Join Date: Nov 1999
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Re: Speedo Gear Replacement
I think you got it right or at least close[img]/wwwthreads_images/icons/smile.gif[/img]. Remember, I didn't change my gearing at the differential. (# of teeth old speedo gear) X (new gear ratio) divide by (old gear ratio)=# of teeth on new speedo gear.
(27) X (4.27) / (2.73)= 42.23 That is assuming you kept tires the same.
It would be more accurate if you measured the tire diameter of old and new tires
since you changed tire sizes. If you went from an actual 28" to 33" (33)/(28)=1.1786 or a 17.86% increase in tire size. Then you need to reduce the above calculation by the same % as your tire increase. I just used the numbers you gave as an example. To figure a decrease in % you multiply by(1- % change)
1 - .1786= 0.8214
(42.23) X (0.8214)=(34.68.) teeth on new speedo gear. Good work!!![img]/wwwthreads_images/icons/wink.gif[/img]Substitue your numbers in and you'll probably be pretty close.
I didn't know my original tire diameter since I bought the CJ with 31's. I put 33's on last fall and I knew I was going faster than the speedometer was reading. I clocked about 10 or 12 miles using highway signs for distance. They vary considerably, so I just used several miles of road holding steady at 52mph to go one mile in each minute--or 60mph actual speed .
That ratio was: old gear X apparentspeed/ actual speed= new gear
or 31 teeth X 52mph/ 60mph = 26.87 or 27 teeth on the new gear.
Chu, you could check your self by finding the speedometer reading for one accurately measured mile or average several like I did. Then you could get a more accurate estimate if you don't have your old tires to measure. You also can have someone ride alongside who has a known accurate speedometer and check your speed when they are going evenly alongside. You need to know their speed and yours to be fairly close to accurate.
I'll bet I made that really more painful than it needs to be and no one should ever ask me a math question again [img]/wwwthreads_images/icons/laugh.gif[/img]. Hope it helps but it looks like you figgered pretty close[img]/wwwthreads_images/icons/smile.gif[/img]
85 CJ7 [img]/wwwthreads_images/icons/tongue.gif[/img][img]/wwwthreads_images/icons/smile.gif[/img]