Help Me Caculate My Gas Mileage On My 80 B

Since I've never checked the MPG on my 80 one of the members suggested I do so before the tune up and then afterwards. It has a Weber 32/36, original distributor, etc.

I've now ran three tanks fulls and I thought I was averaging 22 MPG around town. Today I had to be in Roanoke and the hubby suggested we take the GPS off his boat and see how accurate my speedometer is. Here are the results:

Speedometer 0 --40 MPH --- GPS exactly with Speedometer
Speedometer 50 MPH ----GPS 48 MPH
Speedometer 60 MPH --- GPS 57 MPH
Speedometer 70 MPH --- GPS 65 MPH
Speedometer 80 MPH --- GPS 73 MPH

Hubby wouldn't let me run any faster. Anyway all total on that tank of gas I drove 212 ( by the speedometer ) miles and it took 9.22 gallons to fill it back up.

If one takes in consideration that my speedometer isn't reading right I take it the mileage counter would be inaccurate also.

Math to much for me. How would you caculate my MPG given the above figures?


Your turn :)

speedometer gives mph whereas odometer gives miles.

the odo works directly from the turns in the cable. the speedo uses a magnetic or some other means of moving the needle that can have an error built into it thru slippage, etc.

the speedo can be off when the odo is correct.

or both can be off.

it makes no real difference. any error that is there in your calculation now will be there when you do the calculation later. You'll still be able to tell if there's an improvement and about how much.

Spedo is approx 6% off,

21.5 mpg
This assumes odo is off by same ammount, they are not connected.

Never checked mine, My LS400 is a 4.0 v8 and I get 26!!! Sheesh

That equates to 23 mpg. 212/9.22

If you don't know how far you've travelled,how on earth can you figure out you mileage !!!!!!

You need to know the distance travelled ( get it rom the GPS ),and divide it by the number of gallons you consumed.

Take into account the GPS calculates your speed as if you were going from Point A to Point B on a completely FLAT road. As no actual Point A to Point B trip is completely flat that probably accounts for the difference. You have to go faster (further) on the ACTUAL road to match the distant covered that the GPS calculates from, hence the difference in MPH.
HTH , John

The up and down does not really affest anything unless you are going up and down mountains.

Assuming you drive 1 mile with a 5% adverage incline./decline then the GPS is going to be off by 5-6 feet distance travelled.

1 mile = 5,280'
5% elevation = approx 250 climb

Pythagoream therum using the road as the hypot, you get

a = 1mile calculated by GPS - squared = 27,878,400
plus b= up down distance of 250 squared = 62,500

square root of 27,940,900 is 5,285 feet travelled.

So off by 5 feet per mile x 212 miles = 1,050 feet off for the whole trip, or 1/5th mile.

rickd Wrote:

MPG is an average of how many miles you go in a situation(usually all different speeds). You don't care about the GPS figures. So I am with Horst and his figure.

Don't worry about your mpg in the MGB....just go out and have fun.

Start worrying if it's running too lean (engine temp will get hot) or too rich (black sooty deposits or black exhaust smoke.

Just my philosophy.

23 the number is the I come up with also. I think that's close enough.

George Herschell

One note: most GPS units do take route into account. It is not a direct distance of point A to point B but is based on route.

point being ignored: odo can be accurate and speedo not.

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