So I've got a 76 blazer with 32x11.50 tires and that's obviously an inch or 2 bigger than stock, so what is the formula/calculation to figure out my actual gas mileage with the bigger than stock tires?
CK5
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How do I figure out my actual MPG?
- Thread starter 76k5blazerr
- Start date
use a gps, or a known distance (like highway mile markers) fill up the tank the same way before and after the trip, then do the math, its the most accurate/simplest way of measuring mileage. I would also suggest a minimum of 100mi.
Ok, I'll give that a try next time I take a highway trip, there was a formula I saw on some site the other day but now I can't remember where...
Example using an original 29" tire and a current 32" tire over an 'indicated' 100 mi.
(32/29)*100=110 'actual' miles. Then just divide by fuel usage for mpg.
This will get you close, but there are obviously inaccuracies in tire diameters and factory speedometers/odometers. GPS is most accurate.
(32/29)*100=110 'actual' miles. Then just divide by fuel usage for mpg.
This will get you close, but there are obviously inaccuracies in tire diameters and factory speedometers/odometers. GPS is most accurate.
MajMike
1/2 ton status
Hmmm . . .
Best bet is to change the speedo gears, with that correct everything else is easy.
Best bet is to change the speedo gears, with that correct everything else is easy.
badmix
1/2 ton status
I would imagine there are more than just tire size and gearing that effects MPG, things like carb size, and how efficient your motor is running, etc etc
Like mentioned, you just need to fill up the tank, drive on the highway for a solid 100miles and then refill it, keep your throttle/speed as consistent as possible.
If you drive 100miles and then fill it back up and only put 5gals in to fill it back up, your getting 20mpg. I figure its that simple.
If you drive 100miles and then put 6 gals in, you are now getting 16.6mpg.
Just divide the number of gals you put in to the 100miles and there is your answer.
Like mentioned, you just need to fill up the tank, drive on the highway for a solid 100miles and then refill it, keep your throttle/speed as consistent as possible.
If you drive 100miles and then fill it back up and only put 5gals in to fill it back up, your getting 20mpg. I figure its that simple.
If you drive 100miles and then put 6 gals in, you are now getting 16.6mpg.
Just divide the number of gals you put in to the 100miles and there is your answer.
runamok151
1 ton status
this x2
DEMON44
Low-Tech Redneck
GPS is much easier. Suction cup to the windshield vs messing around under the truck.
Plus it eliminates all other factors.
Plus it eliminates all other factors.
When using the gps you mean tracking how many miles you drive between fillups with the gps instead of the odometer?
DEMON44
Low-Tech Redneck
Yes
4xcrazy
3/4 ton status
Ya don't have to drive exactly 100 miles, can be more, less than may make it a tad more difficult to get a decent reading though.
Top off the tank, record mileage, drive for awhile, top it off again, record mileage again. Subtract new mileage from old mileage then Divide by the amount of new gallons of gas.
Obviously need an accurate mileage reading though.
Top off the tank, record mileage, drive for awhile, top it off again, record mileage again. Subtract new mileage from old mileage then Divide by the amount of new gallons of gas.
Obviously need an accurate mileage reading though.
DEMON44
Low-Tech Redneck
Oh it doesn't have to be a hundred miles for sure. I do full tanks. Divide miles driven. By fuel put in to refill tank.
It's all Very easy really.
It's all Very easy really.
Alright thanks guys, I appreciate it, gonna see if I can borrow a gps from somebody and figure this out.
Ok here's what I did today to find out my true mpg without using a gps. I copied this from another forum. Seems accurate to me, and I came out with 9 mpg. Does this sound right for a 76 blazer with an edelbrock 1406 and 32s? I thought I should be getting atleast 10, this tank was a combo of city and highway driving. I'm not saying I think that my calculations were off, I think my truck might be running rich. What do yall think?
2. Without GPS: Measure the diameter of your old tires and new tires. Divide your old tire diameter by your new tire diameter. Subtract from 1. This will be the % your odometer is off.
Odometer-Calculation-Chart.jpg
If your vehicle odometer registers 250 miles and it takes 16 gallons to fill…
Multiply 250 miles by 10% (amount odometer is off calculated above) = 25 miles. Add 25 miles to your odometer reading = 275 actual miles traveled. Divide 275 miles by 16 gallons = 17.18 MPG
2. Without GPS: Measure the diameter of your old tires and new tires. Divide your old tire diameter by your new tire diameter. Subtract from 1. This will be the % your odometer is off.
Odometer-Calculation-Chart.jpg
If your vehicle odometer registers 250 miles and it takes 16 gallons to fill…
Multiply 250 miles by 10% (amount odometer is off calculated above) = 25 miles. Add 25 miles to your odometer reading = 275 actual miles traveled. Divide 275 miles by 16 gallons = 17.18 MPG
How do you know what the original tire size is, or trans or t-case speedo drive/driven gear ratio?
The only thing you can do without knowing those, and not using a GPS, is to figure out how far off your speedometer/odometer is using mile markers (which are not precise but close enough) then use that percentage to figure true miles driven per fillup.
People generally have a real hard time understanding you can't just assume factors in the equation.
The only thing you can do without knowing those, and not using a GPS, is to figure out how far off your speedometer/odometer is using mile markers (which are not precise but close enough) then use that percentage to figure true miles driven per fillup.
People generally have a real hard time understanding you can't just assume factors in the equation.
That's what this equation does, I used my old 31s as the "old tire" in the equation and my new 32s as the new tire, 31 measured at 29 inches and 32 measured 31. Then when you do the equation it tells you what percent your odometer is off, mines off by 9 percent, cause the new tires are 9% bigger than the old ones, then you can figure how many miles you actually drove and how many mpg you actually get. Maybe not as accurate as a gps but pretty dang close.
So you know for a fact that before you changed tire size your speedometer/odometer was correct?
Yes, and even now it's not far off with the 32s, it's accurate in the lower speeds, I've only checked it with gps at 35 mph and it's maybe 1 or 2 mph off. But yeah, 31 was stock tire size on a k5 right?
I don't know about older, but not on the newer ones. Stock was 235/75R15.
10% difference is "only" 1-2MPG on these trucks, and that's huge.
If speedometer was dead on before, then yes, calculating based on percent difference is valid. If it wasn't dead on, it's invalid.
10% difference is "only" 1-2MPG on these trucks, and that's huge.
If speedometer was dead on before, then yes, calculating based on percent difference is valid. If it wasn't dead on, it's invalid.
