rear end ratios to speedo

[twist]

I replaced my old 32/10 final drive for a 33/11 some time ago. I did not change the speedometer or have it calibrated, I just kept in mind the speedo showed about 10 mph difference to the slow side. My question is this, I want to calculate my mileage but since I can't rely on my odometer to read right. I travel about 10 mph faster than the speedo shows. Math is not my strong suite so I thought I would dare to ask the brainiacs here about figuring the mileage. I hope I'm asking the question right.

[Kurt in S.A.]

You should do a constant speed run down the Interstate using the mile markers...say run 5 or 10 miles at a constant speed. Keep track of the odometer reading versus the mile marker as well as the number of seconds to cover the distance. The odometer reading will tell you what percentage difference there is between speedo miles and "real" miles. As for speed, if run at 60 miles per hour, you should cover 5 miles in 300 seconds or 10 miles in 600 seconds. Take the number of seconds you cover the distance in (say 5 miles) and compute [60 * 300 / x] to get the "real" mph. If you cover the 5 miles in 280 seconds, replace the "x" with 280 to get "real" mph as 64.3 mph.

Kurt in S.A.

[mattcfish]

32/10 = 3.20 ratio use 1.186 ratio speedo
33/11 = 3.00 ratio use 1.112 ratio speedo

A 33/11 with a 1.186 ratio speedo will be off by 7.40%
Every 100 miles you need to add 7.4 miles to your odometer reading.
At 60mph your actual speed is 64.44mph
at 70mph your actual speed is 75.18mph

Of course Airhead speedos are horribly inaccurate, especially after 30 years of use, but in a perfect world this is wat you get. I put a 32/11 on my R90 which had a 32/10. I get to add 10.8 miles every 100.
I made an excel spread sheet to figure out my mileage and speed. You can't upload excel files on here though.

[twist]

32/10 = 3.20 ratio use 1.186 ratio speedo
33/11 = 3.00 ratio use 1.112 ratio speedo

A 33/11 with a 1.186 ratio speedo will be off by 7.40%
Every 100 miles you need to add 7.4 miles to your odometer reading.
At 60mph your actual speed is 64.44mph
at 70mph your actual speed is 75.18mph

Of course Airhead speedos are horribly inaccurate, especially after 30 years of use, but in a perfect world this is wat you get. I put a 32/11 on my R90 which had a 32/10. I get to add 10.8 miles every 100.
I made an excel spread sheet to figure out my mileage and speed. You can't upload excel files on here though.

thanks for that! that's just what I was looking for!

cheers!

[SteveD]

http://www.largiader.com/articles/gearing/

[Kurt in S.A.]

32/10 = 3.20 ratio use 1.186 ratio speedo
33/11 = 3.00 ratio use 1.112 ratio speedo

A 33/11 with a 1.186 ratio speedo will be off by 7.40%
Every 100 miles you need to add 7.4 miles to your odometer reading.

The 1.186 speedo will see the cable turn 1186 times per mile. The 1.112 speedo will have the cable turn 1112 times per miles. The ratio between them is 1.066 or 6.6% difference. Where does the 7.4% value come from?

Kurt in S.A.

[bbelk]

Of course Airhead speedos are horribly inaccurate..........

I quite agree. On my 1975, the only thing they got worse than the instruments was the brakes. The speed is 5 to 10 MPH off across the cruising range and the odometer does not work at all. On my 1979, they did get the speed right and the odometer does actually measure something. That something is longer than a kilometer and shorter than a mile.

As was suggested you can use the mile markers on the road or borrow a GPS as the on-board instrumentation of a pre-1980 airhead is useless. I think the quality of the instrumentation may be the source of some wild claims often seen involving extreme speeds or MPG.

[mattcfish]

32/10 = 3.20 ratio use 1.186 ratio speedo
33/11 = 3.00 ratio use 1.112 ratio speedo

A 33/11 with a 1.186 ratio speedo will be off by 7.40%
Every 100 miles you need to add 7.4 miles to your odometer reading.

The 1.186 speedo will see the cable turn 1186 times per mile. The 1.112 speedo will have the cable turn 1112 times per miles. The ratio between them is 1.066 or 6.6% difference. Where does the 7.4% value come from?

Kurt in S.A.

I based my spreadsheet on this information on the airhead site.
http://www.airheads.org/content/view/222/98/
1.186-1.112=.074=7.4% I'm no math whiz either. How do you get the 1.066 (6.6%)?

[Kurt in S.A.]

How do you get the 1.066 (6.6%)?

(1.186 - 1.112)/1.112 = 0.0665 or expressed as 6.65% higher.

From that link you provided, comparing the RPM at 55mph for the two speedo ratios I get:

(3357 - 3147)/3147 = 0.0667 or 6.67%...the same number.

Interesting, though doing the same thing to speed at 55mph:

(118 - 126)/126 = -0.0635 or 6.35% lower...basically the same percentage within the accuracy of the numbers used.

Kurt in S.A.

[Major Softie]

Mattfish, subtracting one ratio from the other does not yield the percentage difference - you have to do another step. Also, there is not a percentage difference "between the two," but from one to the other (which changes depending on which way you're looking at it - from which to which).

1.186 - 1.112 = .074

.074 / 1.186 = 6.2%

.074 / 1.112 = 6.65%

So, the 1.186 reads 6.2% slower than the 1.112, at same speed, and the 1.112 reads 6.65% faster than the 1.186, at same speed.

The percentage changes depending on which direction you are looking at it because .074 is a larger percentage of 1.112 than it is of 1.186.