When do you change up?

[bmcecosse]

With the 3.7 diff - 2nd gear is good for 50, 3rd for 80 and 4th for well round the fuel gauge. And the speedo is spot on accurate!

[MColes]

The max speeds for a 948cc engine on the link that morrisman1 sent me said

1st = 23.4mph
2nd = 35.2mph
3rd = 60.5mph
4th = 75.1mph

[mike.perry]

Max change up 18, 30, 45 in my Series MM

[alanworland]

Only if you have all your big ends!!

[ndevans]

I tend to go up to 2 at around 10-15, 3rd 25-35 & like to go into 4th at about 48. I don't tend to cruise at more than 60-65, mostly because it's quieter than at 68-70.

[charlie_morris_minor]

i tend to cruise @ 60mph purely because i am a tight git and i get better mpg.. but back on topic..

i have done a few calcs I have included all my calcs in case I have made a mistake and I hope someone will correct me. Also I hope that it will prove helpful to people who are looking to play with dif's / gearbox / wheels etc ..

a 145/14 tyre has a circumference of 1846 mm (according to http://www.alloywheels.com/tyrecalc.asp)

so with a 4.22 dif means that one turn of your prop = 1.85m / 4.22 = 0.437m with a 1098 gear box you have ratios of
1 st= 3.628
2 nd= 2.172
3 rd= 1.412
4 th= 1

to get meters per thousand rpm
(0.437/ratio) *1000
so we 120.57 / 201.4 / 309.8 / 437.44

to get miles per thousand rpm
(m per 1000rpm / 1000)/1.609
1 st= 0.7
2 nd= 0.13
3 rd= 0.19
4 th= 0.27

to get miles per hour @ 1000rpm
miles per thousand rpm * 60
1 st= 4.5
2 nd= 7.51
3 rd= 11.55
4 th= 16.31

so @ 5500 rpm
1 st= 24.73 mph
2 nd= 41.31 mph
3 rd= 63.54 mph
4 th= 89.72 mph

obviously if your tyres are slightly over or under inflated these figures will all be wrong..

so if we apply this to bmc and his car I assume he is running 145/14s and has a 1098 gearbox ( I hope you do not mind me using you as example BMC if you do let me know and I will chop this bit )

2 nd= 50 mph = 5800rpm
3 rd= 80 mph = 6070rpm

so @ 6000rpm he will be doing
1 st= 30.77 mph
4 th= 111.63 mph ( good bye licence )


so now i just need to work out my speedo which is a mile out!

[webbnuts]

My answer is slghtly less technical than above - when the valves are making dents in the bonnet its time to go up a cog!

[ndevans]

i tend to cruise @ 60mph purely because i am a tight git and i get better mpg..

LOL! As good a reason as any!

[MColes]

I'd like to do the above calculations for my car but I'm still not 100% sure what axle I have.

I know there is no filler plug on the prop side of the diff or on the rear of the diff, so can't tell that way. When it was swapped we ended up filling it with oil through the breather as that was the only possible way to get oil into it.

[Blaketon]

I have a rev counter. My engine (1275 Midget) is a bit flat below 3000 rpm and really gets into its stride at 4000. I think the previous owner fitted a 731 cam (The standard 1275 Midget isn't like that) and I am looking to change it for something less wild. It revs happilly beyond 6000 but doesn't need to be revved that far. Unless I'm pressing on, I change by ear.

The Ford box has quite gap between 1st and 2nd, so often (Particularly uphill) it needs to be given a bit in 1st so that you don't bog in 2nd.

Wackiest thing I ever drove had a powerband between 8 and 12000rpm. There was a beeper inside your helmet, which beeped at 11800, giving you just enough time to change up (And the engine was sensitive to being over revved).

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[charlie_morris_minor]

the simple way is to jack up your rear axle and start turning the wheel one full revolution and see how many revolutions the prop has done or do it vice versa i.e. turn the prop shaft and look when the wheel has done one revolution.. probably not the most technical way but.

[PSL184]

I'd like to do the above calculations for my car but I'm still not 100% sure what axle I have.

I know there is no filler plug on the prop side of the diff or on the rear of the diff, so can't tell that way. When it was swapped we ended up filling it with oil through the breather as that was the only possible way to get oil into it.

Sounds like a 4.22 diff in a 4.55 casing.....

[d_harris]

That would be my guess. 4.22 diff in the earlier axle

[MColes]

How good is the setup I got then?

948cc engine and box with 4.22 diff

[PSL184]

Same as my Traveller - works good for me

[MColes]

Do you have any problems climbing hills?

[charlie_morris_minor]

what gear box have you got? a 948 (smooth case) or 1098 (bumpy case) then you can work out your max speed in each gear..

[MColes]

948 box

[PSL184]

Do you have any problems climbing hills?

Noo but then we don't have too many really big hills around the Midlands - Not like Wales or Scotland at least.....

[MColes]

I see. Oh well I get up them eventually