i have an 88 f-150, 302 4barrel holley, edelbrock intake, etc... i have 33 12.5 inch tires, of course it originally had 235 75 r 15s on it, i still have 15 ince wheels on it, speedo is about 5 to 6 mph off, just wondering if anyone knows the best way to calibrate the speedo, the ecm is gone i run it off a crane cams ignition box, got a 355 ls rear in it, any help would be appreciated, thanks

 

I believe your speedo runs off a gear on the input shaft of the BW1356 tcase. To verify this, there will be a small cable that runs from the side of the case up towards the cab. The gear is a plastic variety with a known number of teeth. There are replacement gears you can purchase to compensate for larger tires. I know www.broncograveyard.com carries some of these varieties. Which one would suit your needs.... I have no clue. I reprogrammed the PSOM to fix for my tire size.

Some of the trucks have a speedo gear on the automatic transmission as well... but I've only seen parts for the E4OD. Being that your truck is an 88 and a carb setup, it's either an AOD or a C-6 if it's an auto.

Hope this helps,
Mike