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post #1 of (permalink) Old 12-14-1999, 12:07 AM
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Stock Tire Size, Bronco II For Sale


First, any ideas on the stock tire size(s) on the 1978 Bronco Custom? I'm trying to figure out 1) How far off my speedometer/odometer is, and 2) what axle ratio I need to swap to (4.10/4.11 or 4.56?). I'm currently running 35X12.5's with stock 3.50 gears, and I'd like to be around 3000 RPM @ 70 MPH.

Secondly, I'm selling a 1990 Bronco II XLT. 126,000 miles (20K towed), 4X4 (auto locking hubs), 2.9L V6, 5-Speed, 3.73 axle ratio, 235/75R15 BFG AT's with good tread, factory alloy rims, cruise, AC, AM/FM/Cassette, power windows & locks, privacy glass, new paint (gold) & muffler; $6000.

I'm in the Seattle, WA area. If you're interested, email me at [email protected].

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post #2 of (permalink) Old 12-14-1999, 04:15 PM
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Re: Stock Tire Size, Bronco II For Sale

The stock tire size is a L78/15, which is a 235/75/15.

Go to:

You can calculate what gear ration you need to do what you are trying to do. I calculated that with a 4.4 gear ratio and 35" tires you will be turning 3000 rpm at 70mph. Try it for yourself.

post #3 of (permalink) Old 12-16-1999, 08:41 PM
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Re: Stock Tire Size, Bronco II For Sale

I have an 89 351W XLT and am running the same size tire. I have the C6 tranny and just installed 4.56 gears. I'm running about 3000 rpm's at 70MPH. The speedo was way off. The original gear in my truck had 18 teeth. I got a 21 tooth gear from Ford and it got me pretty close. I have a Magellan 315 GPS and calculated that I'm off by 6.6%. Each tooth on the speedo gear makes a 5% change. If I could get a 22 tooth gear I'd be on the money but Ford doesn't make one. Or at least that's what the guys at the parts counter tell me.

By the way the gear calculaters will tell you that the ideal gear set-up is around 3.88 or 4.10, I'd stick with 4.56 even though highway miles will suffer(city miles will improve because the truck won't struggle to get moving especially with the heavy meat on your truck). Although Thump is right about the 4.4 gears as per the calculator, remember your tires aren't really 35 inches tall more like 34.75.

Good luck,


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