Gearbox ratios

[billwill]

Musing on Elanconvert's topic about diff ratios, can anyone suggest a good method of checking gearbox ratios?

Long ago I exchanged my original gearbox for a refurbished one at an 'ordinary' Ford shop, not a Lotus specialist and on thinking about it I have no idea if it is the proper close ratio box.

It 'felt' the same and I have done many many miles since, but it would be nice to know for sure.

[oldchieft]

The easy way is MPH/1000revs, you need a car with the same final drive ratio and a close ratio box to compare yours to.

Jon the Chief

[Craven]

Hi.
In top gear the ratio is 1: 1 simply do the sums using rev counter and speedo, there accuracy is unimportant as it?s a comparison.
Ron.

[Chancer]

But that is not going to tell him what box he has, all the 4 speed boxes are 1:1 top gear.

Needs to be done in 1st gear, I am assuming you know the diff ratio, follow the procedure I wrote in the diff thread but in 1st gear, divide the final figure by the diff ratio and you will know what 1st gear ratio you have and ergo what gearbox.

[billwill]

But that is not going to tell him what box he has, all the 4 speed boxes are 1:1 top gear.

Needs to be done in 1st gear, I am assuming you know the diff ratio, follow the procedure I wrote in the diff thread but in 1st gear, divide the final figure by the diff ratio and you will know what 1st gear ratio you have and ergo what gearbox.

Ah, no, Ron is right. Once you know one of the ratios i.e. the 1:1 of top gear You can calculate the other gear ratios by observing the speedo & rev counter.

E.g. drive at 2500 RPM in each gear and see what speed it is doing.

[RogerFrench]

Even easier, drive at constant speed and note RPM.

But that is not going to tell him what box he has, all the 4 speed boxes are 1:1 top gear.

Needs to be done in 1st gear, I am assuming you know the diff ratio, follow the procedure I wrote in the diff thread but in 1st gear, divide the final figure by the diff ratio and you will know what 1st gear ratio you have and ergo what gearbox.

Ah, no, Ron is right. Once you know one of the ratios i.e. the 1:1 of top gear You can calculate the other gear ratios by observing the speedo & rev counter.

E.g. drive at 2500 RPM in each gear and see what speed it is doing.

[Chancer]

I understand where you/he were coming from now.

Even easier, - thrash the nuts off it in 1st gear and see how fast it goes

Joking apart even with high numeric diff ratios its easy to tell a 2.54 or a 2.9 first gear, and a 3.3 just feels darn stupid.

[gjz30075]

Quite right. All you need to know is first gear ratio; that'll tell you what box you have. As mentioned in the other thread, pull the plugs, mark a rear tire at 6:00 and mark the crank pulley somewhere. Push the car for one rev of the tire. Check the crank pulley and knowing it'll be about three revolutions, see where the mark ends up; either before three revs and exactly where, or after three.

[Chancer]

Wish I had been able to explain it concisely, however it wont be 3 or so turns of the engine for one turn of the driven wheels but 3 or so times 3 or so turns, the final drive ratio multiplied by the first gear ratio.

Best way though, not relying on speedo's, rev counters, GPS's etc.

[gjz30075]

Whoops! Yep

Greg z

[Craven]

Some Numbers for you,

Semi- Close------------------------Close
Top 1:1------------------------------------1:1
3rd 1.396--------------------------------1.230
2nd 2.009--------------------------------1.636
1st 2.972---------------------------------2.510
If you have a semi-close gearbox your revs will double if you change down from top into 2nd gear for the same road speed. Yes I know don?t do at high speed.
As per my first post Rev counter & Speedo absolute accuracy not involved, if you really want to worry then the Rev counter linearity is a source of error. Diff ratio and tyre size are constants.
Ron.

[gjz30075]

There's also a wide ratio box:

Top 1:1
3rd 1.41
2nd 2.39
1st 3.54

[Chancer]

Please tell me that Lotus never fitted that one as standard!!!!

Isnt it just a normal cooking road car box? I have driven a Westfield with what I thought was a 3.3 first gear and it was appalling, 3.5 doesnt bear thinking about.

[gjz30075]

No, Lotus never fitted one but one could have been fitted to BillWill's car, as he had his fitted by an 'ordinary' Ford shop and who knows what they had in house.

Greg Z

[billwill]

Thanks for putting all the ratios here, saves me from going to look them up.


I know that my diff is 3.54 as I had it rebuilt to that.


I did the actual 'fitting' myself. Took the old one out & took it to the shop & then fitted the newer one into the car myself.