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.

[DRM500RUBYZR-1]

Did somebody say 4.10?

Bolt in and GO!

http://www.zr1.net/forum/showthread.php?t=30195

Marty

[JSad]

Paul, fantastic input and I would agree that it depends on what someone wants that determines the upgrade. I am in the Tulsa area and do some Oklahoma back roads, Arkansas "twisties" and in town driving...and I really enjoy rowing the gears.

Right now the syncro is grinding on a hard shift between 1st and 2nd so that gets fixed first. Then after enjoying the 3:45s for a bit, with the way I drive, the 4:10s are next. I am attempting to keep the budget at $2000+/- every 6 months or so.

From what little I could find, I would agree the 4:30s would be more than I want.

Marty - when I am ready I will check and see if you still have the "bolt and go" 4.10s

[Mystic ZR-1]

From Marc’s chart, with 3:45s, we’d be going
338 mph in 6th at 7500 rpm!!! 🤪
Probably be a little slower when the rev limiter kicks in?
What? Your ZR-1 won’t pull 7500 in 6th?

[JSad]

Well, I hit 300 mph the other day and it scared me almost to death. I am actually getting the 410s to keep the top end under 300...😀

[BigJohn]

Well, here I am complaining about my hood pushing up above 190!

[kluketic]

Doing the Math: Tire Size (mm) /25.4 (mm)/in X (aspect ratio x2) + Rim Dia = Tire height

So: 315/25.4 X (.35X2) + 17 = 25.68 in for tire height for a 315x35x17 tire.

Tire Air pressure could factor when measuring as you did as well as being installed with the car sitting on the tires.

Just sayin'

[JSad]

Good point on using better numbers, but using the 25.68 in place of the 25.25 in my calcs doesn't get to the 3:45 ratio we came up with jacking the car up marking and spinning the tire/drive shaft per Hib's suggestion.
So good news and bad news I guess...bad news is I need to buy 4:10s, good news is I get to buy 4:10s and improve the car even more.

[Paul Workman]

Doing the Math: Tire Size (mm) /25.4 (mm)/in X (aspect ratio x2) + Rim Dia = Tire height

So: 315/25.4 X (.35X2) + 17 = 25.68 in for tire height for a 315x35x17 tire.

Tire Air pressure could factor when measuring as you did as well as being installed with the car sitting on the tires.

Just sayin'

Because air pressure affects the actual radius of the tire, a more accurate way to calculate speed is measure the actual distance traveled for one tire revolution (rear tire inflated with the proper pressure) and use that actual distance in the calculation.

[kluketic]

Certainly circumference (one tire revolution) is the key here [(Pi) X Diameter] => 3.14.16X25.68 in. = 80.68 in., this is probably relatively insensitve to inflation (within limits).

[Paul Workman]

Certainly circumference (one tire revolution) is the key here [(Pi) X Diameter] => 3.14.16X25.68 in. = 80.68 in., this is probably relatively insensitve to inflation (within limits).

Yeah, you would think....

[Gunny]

I worked up an Excel spreadsheet that does all the calculations for you. Download it HERE (https://www.dropbox.com/sh/4sssqzlqw49zfeu/AADkZhkVD6wl05JHqLAjEE3Da?dl=0)

[JSad]

Gunny - thanks for the spreadsheet, that is great! But now I am thinking 4.30's again after looking at the chart and knowing how I drive!!

[lfalzarano]

There are no other factors needed to calculate the theoretical speed. An actual speed calc would bring in other environmental factors and aerodynamics to name a few. But for selecting gear ratios, klukeic and Paul are right on.


Sent from my iPhone using ZR-1 Net Registry (http://r.tapatalk.com/byo?rid=90383)

[Gunny]

Gunny - thanks for the spreadsheet, that is great! But now I am thinking 4.30's again after looking at the chart and knowing how I drive!!

Take plenty of time playing with the spreadsheet & try out as many "what ifs" as possible. The spreadsheet is nothing more than a quick comparison tool but there are so many other factors at play, you need to view with "eyes totally open".

That said, the default values are for a stock ZR-1 with 3.45 gears. If you change the tire size from rear tires to front the results will change slightly because the diameter from rear to front are not identical - close but not exact.

As Paul mentioned you need to carefully analyze your driving style & habits - that is so important. Paul has been with me on a number of mountain runs and I understand completely why he's reluctant to change from 3.45 to 4.10. I've tried both and for mountain driving the 3.45's work much better (for me, at least). I've never tried 3.73's but I imagine those would be a nice compromise.

Once I got the tires, brakes, and go fast mods set up to my taste I felt my '90 w/3.45 gears was as good as it gets for running the mountains ... but then I added an '02 ZO6 to the stable - bone stock with Nitto NT5 tires I began to run the mountains better than ever - instant twisty-curvy nirvana. And now I have a '19 w/Z51 package ... I've managed the Dragon twice before the shut down and I believe the 90Z and 02 ZO6 can no longer hang with the 19 Z51 - close, but the Z51 is so nice in the twisties. Photos HERE (https://www.zrwon.com/cruise/190919/index.html).

Here's a VIDEO (https://www.zrwon.com/events/190815/slides/OneLap.mp4) of what it's like behind the wheel of a Z51. If possible, view on a big screen with sound up.

I've been a legal driver for 62-odd years now & have enjoyed every moment - but with the cars available today that offer such great performance at affordable prices, I likely wouldn't refuse any offers to turn the clock back a few years.

I'm anxious to get back to the mountains ASAP and have tentatively planned to take a short trip back to the mountains beginning on June 12, the day that our governor is supposed to allow my age group to venture forth from the safety of our homes. I've been under house arrest since March and it's probably a good thing they didn't put an ankle bracelet on me or I would have already been in the pokey for good ... just sayin'

[We Gone]

Great tool Gunny, Love my 3:73s (for my driving style) better use of 2nd & 3rd my favorite mountain gears.

[PhillipsLT5]

http://gearcal.com/

[Paul Workman]

I'm anxious to get back to the mountains ASAP and have tentatively planned to take a short trip back to the mountains beginning on June 12, the day that our governor is supposed to allow my age group to venture forth from the safety of our homes. I've been under house arrest since March and it's probably a good thing they didn't put an ankle bracelet on me or I would have already been in the pokey for good ... just sayin'

I hear ya, Gunny! Wish you could have joined us on the 2018 Rocky Mtn Run. It was every bit as challenging, the routes we picked weren't as "knotted" as the "Dragon" is, with some exceptions: more stretched out, I mean to say. Different, but SAME.

There were more open vistas and views of snow-capped peaks with ribbons of blacktop winding through meadows (did I mention elk lounging on the roadside in some meadows?). Many high-speed straights that dove into stands of conifer on the slopes containing "Dragon" style twisties (quick brake and "heel-toe" from 5th to 2nd or 3rd and be glad your new tires are warmed up! As a consequence, probably out of old habit from the Appalachian "MRs", I lingered in 2nd too much - the RPM dwelled too long, too often at 5k+ and the combo of thin air and thermostat bypassing [above 5k RPM], it got hot and overheated. My bad! We ain't in Tennessee anymore Toto!)

So, in deference to the topic, 3.45s or maybe 3.73s for some driving would by my choice.