Rear end gearing calculations

[JSad]

I bought my 91 ZR-1 #210 from a Dodge Dealer about 1 1/2 years ago and joined the forum a couple weeks ago. I have not been able to contact the previous owner for any details. I believe it is a stock motor. The first thing I wanted to do after all the normal maintenance was put 4:10 or 4:30 gears in it for that nice seat of the pants feel. So before I ran out to buy new gears, today my son and I drove it and took some rough readings. Our calcs based on the following shows I have 4:30's already installed. What might I be missing based on the following:

Tire (315x35x17) height 25.25" measured with straight edge from top of tire to garage floor using a tape measure
6th gear, 2100 rpm, 70 mph (car gauges only)
4th gear, 2300 rpm, 40 mph (GPS app and car gauges)
Per online gear calculator that figures out at about 4:30's

Haibeck gear chart for 4:10's
4th gear, 2000 rpm,37.35 mph
6th gear, 2000 rpm, 76.22 mph

[Hib Halverson]

Get the back of the car up in the air.
Put chalk marks on a rear tire and on the driveshaft.


Rotate the rear wheels once.


Count the number of times the mark on the driveshaft goes by.


If it goes by about 4 and a third times you have 4.30s. It it goes by a little more than four times, you have a 4.09/4.10 axle. If it goes by 3 and a half times, you have the stock 3.45 axle.

[JSad]

OK, comparing wheel to drive shaft its 3:45. Double checked my previous numbers and they were correct per the gauges and gps. Speedometer matched GPS speed. Would this indicate my tach is off by about 400 rpm?

[Paul Workman]

It is not uncommon for the printed resistor etch on the tach voltage divider board to have deteriorated, resulting in (typically) the tach gauge to read higher than actual.

It is an easy fix for someone with some basic soldering skills and familiar with very basic electronics. Or, you can send the instrument cluster (board) off to BATEE for repair, OR you can purchase a replacement resistor board from BATEE and install it yourself.

How would you care to proceed?

[JSad]

Thank you both for the input. I think I will look into fixing it myself and if I have trouble get with BATEE...and go ahead and have 4:10s or 4:30s installed.

[ZWILD1]

If you go with the 4.10 gears I would at least go with long tube headers and a free flowing exhaust. Does your car still have the factory exhaust manifolds and factory cat back system. There is extra horsepower to be had by replacing them. After these mods getting a new chip burned also will help a lot. My first ZR1 was a 91. The previous owner installed a set of Watson long tube headers, and a 3 inch early B&B exhaust. He also had a set of 4.10 gears installed. The car ran very well with these mods. I was able to run mid 12's @ 117 MPH. Personally I loved the 4.10 gears. The car sounded utterly amazing with the headers and the big B&B pipes.

https://www.youtube.com/watch?v=A_6CfT21TpU

[JSad]

ZWILD1, you read my mind. Mine appears to be 100% stock
and the 4.10s, long tube headers, performance exhaust, Haibeck chip, plus a short shifter from ZFDoc was what I was planning from reading the forum for the last year or so. Probably be a year or two before I get it all done.

[JSad]

Oh and a great burnout clip!

[Paul Workman]

My wife Ami has 410s, headers, B&B exhaust, conservative porting (by today's standards), short shifter, and Pete's cams. It is a blast to drive.

But, my 90 ZR-1 is set up the same except for stock shifter, 3.45 gear and stock cams. HP is shy of Ami's car by only ~ 6 rear wheel HP (due to her more conservative porting - ordinarily Pete's cams are good for 20-25+ rwhp lift).

So! I get to compare the two cars with comparable HP but different gear ratios. And, I have to tell ya, it depends on what kind of contests you want to indulge in as to which setup is better.

To be honest, I'm on the fence. A LOT can be said for the 410s - city driving, drag racing, etc. However, I've had the chance to upgrade my gears to 410s a couple times and have declined b/c (frankly), with the light flywheel, it gets loose as it is in 1st gear by simply pushing the "peddle to the metal". Once the tires slip, there's no added "seat of the pants", except on drag strips, of course. And, for top end or mountain twisties, the 3.45s behind a 6 speed trans gets the job done better. I think - less 'rowing' and 2nd is better than 3rd in the twisties, I find. And, combining a ported/headered LT5 with the 3.45 gear, there is the 45-50 mph downshift to 2nd and standing on it all the way to 90 mph that will shock the hell outa would be challengers you might meet!

So, yeah...I'm shaded toward the 410s. But, evidently not enough (yet) to make the switch! YMMV!

As for 430s, the feedback is pretty slim. And, as for what little I've seen here on the FORUM in the last 13 years, the one testimonial was (coupled with the M6 manual) that 430s was a bit too much - would perhaps be much better if coupled with an automatic, if a dragster is what you want. You'd have to be the judge on 430s. ALSO...the experience - especially with big inch LT5s - that spindles have been known to twist off. There is an upgrade for that, but be ready to make the upgrade if 430s on a sticky track is in the cards...Just sayin.

[We Gone]

I've had 3:45, 4:10 and now 3:73 so far I like the 3:73 best as I do a lot of back road and mountain driving.