Spedometer driven gear size?

[staciataku]

I have a stock '74 Bronco that has not had the spedo driven gear since I bought it. I am looking at putting 31s on it and I am trying to guess which spedo driven gear to use. Any educated guesses or suggestions out there? Oh yeah, I'm leaving in 2 weeks with the bronco to move to Italy, so I don't have much time to play around. Any help would be greatly appreciated.

Thanks,

Brendan

 

[S_bolt19]

you need to call a speedo shop, tell them your original factory specs and then tell them what size tire you are going to. If you have a 3 speed with 3.50 gears in the axles then you are probably looking at gear that will give you 5% reduction. I know that when I ran 3.50s w/my NP trans I ran about 10% faster than what the speedo showed. So if I was doing 30 I was actually running about 33 mph. Right now, I am running 4.11 and I am +/- 3%.

 

[Seabronc]

Speedo gears run around $10.00 each. I'd buy a 18 tooth gear (this is about the middle of the range of gears), and then try it on the highway. Drive 10 miles according to the mile markers noting the odometer reading at the berginning and end, (no particular speed need be held with this method and mile markers are placed very accurately). If your odometer says you went 9.3 miles for example, you are reading 7% low, (if your odometer is reading accurte, your speedometer is also reading accurate). Each speedo gear tooth added or subtracted from the gear makes about a 7% change. In the case of this example you would then get a speedo gear with 17 teeth, removing teeth increases the indicated speed.

Good luck,

>-

 

[walkercreekbronco]

If you can get access to a handheld GPS, that would be a handy way to see how far off your speedometer is. The GPS is very accurate, and you can quickly see how far off you are over a variety of speeds.

 

[staciataku]

Thanks for all the advice - I went ahead and bought an 18 tooth driven gear, and when it gets here I'll experiment and find out what I'm dealing with. Thanks again,

Brendan