Changing rearend gear ratio

[BamaMan]

Right now I have a 3.73 standard 7.5 inch rearend. I want to go to a 8.8 inch rearend. If I drop to a 3.55 gear ratio what will change? Will I have to somehow recalibrate my speedometer? How much will it affect my acceleration?

[Henson]

I put an 8.8 in. I installed 3.73 back in it. Thinking of changing it later. If I remember right… this limited me to about 130~140mph with my current 5th. The 0.90 5th will force me to go 3.55 or smaller. I’m shooting for ~160 on the back of Mid-Ohio. Currently I’m about 130~135 depending on how big my nuts inflate. The 0.78 bogs the speed.

Unsure what your design intent is. You can always change it later.

If you change ratios you change speedos. Remember, tire diameters.



G-FORCE
RPM REAR END TIRE DIA
6000 3.73 26.04 (mine)

GEAR 1 2 3 4 ORIGINAL 5B
GEAR RATIO 2.95 1.94 1.43 1.00 0.78 0.90

SPEED 42.26 64.26 87.18 124.66 159.83 138.52


PERCENTAGE 0.34 0.26 0.30 0.22 0.10

SPEED RPM
15 2129.7 1400.6 1032.4 721.9 563.1 649.7
25 3549.5 2334.3 1720.6 1203.2 938.5 1082.9
35 4969.3 3268.0 2408.9 1684.5 1313.9 1516.1
40 5679.2 3734.8 2753.0 1925.2 1501.6 1732.6
45 6389.1 4201.7 3097.1 2165.8 1689.3 1949.2
55 7808.9 5135.4 3785.3 2647.1 2064.7 2382.4
60 8518.8 5602.2 4129.5 2887.7 2252.4 2599.0
65 9228.7 6069.1 4473.6 3128.4 2440.1 2815.5
75 10648.5 7002.8 5161.8 3609.7 2815.5 3248.7
85 12068.4 7936.5 5850.1 4091.0 3191.0 3681.9
95 13488.2 8870.2 6538.3 4572.3 3566.4 4115.0
105 14908.0 9803.9 7226.6 5053.5 3941.8 4548.2
115 16327.8 10737.6 7914.8 5534.8 4317.2 4981.4
125 17747.6 11671.3 8603.1 6016.1 4692.6 5414.5
135 19167.4 12605.0 9291.3 6497.4 5068.0 5847.7
140 19877.3 13071.8 9635.4 6738.1 5255.7 6064.3
145 20587.2 13538.7 9979.6 6978.7 5443.4 6280.8
150 21297.1 14005.5 10323.7 7219.4 5631.1 6497.4
155 22007.0 14472.4 10667.8 7460.0 5818.8 6714.0
160 22716.9 14939.3 11011.9 7700.6 6006.5 6930.6

[Henson]

What am I thinking, stay in first. You auto X'ers.

I was look at the sheet. I think I'll go 3.27. Makes a good highway rpm with the 0.90 OD and not bad top speed.

G-FORCE
RPM REAR END TIRE DIA
6000 3.27 26.04

GEAR 1 2 3 4 ORIGINAL 5B
GEAR RATIO 2.95 1.94 1.43 1.00 0.78 0.90

SPEED 48.20 73.30 99.44 142.20 182.31 158.00


PERCENTAGE 0.34 0.26 0.30 0.22 0.10

SPEED RPM
15 1867.1 1227.8 905.1 632.9 493.7 569.6
25 3111.8 2046.4 1508.4 1054.8 822.8 949.4
35 4356.5 2864.9 2111.8 1476.8 1151.9 1329.1
40 4978.8 3274.2 2413.5 1687.7 1316.4 1519.0
45 5601.2 3683.5 2715.2 1898.7 1481.0 1708.8
55 6845.9 4502.1 3318.5 2320.6 1810.1 2088.6
60 7468.3 4911.3 3620.2 2531.6 1974.7 2278.5
65 8090.6 5320.6 3921.9 2742.6 2139.2 2468.3
75 9335.3 6139.2 4525.3 3164.5 2468.3 2848.1
85 10580.0 6957.7 5128.6 3586.5 2797.4 3227.8
95 11824.7 7776.3 5732.0 4008.4 3126.5 3607.5
105 13069.5 8594.8 6335.4 4430.3 3455.7 3987.3
115 14314.2 9413.4 6938.7 4852.3 3784.8 4367.0
125 15558.9 10231.9 7542.1 5274.2 4113.9 4746.8
135 16803.6 11050.5 8145.5 5696.1 4443.0 5126.5
140 17425.9 11459.8 8447.1 5907.1 4607.5 5316.4
145 18048.3 11869.0 8748.8 6118.1 4772.1 5506.3
150 18670.6 12278.3 9050.5 6329.0 4936.6 5696.1
155 19293.0 12687.6 9352.2 6540.0 5101.2 5886.0
160 19915.4 13096.9 9653.9 6751.0 5265.8 6075.9

[mutant]

Will I have to somehow recalibrate my speedometer?

If you change ratios you change speedos.

The speedometer is recal'd by changing the gear on the transmission end of the speedometer cable. They sell gears with differing number of teeth for this very purpose. They make them in different colors for identification. This is somewhat hit or miss, though. You could have one gear be a little too low and the next gear with only 1 tooth difference be a little too high.
The gears are very easy to change. Undo one bolt on transmisssion, pull cable out. Remove retaining clip, slide gear off, do steps in reverse order to install.

[DAVEZ84SVO]

1 tooth on the gear of the speedo cable is = to 3-4 mph in high gear.. More teeth = lower indicated speed.. Less teeth will indicate a higher speed.. The best way to determin how much is needed, is to set a pace with another car before you change the rear gear.. Then test after & see what the difference is.. On my dad's t-bird, we went from 3.27 to 3.73's.. The factory was a 19 tooth gear.. After the rear gear change it was 7 mph off at 60.. We went to a 21 tooth gear & it was perfect.. It is dead on at 100 mph.. As it was before the change..

Dave

[DAVEZ84SVO]

Dude, this is the age of computers, satellites and hand-held GPS units. Go buy one of those spiffy things at the hiking store that reads MPH and test/set the calibration. Couple hours of work. Then take it back when you're done.

True statement, I use one of those radar boxes on the side of the road.. But they seem to flash your speed when ever you go more than the posted limit..

P.S. Those boxes do not display triple digits.. Not that I have ever seen..

[BamaMan]

Dude, this is the age of computers, satellites and hand-held GPS units. Go buy one of those spiffy things at the hiking store that reads MPH and test/set the calibration. Couple hours of work. Then take it back when you're done.

I have a MS GPS Unit and software on my laptop. One of the readouts is MPH. Seems pretty accurate.

[Henson]

Just look at your color you’ll know what speedo gear you have. There is a simple calculation for finding your tire height. Compare it to stock and see what you have. If you’re close then just go to the market and buy the correct color for your rear end ratio.


Existing 16" SVO, tire size 225, ratio 0.50, height mm 631.4, height in 24.86, width 8.86", over hang 0.93"

[DAVEZ84SVO]

Be careful - some of them have cameras - so you'll have to plan ahead.

I always do a drive-buy first.. Then test it..

Did you know that they work both ways.. Going at it or away from it..

Dave

[DAVEZ84SVO]

Me to Mike.. Just thought I would everybody know of that fact!!

Dave