I installed some 4.88 gears in my truck with 3rd members out of a 4-runner and I have a couple of questions. I think my new rear diff is a limited slip if you drive it in the air and both tires spin but if you shut the truck off and spin one tire by hand the other tire spins the other direction doesn't that mean that it's a limited slip? If so should I put in a limited slip additive like in my newer truck? and also after installing my gears it seems that my speedo reads the correct speed with 33" tires is that true? Sorry for the newbie questions

Thanks,
Brian

1. No, that means it's an OPEN diff.

2. Yes, 4.88's with 33's would read the proper speed.

Yep, sounds like an open diff as mentioned above. Tires will spin the same direction (in the air) with a limited slip. ANd with 4.88s and 33s you'll be close on the speedo and odo. On my '85, I find the odo is just about right (maybe 1% off) but the speedo is about 8% slow. So I need to have it indicate 70 to get an actual speed (via GPS) of 65. With 35s, the speedo is just about correct, but the odo is off. All depends on how yours are calibrated.

How is that? Unless you mess with the spring tension on the speedo needle, your error percentage should be the same between speedometer and odometer.

My speedo is off from my GPS by about the same in my 90 with 4.88s/33's. GPS reads the same as the side of the road radar thingys.

70mph on speedo is 65mph on GPS. Never thought to check the
odometer.

Never messed with the speedo spring tension, that is just the way it is. Not uncommon for the two to be off. The odometer is a purely mechanical gear driven device, the speedometer uses a spinning magnet and spring to display the speed. So if it is not perfectly calibrated, it can read faster or slower than the odometer.