How do you choose gear ratios?

This is how I do my choosing of gear ratios.
There are many ways to get there, and the only right ratios are the ones that make the customer have a happy driving experience.

I use the below listed calculator.
http://locost7.info/gearcalc.php

1st step is to find the parameters of the car.
tire size -if its a paddle style tire we are only interested in the carcass size not the paddle.Warning the #'s on the tires some time lie to ya, so tape measure
the height of the tire. Ex. a 33 is some times a 31.5 or 32

weight of vehicle - this will be needed in selection of how much torque will be needed to pull the car. How low of a gear to choose.

engine #s -what engine,we need redline #,hp #, torque #, turbo or blower.

2nd step is to get to know the driver.(I catch a lot of flack for this when I ask questions, but you need to know)

Who's driving- owner & his wife,son, daughter,second cousin

Where they will be driving - glamis,pismo,ocotillo,hard pack, fire roads, death valley
Ex. Glamis- dunes 75% of time drags 5% of the time, shoot Comp 20% of the time

How long they have been driving- believe it or not there are customers that have never driven in the dunes or are bike riders and moving in to a car.
(there are no bunny slopes in the dessert)

What they have previously been driving -2 seat mid eng. VW powered car, quad or bike, asphalt drag car,hydro-boat.(what kind of speed are they used to)

How fast and slow they want to go and @ what rpm- top speed, cruising speed,ect.

Ok lets get gear ratios.
Parameters: 2 seat, 1900Lbs, turbo V6, wants to go 85Mph @ 6500Rpm, redlines motor @6600Rpm

What I do is work backwards with the given info.
We will use a 5.14 Ring & Pinion because this is a strong ratio to use & low enough to make the driving gears we will use strong as well.
To go 85Mph @ 6500Rpm with a 5.14 R&P with a 31" tall tire will take a 1.35 4th gear = to 86 mph @ 6500Rpm, cruising speed is 40Mph @ 3000Rpm
Will the customer be happy with these two #'s?
I like to give a customer some room to grow in top speed so I will suggest a 1.26 4th gear = to 88Mph @ 6150Rpm & 93 Mph @ 6500Rpm. Cruising speed is 44Mph @ 3000Rpm,
From here it is a matter of stacking the rest of the gears from 3rd down to first. You do this by picking the amount of Rpm drop you want.
The key to this is remembering a driver drives by the seat of his pants. If it does not feel like its pulling from the last gear choice than it feels like it fell to far.Granted different engines have different torque bands and this is where you need to consider how far the gears can be spread apart from each other to moving the vehicles weight with the torque the engine has.
My 3rd gear choice is a 1.56 3rd gear. rpm drop when shifting into 4th is -1250rpm
My 2nd gear choice is a 2.08 2nd gear. rpm drop when shifting into 3rd is -1625rpm
My 1st gear choice is a 2.90 1st gear. rpm drop when shifting into 2nd is -1838rpm

I feel the engine should pull these gears adequately.
My reasons for this gearing are:
The car is upgrading to a -2D from An E box maybe do to an added turbo or engine change.
It is a 2 seat sand car @1900Lbs, and generally 2 seat cars are 95% dunning cars and may see very little of the hill or drags.
6500 Rpm was to close to the rev-limiter and 6000 Rpm shifts give plenty of reaction time before the limiter.
Being that it is a V6 with a turbo it has torque and lives closer to the top end of the Rpm range.
These drops between gears may seem close but here is something else to consider gears have duty cycles and the more you use the gear the faster it wears out (Duty cycle is the proportion of time during which a component, device, or system is operated.) So the people with second gear noise how often is it in second gear?
The ring and pinion works 100% of the time.not unless your stopped.
1st gear 10% of the time
2nd gear 42% of the time
3rd gear 45% of the time
4th gear 3% of the time
This is just a generalization most use 2nd 50%+ and never see 4th gear but on sand highway.
In my world I try and get people geared to use 3rd & 4th gear much more then they have in the past.

the below is the program calculations SORRY it does something when placed in the post.
Gear 1 2 3 4 Final
Ratios: 2.900 2.080 1.560 1.260 5.140

RPM Limit: 6000
RPM @Peak Power: 3000
Tyre Diameter: 31.00 in


Gear Mph per 1000 RPM Mph @3000 RPM Mph @6000 RPM
-------------------------------------------------------------------------
1 6.19 19 37
2 8.63 26 52
3 11.50 35 69
4 14.24 43 85

Mph RPM (in Gears)
--------------------------------------------------------
1 2 3 4
--------------------------------------------------------
5 808 580 435 351
10 1616 1159 869 702
15 2424 1739 1304 1053
20 3233 2319 1739 1404
25 4041 2898 2174 1756
30 4849 3478 2608 2107
35 5657 4057 3043 2458
40 4637 3478 2809
45 5217 3912 3160
50 5796 4347 3511
55 4782 3862
60 5217 4213
65 5651 4565
70 4916
75 5267
80 5618
85 5969

Gear Change RPM drop (change @3000) RPM drop (change @6000)
-------------------------------------------------------------------------------
1 -> 2 -848 (to 2152) -1697 (to 4303)
2 -> 3 -750 (to 2250) -1500 (to 4500)
3 -> 4 -577 (to 2423) -1154 (to 4846)

Thank you (mkenm) for the use of your car as an example.

Thank You very much.
These ratios are very close to what i was thinking.
1--3.110
2--2.08 or 2.170
3--1.60
4--1.310
I'm glad to see that i should be able to pull an even higher
first gear i think that is important!

This calculator is kinda cool also. Just input your tire size, ring and pinion and rear ratios and it gives you a numerical value as well as a chart.

Is the 5.14 the best overall R&P to go with?

How does this sound for a 2000lb car with a stock LS1 and 35" Extremes?
R&P: 5.14
1st: 3.11
2nd: 2.17
3rd: 1.6
4th: 1.22

I feel that this ratio gives me good 2nd and 3rd gear duning speeds and a decent 4th for the sand hwy.
What would you change if anything?

Here is what I had in my old car with 14.50 Extremes. A bit too tall as I always duned in 2nd.
R&P: 4.63
1st" 3.11
2nd: 1.93
3rd: 1.50
4th: 1.14

The above 4 gear ratios are good usable ratios all the way to 4th being usable.
Use them, or this is my suggestion.
5.14
2.90 1st is usable for putting around camp closer to 2nd
2.08 2nd dunning quick short bowels, just closer ratio in RPM than your pick to get you in to 3rd sooner
1.56 3rd dunning in longer bowels top speed 78@6000
1.18 still a good sand highway gear and usable

The main thing that is different is that we don't stay in 2nd gear for dunning, and are able to use 3rd

Great, thanks for the feedback. However, unless I am doing the gear calculator incorrectly, you ratios are slightly higher than mine, not lower. Lower number = taller gear correct?

Yes you are right I have them slightly higher than yours,but if you look at the RPM drops this is where the difference is.