Contemplating Rear Gear change
 

I have a Pioneer AVIC in the car which is pretty neat. One of the features it has us a vehicle dynamics readout for stuff like amps, speed, lateral g, braking g's, compass. I rely on the speedo since mine is off due to the taller wheels and tires. I know that the rear gear is a 3.45. Comparing the speedo to the GPS, I calculate a 12-15% error. The tires are making the rear gear act as a 3.08. It looks like swapping a 3.90 gear would bring things back to operating as 3.45 gears would with 17" wheels. Thoughts?

Re: Contemplating Rear Gear change
 

i like the 392 gears that are in my white car. but you might want to consider 373s .if you are going to drag race her with your street tires so you can go thru safely in third

Re: Contemplating Rear Gear change
 

Yours is a very interesting approach to correcting a speedometer error.

It would be much easier to calculate if the "new" tire size/diameter was a "known" value. Then you could use an RPM calculator to get "CLOSE". I mention close because actual speedometer calculations are for "rolling radius" and not based on advertised tire diameter.

You could do it more accurately measuring your present tire "loaded radius" and then using this "speed calculator": http://wahiduddin.net/calc/calc_speed_rpm.htm

I've used this after measuring and it's pretty damn accurate. I don't recall the "loaded radius" dimension from my 315's and I can't measure it presently.

Here's an interesting way to approximate the "rolling radius": http://www.vibratesoftware.com/html_...ded_Radius.htm

To get "CLOSE" use advertised diameters, trans rato 1:1 and this calculator to compare: http://www.csgnetwork.com/multirpmcalc.html

I've used all of these for speedometer, tire or axle ratio comparisons. I always just use a trans ratio of 1:1.

Re: Contemplating Rear Gear change
 

You can check here to see how your tire/wheel combinations compare: http://www.rimsntires.com/specs.jsp


Error is "approximate"! This calculator used to be "FREE" and it could be used rather effectively to determine chassis/body clearances for various tire/wheel fitments.

I'd be interested in the percentage comparison of the calculator vs. your GPS. I don't know that they've compensated for "rolling radius" or just use advertised diameters.

Re: Contemplating Rear Gear change
 

Dom,

Well, is the question regarding just changing ratios to "recalibrate" the rear gear i.e., restore 3.45 like performance, or are you asking about boosting the ratio further to complement dragstrip performance??

Once again, Ami and I had a chance to do the Utica run. And, and once again I find 6th gear to be annoying for anything other than expressway travel. So, I'm contemplating a ratio that would put me pretty close to max torque in the trap in 4th gear (or should I go for max HP in the trap...Haven't decided,...)

IMO, at the very least, the ratio needs to be higher to better utilize 6th in normal highway driving, drag racing not withstanding. Make sense??

P.

Re: Contemplating Rear Gear change
 

still free for basic stuff

going from the 315/35R17 on a 17x11 to a 335/30R18 on a 18x12 diameter goes from 25.7 to 25.9

Speedo would be off by 1% at 60 so it would read 60 but actual is 60.6.

If Dom's rear tire is a 325/30R19 on a 19x12 then his Dia is 26.7 and according you the link WVZR-1 posted it is a 4.2% speedo offset. So when the Speedo is showing 60 it will actually be 62.5.

So I am not sure where the 12-15% is exactly

Re: Contemplating Rear Gear change
 

Jeff,

Actually you just reminded me that I did observe speed discrepancy on 474. I had same wheels and size tires on that car and found speed delta to be about 3-4mph at 60. So I suspect that #458 likely had 4.10's along with the required speedo gear in the ZF. That would explain the larger discrepancy. in fact, I used my standalone GPS on 474 to measure the speed differential. At the time, I thought it wasn't worth the bother in terms of correcting the speedo.
A 5-6% differential in speed would suggest a 3.73 rear to get things back to acting "as if" it's a 3.45 with the 19's.

Paul,

The objective at this point is to get things back to "stock". That speed differential would explain the lower trap speeds with 474 once I put on the 19s. At the big end, I was coming thru at a somewhat lower rpm altho the ets were the same.

Re: Contemplating Rear Gear change
 

If the gear change was done and the proper drive/driven used to correct the speedometer the tire diameter change would be constant regardless of ratio.

I believe I'd start with the stand alone GPS unit and attempt to confirm the speedometer error as it is now. I'd use a Tech 1 (or similar) to confirm RPM's at speed and use a speed/rpm comparator for an easy check. The Tech 1 would also display speed.

Maybe the best and easiest is the tire/drive shaft revolution comparator done by "the eye" to determine axle ratio!

Re: Contemplating Rear Gear change
 

WV,

The tire/diameter delta would be a constant %, but the actual mph delta would vary depending on gearing. The rear gear ratio was verified by Pete when we had car at his place for motor swap. So we are certain about that. I suspected that the speedo was reading low even before the 19's went on. As I posted earlier, using my in dash GPS, speedo reads 41 while GPS shows 47.5mph.
So I would probably install the 3.45 trans gear for the speedo, then
install the 3.73 rear gear to bring things into closer alignment both from readout and performance standpoint.

Re: Contemplating Rear Gear change
 

Dom,

I use a Dakota Digital speedo calibration unit. You dial in your speedo prefectly against your GPS no matter what combiantion you're using. Very simple to install and use. Change anytime you want. Mine rides in the center console for easy access. On ebay the go for about $75. Here's a link to their web site.

Hope all is well otherwise.

H
:cheers:

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