Help Required, Gearbox Ratio Calculation.

[Richie-Van-GTi 74 2 Cars]

I cant seem to get my head round this, dont know why, must be having a blonde day

I want to replace the gearbox in my Zafira for one with a slightly taller gearing to save fuel. I know the final drive ratio, RPM, tyre size and MPH in all 5 gears and want to convert backwards to work out the actual gear ratios. Replacement boxes will have same gearing but differ in final drive ratios.

If anyone can do some example calculations it would be very much appreciated

 

Final drive ratio 4.19:1

RPM 3500

1st gear 18mph

2nd gear 30mph

3rd gear 47mph

4th gear 55mph

5th gear 70mph

 

on 195/65/15 so 199.33 cm rolling circumference.

 

options I have are final drives of 3.94 or 3.74 unless I go for a wide ratio derv box but downsides would outweigh any benefit on long journeys.

Edited July 11, 2012 by Richie-Van-GTi

[Anthony 1,003]

I worked backwards using the 205 gear ratio spreadsheet, I reckon the following ratios are about right for the given speeds at 3500rpm wih a 4.19 FD.

 

1st = 3.40

2nd = 2.05

3rd = 1.33

4th = 1.13

5th = 0.89

 

There's probably a couple of percent margin for error in that, but should be in the right ballpark.

 

3.94 and 3.74 FD's increase the speed in each gear at 3500rpm as follows:

 

1st = 19 and 20 mph

2nd = 32 and 34 mph

3rd = 50 and 52 mph

4th = 58 and 62 mph

5th = 74 and 78 mph

 

As an aside, I'm personally far from convinced that longer gearing always equates to better fuel economy!

[Tom Fenton 1,548]

Start with your speed. Convert this into kilometres per hour. Divide by 60 to get kilometres per minute. Multiply by 1000 to make metres per minute. Divide metres per minute by rolling circumference to get how many revs per minute your road wheel is doing. Multiply this by the diff ratio to get your gearbox output to diff rpm. Thus divide your input shaft rpm (from engine 3500rpm) by your output to diff rpm to get your overall gear ratio. Note that depending on how the box works this may be a combination of a train of 2 gears as the drive comes in on the input shaft, onto the laygear, which drives the output, apart from if it has a direct driven 4th and an overdrive 5th.

 

I quickly got these for 1st to 5th so close to Anthony.

 

3.468849 2.08131 1.328496 1.13526 0.89199

[tri_longer 25]

Just using trial and error on the gear calculator Gearboxman it comes out as the following

 

4.2 2.1 1.33 1.12 0.89

 

Cheers

Chris.

[DamirGTI 344]

http://zoldalma.webzona.hu/gear.html

 

Damir

[Richie-Van-GTi 74 2 Cars]

excellent thanks for all the help, I found some data on vauxhall boxes and the figures here tally to the data from vauxhall. Now just need to find the 3.74 final drive box and it should work out ok. I get it to around 3100 rpm at 70 mph as opposed to current 3500 rpm. Or I could stick an F13 box in with similar gear ratios and 3.55 final drive to achieve 2800 rpm but its a weaker box and I reckon round the streets it wil suffer.