papcie03

I just bought a 1987 F-150 with the 4.9 liter inline 6 for a few hundred dollars and I have a few questions for you.

What are some common rear end ratio's. I have the correct tire size but the speedometer reads 56-60 when I'm going low to mid 40's. It has a 5 wire trailer harness and electric brake controller so I suspect the rear end was swapped.

Next, I have a pretty loud knocking noise at speed and typically above 2000 rpm. It is mainly when under light throttle such as cruising and is constant. At first I thought this might be valve noise, but a friend thinks main rod bearings. Any thoughts? If it is rod bearings, is it dumb to just through new bearings in and calling it good without replacing the crank. I understand that pulling the crank is quite a bit more work.

Thanks for any help

Just call me Sean

iTrader: (1)

If the bearing hasn't spun yet then you can get by with just replacing the bearings, but you need to check tolerances. Once the bearing spins then the crank and rod gets damaged. I have a seven prong trailer connector and electric brake and haven't changed my rear to a different ratio, but I can't remember what my ratio is.

ford fan

Sounds like you scored the truck on the cheap side. So it won't be such a kick in the ***** if you end up needing to swap out your motor. I have a friend who scored a 80k 93 4.9 for only $275!

papcie03

Thanks for the advice. I have access to a shop so hopefully I'll get a chance in the next few weeks to check it out.

wde3477

IIRC, the common ratios for that year were the 3.08 and the 3.55.

papcie03

Just crawled under and rearend says 355, any idea what rpms you run at say 65 mph in 4th gear? I'm around 3000 which seems a little high to me.

Thanks again for the input

wde3477

If you have a regular manual tranny without overdrive - don't know that 3000 rpm wouldn't be reasonable at 65mph.

Had an F100 that the previous owner had swapped the 3-on-the-tree tranny out with a 4-speed - only it was the four-speed with a granny low and three regular gears. In high gear, the motor sounded like it was going to flat come unglued above 70mph.

Still can't explain the speedo difference.

aliens8mycow

I have the E4OD auto, and 3.55 rear. In 4th gear at 65mph, I'm sitting right on 2000rpm.

If someone was pulling a trailer, they might have gone with 4.56 gears (or maybe more based on what mines revs out at) - could have just swapped the ring and pinion inside the stock rear diff.

andyman7931

remember that you souldn't tow a trailer with the e4od in overdrive. My truck has 3.07 gears and I'm getting ready to swap 3.73 to help towing, that'll put me closer to 3000 rpm on the highway instead of 2000.

If you are unsure of the gear ratio, lift the rear of the truck, mark the tire and driveshaft and count revs.

dewman

My 96 F150 has the 3.55 gears and turns about 1800 rpm @ 65mph when in OD.