C409

...... Does anyone have a chart indicating which speedo drive gears go with what rear axle ratio ? ... I'm looking specifically for gears for a 700R4 with 3.90 rear end ... I currently have a 3.45 and the meter is very close to actual speed ........ part numbers would be especially helpful ..... Thanks , Bob ...........

toptechx6

See the link then plug in your information:

http://webpages.charter.net/khasting...peedocalc.html

C409

................ Thanks ... ............

WVZR-1

The problem with that information/calculator is when you supply it with information that is NOT OE it usually defaults to a 27.1 inch tire. That's the way I recall it being years ago and I ran through some combinations that showed similar results.

WVZR-1

What you need to do for a start is drive it with a GPS using the GPS at 40 - 50 - 65 -75 record the speedometer at those speeds. You're attempting to find the percentage of error. Once you find out the percentage of error you need the tooth count of your present driven gear. The drive gear can be "assumed" but when you check the driven you might note the color of the drive. Tire size is sometimes misleading and if you can supply the "rolling diameter" or "rolling radius" then it's easier. Speedometer gears aren't as easy to get as they have been in the past, do it right "ONCE"!

In your case it might be easier if the tire is a standard corvette size. What is it?

Cliff Harris

This chart is from the parts book. Since you didn't mention a year, and I used this to answer somebody else's question several months ago, it may not apply directly to you. The options on rear axle ratios changed over the years. Just for reference, my '86 with 3.07 has a red trans output shaft gear and a green VSS gear.

WVZR-1

Theoretically with a 3.90 gear and a 275/40R17 or 315/35 tire you would require a 13 drive and a 42 tooth driven. That's using a "rolling diameter" of 24.750**. The math calls for 41.27xxx so it's generally assumed you +1. Marginal error.

There's no off the shelf available for the 13.

Now for a 3.73 it's generally accepted that a 15 drive and a 45 driven will do well. There is a 15 drive available for the 700R4 and the 45 is max driven. Close enough? That's something near 4% high. Speedometer would be 62 at 60MPH.

**The 24.750 was done on my 3.15's several years ago. It's "best of recollection" and is a static measurement. I had never compared it to the math of the "General". The general apparently allows for the tire growth maybe at what would be "speed limits" of the day. It seems the "General" likes 24.89 for the 275-315 tires.

C409

......... The factory parts list stuff doesn't work since they won't admit to offering the C4's with a rear axle ratio higher than a 3.45 ... but performance folks tend to gather their own info and make lists of parts that accomplish the mission ... ... my vette is an '87 and I'm running 335-35-17's out back that measure out at 25.5" tall ... I currently have the gray drive gear but would have to look at the driven again to be sure which it is ... my memory is on bits of paper that get strewn about ..........

WVZR-1

Measure from center of rear axle to the ground to get a "loaded/rolling" radius or for diameter confirmation, chalk the tire and ground at 6 o'clock rotate it one time and measure, that's the rolling diameter.

Post back!

*If the 25.5 you posted is "static mounted height" then you would need a 46 tooth that's not available. The 45 I mentioned will get you as close as possible without going to a DRA. The part# of the 45 tooth is 25513052.

Not to aggravate but performance guys "do the math"! The math always works, maybe not an answer that one wants but math says it's so!

Cliff Harris

There is an electronic VSS converter available that will convert the VSS pulses to the correct number for your rear axle ratio. Check out this guy:

http://www.dakotadigital.com/index.c...rod/prd126.htm

WVZR-1

Using DRA's can have an adverse effect on "everything"! That's a newer DRA and it certainly is an option but I'd think for the OP's requirement he'll find the 15/45 combination satisfactory.

He asked for "gears" I gave him gears! A DRA needs to be a last resort and I understand it's more troublesome maybe on the early model cars. Don't know, never needed one for myself but it's not the branding that I'd suggest. I reread the link and at least they've included now the possible ramifications of their install on some applications. SLP I believe had to do a recall on a few thousand DRA's.