Jays99gt

I have a '99 GT with a 5 speed, stock rear end, 18" Bullitt wheels, and 295/35 rear tires. I checked an app on my iPhone and the speedo seems to read at least 10 mph too slow, even more at higher speeds. I'm fairly new to newer cars so I fig I'd ask for some help here. Any input is greatly appreciated!

unitedwestang

The slightly taller tire would cause the speedo to read a little slower, but we're only talking about a couple percent--certainly not the 10mph difference you are claiming.

Are you the original owner? Are you 100% sure it has the stock diff. gear ratio? Simple test is to lift the rear of the car. Place a piece of tape on the drive shaft. Rotate the rear wheel exactly one revolution and count how many revolutions the driveshaft makes. Bam, you have the gear ratio of the rear end. I believe stock is 3.27:1, meaning the driveshaft makes about three and a quarter revolutions for every one revolution of the rear wheels.

Jays99gt

I'm not the original owner so I'm not sure if the gears have been changed or not. I'll check that out today, thanks for the tip!

unitedwestang

No problem. In my particular case I didn't know if the rear gears had been changed either. So I did exactly as I described above. I counted about three and three fourths revolutions of the drive shaft to one revolution of the wheel. A common aftermarket gear ratio is 3.73. Once I took the rear end apart to replace a bent axle shaft my suspicions were confirmed as the gear ratio is usually stamped on the ring gear.

Jays99gt

I'll give that a shot, maybe I'll pull the cover and check too. I don't know how long it's been since the gear oil had been changed anyway. Is there a certain type of gear oil these diffs like best by the way?

unitedwestang

Oh boy, now you're really testing my memory. You'd have to search on here or check a manual to find the weight. I do know you need to get the bottle of friction modifier because of the clutch plates in the limited slip diff.

If you do decide to pull the cover, there is actually very large debates online about the best way to seal the cover back up. Some claim just the rtv gasket maker stuff, some claim just a gasket, some say both. I will say this:
My diff cover was leaking slightly when I got it. Whoever had been in there when they did the gear install only used the rtv gasket maker stuff. I used a gasket with a very small amount of rtv on both sides and around all holes and its held up great with no leaks so far.

Jays99gt

I've always used the permatex 518 anaerobic sealer on all my Vw engines, I'm not sure how it would hold up on the diff cover though. I like it because it doesn't dry right away and seems to seal Vw engine case halves pretty well.

unitedwestang

I have no experience with that particular sealer, but in my opinion if it seals the engine case halve together it should be fine for the diff cover. But like I said, I have no experience with it, nor am I a sealant expert by any stretch of the imagination. I just wanted to report my experience.

Because redoing the diff cover would be a real pain...

ESABATM

I used to use black rtv with no gasket. Let it cure for 24 hours and service the oil never leaked. Since the rear was built I use a steel core silicone imprinted gasket with my trickflow cover. No leaks here either.

Torque wrench is your friend.

Jays99gt

I feel like I'm having to learn things all over, lol. I've been a die hard air cooled Vw guy for 20 years, but I'm very mechanically inclined and just have to learn the car. Next thing to do research on is the suspension and clutch cable!