18 MPG? is my math correct?

[suburban99]

1980 CJ7 AMC 304 edelbrock 4 barrel, and manifold, fenderwall headers and side exhaust with short mufflers. Stock gears CJ7 is a Laredo . 33" tires

Filled tank with a gas can till I could see the gas at the fill tube. Was expecting it to take more but it did not. Odometer read 55774 kilometers as it is a Canadian jeep. Drove till Odometer said 55878 so 104 kilometers.
Added 4 US gallons of gas and tank was full to same level.

104 km x (tire diameter factor for 33" versus 29" stock) 1.13= 117.52 kilometers which is 73 miles...on 4 gallons of gas or 18.25 MPG...
Driving to work and back, around 50 mph on country roads. Had it on the freeway up to 60 mph for a short time.

 

[BusaDave9]

I have a '79 CJ5 with a AMC 304 Edelbrock 4 barrel, and manifold.
I get 13 MPG.
There is a lot of math in your calculations. I would try to narrow it down. First off, your speedometer. I would test that by driving by one of those speed signs that have a radar.

These signs are very accurate for testing.
Another way to test your speedometer it to test your odometer. Use the mile markers to see how far off the odometer is. Drive 10 or so miles before comparing mileage.

 

[1986cj7]

1980 cj7 304 edelbrock 4 barrel, and manifold, fenderwall headers and side exhaust with short mufflers. Stock gears CJ7 is a Laredo. 33" tires

Filled tank with a gas can till I could see the gas at the fill tube. Was expecting it to take more but it did not. Odometer read 55774 kilometers as it is a Canadian jeep. Drove till Odometer said 55878 so 104 kilometers.
Added 4 US gallons of gas and tank was full to same level.

104 km x (tire diameter factor for 33" versus 29" stock) 1.13= 117.52 kilometers which is 73 miles...on 4 gallons of gas or 18.25 MPG...
Driving to work and back, around 50 mph on country roads. Had it on the freeway up to 60 mph for a short time.

As Dave said I believe your speedo is off with stock gears and 33" Tires You will be going faster then what your speedo says. My 86 had stocks gears 2.73 and 33" tires on it when I check my speedo by driving 10 miles on the highway the speedo read 12.2 miles. So this told me the speedo was 20% faster. Now I have 4.10 in it and 33" tires and checked it again by driving 10 miles on the highway and speedo registered 8.2. So now I'm about 20% slower. To get an accurate reading you will have to do one of the two tests Dave suggested. And do the math if your speedo says 100 miles or Kilometers and your test shows your speedo is off by 20% you will have to Multiply your miles by 20% to get the amount of your speedo is off and add or subtract this amount to your mileage to get an accurate measurement of you gas mileage. Hope this helps.

 

[Manhattan]

Your math is correct - I came up with 18.11 mpg. There are a few assumptions here though...
1. Your CJ came with 29" tires - actual
2. Your 33s are 33" - actual - if they measure closer to 32", your MPG drops into the 17s
3. You actually did burn 4 gallons. If you'er off by .2 gallons, you're into the 16 mpg range... add in a 33 that's not really 33... and you're into the high 14s...

A handheld GPS unit is great for this - I have a Garmin 500 (cycling computer) and have used it with a couple of my trucks with "over-size" tires. Each time, I've found that the delta from stock to current is NOT as great as calculated, e.g. dividing the 29 by 33 x shown speed... overstates the difference.

Also, I'd guess the biggest factor may be that amount of fuel - you'd be better off working from full tanks. Fill the tank (as full as possible on flat surface). Drive jeep to empty (doesn't have to be fumes or even close). Fill tank at same pump as before. It just gives a safer/more confident number, e.g. 15 vs. 4 gallons. If you're off by .2 gallons in the refill, the error factor goes from .05 percent to .013.

And the part that sucks... you could do ^that^ 10 times and get 10 different results. But, you'd have an average... or a range... or an idea.

For what it's worth, my only stock CJ7 (all stock, hard top, AMC 258 i6 / 4.2l ) got 19.1 mpg on a 500 mile trip - averaged over a couple fillups. I was shocked, impressed... then put 35s and 4.56 gears on it

 

[Razorfish]

To add to the above post, a cell phone GPS app will work too. However side-by-side comparison my Garmin Dakota 20 GPS is 10x better than an iPhones internal GPS regardless of app. Don't get me started how awful the phone's GPS is for hiking. However in the car they both indicate the same speed.