Post by TexasG8 on Jul 10, 2009 13:08:05 GMT -6
The old addage about gearing(60's and 70's) was that the more gearing a car had,the faster it was.That was true,unless the car spun the tires,or the motor was overreving at the end of the quarter.Most of those cars of the era had big blocks and narrow hard compound tires,so they burnt the tires for blocks and ran slow times at high mph.
Today's cars use smaller,more efficient motors that make more hp/ci than previous models.To make a car run to its potential it needs the proper gearing,and I will tell you how to do that.
Most important item:
You want to cross the 1/4 mile mark at either peak power(rpm)in your 1:1 gear,or slightly below it.Why?Your car will pull only until the peak,then it dies regardless. Unfortunately the gear selection for 10 and 9 bolt rears that came from the factory is limited,so choose carefully.
An Example:
A car that has 500hp on the bottle at 5500rpm, 25.5" tires,and runs about 121mph in the quarter. I looked up the stock gear ratios in a Firebird manual.It gave the mph/1000rpm for every gear available.I proceded as follows:5500rpm with 3.42 gears gave 120mph,based on 21.82mph/1000rpm.You just multiply 5.5 times 21.82,and you get 120mph,right on the money.I didn't mention that off the bottle that the car runs 108mph,at the same rpm.If I had done the gearing just based on the motor,I would have picked 3.73 gears.Why?Because the 3.73 gears are 20mph/1000rpm,making 110mph(just slightly under).
The following list comprises gear ratio and mph/1000rpm with 25.5" tires(stock height).
2.73 gears: 27.35mph/1000rpm
3.08 gears: 24.24mph/1000rpm
3.23 gears: 23.10mph/1000rpm
3.42 gears: 21.82mph/1000rpm
3.73 gears: 20 mph/1000rpm
4.10 gears: 18.20mph/1000rpm
4.56 gears: 16.36mph/1000rpm
Of course.not all tires are 25.5" tall,all slicks and street slicks measure 26" and up. Tires taller than 29" don't fit,so I will give ratios for 26,27,28,29" tires.I also will show how to calculate based on different tire sizes and gearing for any application.
26" tires
2.73 gears: 27.89mph/1000rpm
3.08 gears: 24.71mph/1000rpm
3.23 gears: 23.55mph/1000rpm
3.42 gears: 22.24mph/1000rpm
3.73 gears: 20.39mph/1000rpm
4.10 gears: 18.55mph/1000rpm
4.56 gears: 16.68mph/1000rpm
27" tires
2.73 gears: 28.96mph/1000rpm
3.08 gears: 25.66mph/1000rpm
3.23 gears: 24.45mph/1000rpm
3.42 gears: 23.09mph/1000rpm
3.73 gears: 21.17mph/1000rpm
4.10 gears: 19.26mph/1000rpm
4.56 gears: 17.32mph/1000rpm
28" tires
2.73 gears: 30.03mph/1000rpm
3.08 gears: 26.61mph/1000rpm
3.23 gears: 25.45mph/1000rpm
3.42 gears: 23.94mph/1000rpm
3.73 gears: 21.95mph/1000rpm
4.10 gears: 19.97mph/1000rpm
4.56 gears: 17.96mph/1000rpm
29" tires
2.73 gears: 31.10mph/1000rpm
3.08 gears: 27.56mph/1000rpm
3.23 gears: 26.36mph/1000rpm
3.42 gears: 24.80mph/1000rpm
3.73 gears: 22.73mph/1000rpm
4.10 gears: 20.68mph/1000rpm
4.56 gears: 18.60mph/1000rpm
If you have gears that are imbetween,smaller,or larger than the numbers,or if your tires are a different height,use this formula:
rpm(at power peak) * tire height (inches)
------------------------------------------ = optimum gear ratio
mph(at end of quarter mile) * 341.73 (constant)
I'll do an example:
5500rpm * 25.5"
----------------- = 3.39 gears, a close match for what I picked
121mph * 341.73
Most street cars do not need over 3.73 gears for stock height tires,usually 3.23 to 3.73 works best.Do not overlook the fact that the higher the gear,the more wheelspin you will get,don't expect a car with 4.56 gears to hook up on street tires as well as a car with 3.73's.
Higher powered cars need larger(taller) tires to hook well,this is because taller tires allow more tire patch on the pavement.You need to regear the car for the tall tires you need at the track,but live with higher rpm around town.
When you change gears and or tire height,you need to recalibrate the speedo gear so that the speedo is correct,and the computer functions correctly.I know for a fact that TH700R4 transmissions cars came with 2.73,3.08,3.23,and 3.42 gears from the factory,some might have come with 3.73's also(I am not definitely sure).To get the correct gears(2),go to a Chevy(or pontaic) dealer and ask them to look up replacement speedo gears for 3rd generation f-bodies,they have the numbers in the computer for the forementioned ratios.Those with T-5's can do the same thing,because GM stocks those gears also.Those with 4.10 or higher gears will have to ask questions and do research to find the correct speedo gears.
One last thing:
Changing the rearend gears to a ratio that suits quarter mile performance will sometime cut top speed(in OD).To figure out if this will happen,just leave the mph blank(at end of quarter mile) and multiply the gear ratio(you have) times the OD gear ratio.Working backward,you can find the mph(top end now,not quarter mile!).
Example:
5500rpm * 25.5"
------------------ = 3.42 * 0.70
Mph *341.73
You get 171mph,probably about 5mph faster than the actual top end.I was just lucky that the quarter mile gearing was the optimum top end gearing also,others may not be so lucky.
Today's cars use smaller,more efficient motors that make more hp/ci than previous models.To make a car run to its potential it needs the proper gearing,and I will tell you how to do that.
Most important item:
You want to cross the 1/4 mile mark at either peak power(rpm)in your 1:1 gear,or slightly below it.Why?Your car will pull only until the peak,then it dies regardless. Unfortunately the gear selection for 10 and 9 bolt rears that came from the factory is limited,so choose carefully.
An Example:
A car that has 500hp on the bottle at 5500rpm, 25.5" tires,and runs about 121mph in the quarter. I looked up the stock gear ratios in a Firebird manual.It gave the mph/1000rpm for every gear available.I proceded as follows:5500rpm with 3.42 gears gave 120mph,based on 21.82mph/1000rpm.You just multiply 5.5 times 21.82,and you get 120mph,right on the money.I didn't mention that off the bottle that the car runs 108mph,at the same rpm.If I had done the gearing just based on the motor,I would have picked 3.73 gears.Why?Because the 3.73 gears are 20mph/1000rpm,making 110mph(just slightly under).
The following list comprises gear ratio and mph/1000rpm with 25.5" tires(stock height).
2.73 gears: 27.35mph/1000rpm
3.08 gears: 24.24mph/1000rpm
3.23 gears: 23.10mph/1000rpm
3.42 gears: 21.82mph/1000rpm
3.73 gears: 20 mph/1000rpm
4.10 gears: 18.20mph/1000rpm
4.56 gears: 16.36mph/1000rpm
Of course.not all tires are 25.5" tall,all slicks and street slicks measure 26" and up. Tires taller than 29" don't fit,so I will give ratios for 26,27,28,29" tires.I also will show how to calculate based on different tire sizes and gearing for any application.
26" tires
2.73 gears: 27.89mph/1000rpm
3.08 gears: 24.71mph/1000rpm
3.23 gears: 23.55mph/1000rpm
3.42 gears: 22.24mph/1000rpm
3.73 gears: 20.39mph/1000rpm
4.10 gears: 18.55mph/1000rpm
4.56 gears: 16.68mph/1000rpm
27" tires
2.73 gears: 28.96mph/1000rpm
3.08 gears: 25.66mph/1000rpm
3.23 gears: 24.45mph/1000rpm
3.42 gears: 23.09mph/1000rpm
3.73 gears: 21.17mph/1000rpm
4.10 gears: 19.26mph/1000rpm
4.56 gears: 17.32mph/1000rpm
28" tires
2.73 gears: 30.03mph/1000rpm
3.08 gears: 26.61mph/1000rpm
3.23 gears: 25.45mph/1000rpm
3.42 gears: 23.94mph/1000rpm
3.73 gears: 21.95mph/1000rpm
4.10 gears: 19.97mph/1000rpm
4.56 gears: 17.96mph/1000rpm
29" tires
2.73 gears: 31.10mph/1000rpm
3.08 gears: 27.56mph/1000rpm
3.23 gears: 26.36mph/1000rpm
3.42 gears: 24.80mph/1000rpm
3.73 gears: 22.73mph/1000rpm
4.10 gears: 20.68mph/1000rpm
4.56 gears: 18.60mph/1000rpm
If you have gears that are imbetween,smaller,or larger than the numbers,or if your tires are a different height,use this formula:
rpm(at power peak) * tire height (inches)
------------------------------------------ = optimum gear ratio
mph(at end of quarter mile) * 341.73 (constant)
I'll do an example:
5500rpm * 25.5"
----------------- = 3.39 gears, a close match for what I picked
121mph * 341.73
Most street cars do not need over 3.73 gears for stock height tires,usually 3.23 to 3.73 works best.Do not overlook the fact that the higher the gear,the more wheelspin you will get,don't expect a car with 4.56 gears to hook up on street tires as well as a car with 3.73's.
Higher powered cars need larger(taller) tires to hook well,this is because taller tires allow more tire patch on the pavement.You need to regear the car for the tall tires you need at the track,but live with higher rpm around town.
When you change gears and or tire height,you need to recalibrate the speedo gear so that the speedo is correct,and the computer functions correctly.I know for a fact that TH700R4 transmissions cars came with 2.73,3.08,3.23,and 3.42 gears from the factory,some might have come with 3.73's also(I am not definitely sure).To get the correct gears(2),go to a Chevy(or pontaic) dealer and ask them to look up replacement speedo gears for 3rd generation f-bodies,they have the numbers in the computer for the forementioned ratios.Those with T-5's can do the same thing,because GM stocks those gears also.Those with 4.10 or higher gears will have to ask questions and do research to find the correct speedo gears.
One last thing:
Changing the rearend gears to a ratio that suits quarter mile performance will sometime cut top speed(in OD).To figure out if this will happen,just leave the mph blank(at end of quarter mile) and multiply the gear ratio(you have) times the OD gear ratio.Working backward,you can find the mph(top end now,not quarter mile!).
Example:
5500rpm * 25.5"
------------------ = 3.42 * 0.70
Mph *341.73
You get 171mph,probably about 5mph faster than the actual top end.I was just lucky that the quarter mile gearing was the optimum top end gearing also,others may not be so lucky.