Odometer calculations

[regal403]

I'm pulling a major brain fart today and can't seem to figure out how to calculate what my actual distance traveled is. I want to be able to determine my mpg in order to approximate my fuel level based on how many miles I've driven on a full tank. The gas gauge doesn't work.

I know buying a new fuel gauge sender would probably be best, but in the mean time I want to know roughly how much farther I can last on a given tank based on how far I've driven so far.
Make sense?

Stock
235/75-15 tires
2.73 gears

Currently
33x12.5-15 tires
3.42 gears

The odometer says I drove 101 miles and the gas pump says I out 9.5 gallons in.

What's the secret equation I need to use? It's probably a simple answer but I couldn't find what I need on Google.

 

[Fastereddie]

I'm pulling a major brain fart today and can't seem to figure out how to calculate what my actual distance traveled is. I want to be able to determine my mpg in order to approximate my fuel level based on how many miles I've driven on a full tank. The gas gauge doesn't work.

I know buying a new fuel gauge sender would probably be best, but in the mean time I want to know roughly how much farther I can last on a given tank based on how far I've driven so far.
Make sense?

Stock
235/75-15 tires
2.73 gears

Currently
33x12.5-15 tires
3.42 gears

The odometer says I drove 101 miles and the gas pump says I out 9.5 gallons in.

What's the secret equation I need to use? It's probably a simple answer but I couldn't find what I need on Google.

I think the easiest is to look at the mileage of your trip on your favorite navigation app

 

[Blue85]

101 miles x (33"/28.9")x(2.73/3.42)/9.5 gallons = 9.7MPG. Your odometer/speedometer under-report by 8%

EDIT: this is easy to conceptualize. Going to a bigger tire means you travel further for each tire revolution. A deeper gear means the driveshaft has more rotations per mile. Just multiply by the ratios. To calculate it from scratch requires you to know the speedometer gear tooth counts.

 

[regal403]

Thanks guys, appreciate the quick responses!

 

[kennyw]

101 miles x (33"/28.9")x(2.73/3.42)/9.5 gallons = 9.7MPG. Your odometer/speedometer under-report by 8%

EDIT: this is easy to conceptualize. Going to a bigger tire means you travel further for each tire revolution. A deeper gear means the driveshaft has more rotations per mile. Just multiply by the ratios. To calculate it from scratch requires you to know the speedometer gear tooth counts.

However, if he calculates the MPG without any conversions, he can calculate how many miles he can travel on his odometer as shown by simpler math.

Gallons in tank × Miles driven / gallons used = range in miles on your odometer as you see it.

 

[Blue85]

However, if he calculates the MPG without any conversions, he can calculate how many miles he can travel on his odometer as shown by simpler math.

Gallons in tank × Miles driven / gallons used = range in miles on your odometer as you see it.

Basically true, but not the question asked.

 

[kennyw]

Basically true, but not the question asked.

But not a bad idea to track that way with a broken fuel gauge.

 

[ZooMad75]

I looked at mine this way when my sender was broken. I tracked my mileage in every tank. My speedo/odo is accurate, but if you just go off of the uncorrected odometer it would still work. But I knew by the app I tracked my MPG with I normally got say 13 mpg. 13 mpg x 31 gallon tank put me at a range of 400 miles approximately. So as long as I stayed consistent on my right foot and knew the mileage from the last fill I could figure pretty quickly where my fuel level was.

Worst case if I figured 10mpg would be 300 miles of range on a full tank. I really didn't have a problem operating that way. I'd typically fill up after using 200 miles of range as I knew the tank would be closer to 1/2 on conservative driving or a 1/4 if I had my foot in it. Never ran it out of fuel.

The difference in the cruise RPM between the 2.73/28.9" tire (2,064 rpm) to the 3.42/33" tire (2,264 rpm) is 200 rpm in direct drive at 65 mph. So I think the difference is negligible. I'd bet if you compared the speedo/odo to GPS or even measured off of a mile by a stopwatch the stock speedo/odo is going to be close.

Though the OP is right, fixing the fuel level sensor is the primary fix. But if you got a good handle on average fuel use it's pretty easy to base it off of miles of range for a full tank.

 

[Blue85]

Yeah, why not run a GPS app on your phone for a bit and see how far off it is? Might be closer than the theoretical 8% difference. It's pretty common for stock speedos to read a little high.

 

[82355]

I just keep a 2 gallon gas can in my '75 Blazer.

Martin

 

[regal403]

Yeah, why not run a GPS app on your phone for a bit and see how far off it is? Might be closer than the theoretical 8% difference. It's pretty common for stock speedos to read a little high.

My speedo is way off, apparently.
It's erratic and a little noisy at times, but it bounces around 55 when my phone says I'm doing 45.