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Calculating speeds from gear ratios
scoobyis2cool - 13/7/04 at 09:04 PM

I'm having a bit of trouble calculating what speed I will be able to do in each gear (type 9 box)

Assuming a diff ratio of 3.62, wheel circumference of 1.8m and gear ratios as follows:

1st - 3.65
2nd - 1.97
3rd - 1.37
4th - 1
5th - 0.82

I get the following speeds at 6000rpm in each gear:

1st - 31mph
2nd - 57mph
3rd - 82mph
4th - 112mph
5th - 136mph

Now having driven a sierra as an everyday car for some months I'm sure it won't do anything like these speeds in these gears, so where have I gone wrong?!

Pete

Mk-Ninja - 13/7/04 at 09:07 PM

Ill send you a gear calculator if you want one, but I bet your not far off if you rev it to 6000.

scoobyis2cool - 13/7/04 at 09:10 PM

quote:
Originally posted by Mk-Ninja
Ill send you a gear calculator if you want one, but I bet your not far off if you rev it to 6000.


Yeah that would be good if you've got one. I spose maybe they might not be too far out but I'm pretty sure I had to change into 4th just before hitting 60 and I was revving it pretty high, maybe just not as high as I thought!

Pete

trueblue - 13/7/04 at 09:55 PM

Regarding calculating of speeds from gears.
If you have access to a copy of :
Rebuilding and Tuning Pintos and CrossFlows
(Blueprint Books : isbn 1 899814 21 3)

It gives data on calculating speeds taking into account laden tyres diameter etc.

The example given has 1000 rpm for 20 mph in fourth gear so 112 in your list is
not too far off.

white130d - 13/7/04 at 10:34 PM

Pete what is your tire size and I will tell you the speed


David

Surrey Dave - 13/7/04 at 10:58 PM

Where do you allow for the drag factor?

My car is geared at about 23mph per 1000 revs in 5th

But it will never do 138mph (6000)

In fact because of the long gearing and drag it has a similar top speed in 4th or 5th!!!!!!

scoobyis2cool - 14/7/04 at 12:22 AM

quote:
Originally posted by white130d
Pete what is your tire size and I will tell you the speed


David


I haven't got a tyre size yet, I'm still deciding on one and wanted to look at the gearing to help me decide

The value that is really bothering me is third gear. Even though 6000rpm is over optimistic, it still sounds much faster than I ever managed in third! I suppose time will tell

Pete

scoobyis2cool - 14/7/04 at 12:26 AM

quote:
Originally posted by Surrey Dave
Where do you allow for the drag factor?

My car is geared at about 23mph per 1000 revs in 5th

But it will never do 138mph (6000)

In fact because of the long gearing and drag it has a similar top speed in 4th or 5th!!!!!!


I suppose drag could maybe limit the maximum revs in a particular gear. For example I know I've revved the engine as far as it would go in third and that reached just under 60mph, but I suppose wind resistance could limit the maximum revs and prevent the car reaching the theoretical maximum in that gear

Yeah, that sounds about right to me, just wish I had a spare sierra to test it out on!

Pete

Mk-Ninja - 14/7/04 at 07:31 AM

Pete

You have Email