you change to 31x10.50's. Use a gps or compare to a knowm accurate car at 60 mph. If you are actually going 66 when 60 is indicated note that.

I then take indicated miles travels since last fill up. Say 277 miles. I then divide this by 60. This = 4.61. Now do 4.61 x The missing 6 mph. As in for every 60 miles you show on the ODO you are missing 6 miles because you were really traveling at 66mph. So in one hour of driving at 60 you really went 66 miles or 6 more than the odo shows.

So 4.61x6=27.69. You are missing 27.69 miles on your odo for that last tank full. So add 277+27.69=304.6 miles actually traveled.

Ok so you filled up 17 gallons. 304.6 miles divided by 17gal=17.92 mpg!

If you had used the indicated 277 divided by 17 gallons=16.29mpg. Lot's of folks do this and that's why they say their mileage went through the floor when they went to bigger tires.

Kinda hillbilly math compared to the schoolboy increase of XX% tire diameter thing. But just as or more accurate because it's real world measurement vs paper tire size.

Using the same formula. 3,000 indicated miles since last oil change is actually 3,000 Divided by 60=50. 50 x 6 missing miles =300 miles. So you actually have 3,300 miles since last oil change. This adds up after a while.
100,000 miles indicated is actually 109,999 miles travels. Those 31's with stock speedo gear are off by 10,000 miles over 100,000!
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Re: Hillbilly MPG calculator!

Postby asghilo » Tue Jan 28, 2014 1:56 pm

you sir...have got a lot of time on your hands :lol: :lol:
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Re: Hillbilly MPG calculator!

Postby DSUZU » Tue Jan 28, 2014 3:52 pm

Thanks for sharing. per my other post, I know my Trooper is reading about 1/10th of a mile low per mile. Any time I fill up, I add 10% to my mileage. Dennis
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Re: Hillbilly MPG calculator!

Postby squatch » Tue Jan 28, 2014 9:22 pm

asghilo wrote:you sir...have got a lot of time on your hands :lol: :lol:

Not really. I figured that one out with some help 30 + years ago. I've used it on 3 or 4 lifted trucks that didn't have speedo correction. Saved me a lot of tickets.
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Re: Hillbilly MPG calculator!

Postby Selahdoor » Wed Jan 29, 2014 12:49 am

To find actual miles vs odo miles, I look for mile markers on the side of the highway.

They aren't extremely accurate, but they are accurate enough if you are careful, and you go enough miles.

They'll be accurate to at least a tenth of a mile, and if you go at least ten miles, it will average out close enough for what we need.

It won't matter whether you speed up, slow down, etc. (For the neophytes.) So you could actually pull over at each marker, if you wish, and if you don't think you can 'guess close enough' about what your meter says at the instant you are beside the marker. Just don't back up.

Use the trip meter. Hit the button to zero the meter, when you are exactly beside the beginning marker. If you think you hit it too soon, or too late, start again at the next marker.

When you pass a mile marker, make note of the number that is on the meter. Don't push the button or zero the meter again. Let it continue.

If you miss a marker, no biggie, just keep a better eye out for the next one.

You should have made a note of the number on the first marker. And make note of the differences, if there are any, at each marker from that point on.

Once you have gone at least ten miles, you should be able to come up with a pretty close guesstimate of how many miles you actually travelled, versus how many miles the odo claims you traveled.

If I change to a new tire size, I will do this at least three times, in different conditions, and use the best average of the three questimates.
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Re: Hillbilly MPG calculator!

Postby wbuz5707 » Sun Mar 15, 2015 5:35 pm

does anybody know how to remove speedometer
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Re: Hillbilly MPG calculator!

Postby One_Big_Game » Tue Apr 03, 2018 11:11 pm

Thanks for showing this, I always forget to include the 10% (really about 12.5%) increase for my tires and I'm always confused when I get 13.8 for 100% city. 15.3 sounds about right.
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