question for fred

[bigtow]

hi fred i think i will get a eaton diff to replace my worn unit if i fit 3.73 while i am at it will it throw my speedo out by much? i am running 275/40/17 tyres also is it worth upgraiding the diff cover for one of the cast ones that they list in summit thanks in advance

[Injuneer]

Changing the gears from the "stock" ratio to 3.73 will cause the speedometer to show higher than the actual kM/HR. The error will depend on what gears you are starting with:

2.73 -> 3.73 = 36.6% higher than actual speed
3.23 -> 3.73 = 15.5% higher
3.42-> 3.73 = 9.1% higher

In addition to the speedometer showing higher than actual kM/HR, the odometer will add kM at that same elevated rate. With an automatic, it can affect shift points.

If your car came from the factory with the Z-rated option (245/50-16) the 275/40-17 tires will have no affect on speedometer. They are both the same outside diameter.

If your car came from the factory with the 235/55-16 tires, the use of the 275/40-17's will cause the speedo to read about 2% higher than actual speed. That difference would be addititive with the difference for the rear gears.

You can correct for the gears in one of several ways.... 1) Get a Hypertech programmer (used can be a good deal), 2) get LT1_Edit and program it with a laptop, 3) get a "mail order" tune on your PCM (not sure how easy that is to do with the Atlantic Ocean in the way ), 4) get an external speedo correction box that fits in the VSS circuit.

The aluminum cover will probably help. Theory is that it is stiffer and keeps the housing from distorting. And be sure to get one with the two brearing "pre-load" bolts, to help heep the bearing caps in place. I have a LPW aluminum cover on my 12-bolt.

[bigtow]

thanks`fred

[Performin Norman]

I'm jealous. Fred gets his own threads.

[Injuneer]

I'm jealous. Fred gets his own threads.

Yep..... gonna add a "PayPal" button like Rob/Shoebox has, 'cept I'll put mine in my signature.

But currently, all advice is free, and guaranteed for 12 months or 12,000 miles to be worth what you paid for it .

[Performin Norman]

Yep..... gonna add a "PayPal" button like Rob/Shoebox has, 'cept I'll put mine in my signature.

But currently, all advice is free, and guaranteed for 12 months or 12,000 miles to be worth what you paid for it .

I have the driveway guarantee. It's good til you leave the driveway.

[bigtow]

opps sorry norm if offended you it only fred was on line at the time i posted my question by the way fantastic burn out in the vet

[Performin Norman]

opps sorry norm if offended you it only fred was on line at the time i posted my question by the way fantastic burn out in the vet

You didn't offend me. I was just having fun.