Gearing and tire size

So, challenge car. 

2000 miata. 

2004 Buick rendezvous 3400 v6

1999 camaro 3800 t5 transmission 

4.10 torsen rear

My tire options: 

21 inch tall avon medium compound slicks

205/50/15 spec miata sm7 slicks

I know a few of you are really good at gear/mph stuff for autocross and drag racing. Im trying to figure out which set of slicks will be faster on paper prior to finalizing the budget. The avons were free, on free wheels. The sm7 are on $75 wheels, wuth 65 in slicks/mounting. (Leaving the budget free tires for drag slicks). Obviously I WANT the avons to be faster. But....

So, who feels like being a hero with math?

Great gearing calculator

Seems to be pretty close either way until fifth gear (im comparing 3rd and 4th, as i figure one of those two will be where its happy on autocross)

A ~5mph difference between the two.

The questionis, is that a big enough difference to make a difference?

What's your redline and what are the gear ratios?

How wide are the Avons?

Last year (2017) my Miata ran the course in second gear.

1st 3.75 
2nd 2.19 
3rd 1.41 
4th 1.00 
5th .72

Redline is 6000

Avons are 7 wide on 14x5.5 steel wheels 

Then I would run the SM7s, they're wider and you'll probably be in 2nd gear.

Edit: Your 2nd gear is part way between the stock 5 speed 2nd and 3rd gears. 3rd might come into play if the course happens to be fairly open.  

https://drive.google.com/file/d/1lW4baywVA7gJS9IlkeIn-O62xxHHaI1M/view?usp=sharing

Here's one that I've modified to my own liking.

I'm seeing a realistic top speed in each gear for the Avon's to be roughly 23, 40, 62, 88, and 123...And for the SM7's to be roughly 26, 44, 69, 97, and 135.

How fast is the autox course usually for a Miata on race tires?  Do you have any ballpark idea what kind of trap speed you're targeting? Between the two, the shortest gearing that will get you to the finish line without needing the next gear will probably be more desirable from a purely gearing standpoint. But then of course not only do you need to factor in the width difference, but also how the compounds compare.

I think with that engine you'll be trapping ~95mph and you should pick whichever tire is not going to require you to shift to 5th. Although I'm not sure what RPM Driven5 used, you will probably be shifting a stockish 3400 around 5500 rpm. 

As far as drags go, there will be a 100 shot used. 

But at the moment, im more concerned about the autocross.  Drags are a whole different tire. 

Deadskunk brings up the width vs compound question. Fingernail test says the tures are about equally hard, with a slight edge to the avons. This is before any tire softener is applied. The sm7 are almost worn out, and the Avon aren't far behind so heat cycling and age are a factor. The avons are bias ply, unsure about sm7.

The question is if the wider/taller will be faster than narrower/shorter. Gearing vs ultimate grip.

The sm7 are on factory nb1 15s. 

Vigo said:

...you will probably be shifting a stockish 3400 around 5500 rpm. 

Why? Based on this GM published stock 3400 dyno, unless the curve sharply falls off rather than continuing as curves, all shifting before 6k will do is reduce the area under the portion of the power curve (or torque curve as applied at tires after gearing multiplication) being used.  

In reply to Driven5 :

200 ft/lbs in a miata with stiff gears is goung to be FUN. 

And ive been looking at the sweet spots of the curves while looking at the gearing/diameter. 

The way i read it, power will be best from about 2300-5500. Seems pretty broad, and like most courses would be run in third regardless of tire diameter. Right?

In reply to Dusterbd13 :

Based on the Town Hall meeting this year, it sounds like next year’s autox will be larger(and have the paddock at the far east end of the facility) again next year. That said, you may still be within 2nd gear depending on how tight the course is. 

I think that in either case, from Driven5's numbers you'll be using mainly 2nd and 3rd on the autocross course, that 1st is going to be uselessly short for anything but launch. With a big torquey engine the taller gears might be more useful. I think the width will actually make the bigger difference in times than gearing.

My Corolla goes to ~44mph in 1st and ~55mph in 2nd and that tends to work well for autocross, 3rd goes to ~72mph but I don't need it often. And that's for a car powered by ill-tempered Japanese hamsters that's only slightly lighter.

Dusterbd13 said:

In reply to Driven5 :

200 ft/lbs in a miata with stiff gears is goung to be FUN. 

And ive been looking at the sweet spots of the curves while looking at the gearing/diameter. 

The way i read it, power will be best from about 2300-5500. Seems pretty broad, and like most courses would be run in third regardless of tire diameter. Right?

5500 is right, but it isn't making any power at 2300.  4000-almost 6000 is your sweet spot.

The key thing to do here is graph predicted accel in each gear and overlay the curves.  If they cross anywhere... that's your shift point.  Easily done in excel. 

If need help, just shoot me a PM and I'll make the chart(s) for you.

In reply to ProDarwin :

Ues please!!!!

Pm incoming!

And i have an adjustable shift light for just this kind of thing.....

Something to think about, or calculate. The car is going to have to be shifted all the way to 5th on the drag strip according to some online calculators I used, and you'll need 26-27" slicks at that. It may be between 2nd and 3rd on the autocross course. Maybe, just maybe you might want the rear end ratio to be the 3.90 from an automatic car mounted to your Torsen. That needs to be calculated out to be sure, but the calculator got an ET of less than 12 seconds @115 mph if you can get 300 HP and 2500 lb on track.

If you download and edit my excel file you can look at the graph and see your shift points. Or i can do that tomorrow during the day.

EDIT:

https://drive.google.com/open?id=1lW4baywVA7gJS9IlkeIn-O62xxHHaI1M

This is what I got using a weight of 2300lb with driver, the posted power chart above, but at the wheels, A drag coefficient of 0.38 (this is going to be a lot higher if you run top down).  

Drag he said he has different tires for, but I don't know the details.  Estimating drag on these tires, I came up with a 12.29 @ 108mph (3000rpm launch/clutch slip, ~10ms^2 peak accel allowed -I don't know what typical drag radials are capable of here)

Accel by gear below, speeds are with 23" dia tires (205/50-15).  No crossing, so run it to redline every time.

In reply to ProDarwin :

If I'm interpreting your graph properly Michael will be making the 4-5 shift very near the finish line. I'd try and avoid that shift with a 3.90 gear or slightly taller drag slicks. 

Yeah that is with the 23" tires.  I doubt his drag tires are that short.  Anyway, that's without Nitrous which totally messes up my chart :P

But on the subject of gearing, that gearing is really short.  Like he would be will into 3rd in most autox courses, but still need 2nd quite a bit too.  The avons shift the 3rd redline down to ~64mph (and provide more accel).  i suppose running 3rd would be fine... it would just feel weird to me.

So, realistically theres going to be very little speed/quickness differential on the autocross ciurse between the two, correct?

And i cant break 13.50 on the strip. Dont have tbe safety gear for it.

Avons may squeak out a little faster due to more acceleration force out of corners in third, with potentially less shifting. Hoeever, may be offset by less contact patch, or be harder to control with the throttle. Right?