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Axle Ratios and Wheel Sizes

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James Alford02/07/2018 22:44:20
341 forum posts
72 photos

Good evening.

I am getting muddled, so should appreciate any help, if possible.

If an engine and gearbox which is normally matched to an axle with a differential with a ratio of 3.23 and 10" wheels (tyre OD of 19.1" is fitted to an axle with a differential with a ratio of 5.25 and 17" wheels (tyre OD of 25.6", what is the likely result?

I cannot work out whether the larger wheels and 5.25 differential will result in the engine over-revving, under-revving or whether there is likely to be little difference.

Any suggestions?

Regards,

James.

Martin W02/07/2018 23:13:14
785 forum posts
29 photos

James

The 19.1 tyre OD with the 3.23 ratio for 1 rev of the input shaft will move 18.58 in.

The 25.6 tyre with the 5.25 ratio for 1 rev of the input shaft will move 15.32 in.

The difference between the two is 18.58/15.32 = 1.21. So the drive shaft will be rotating 1.2 times faster for the larger wheel with the 5.25 ratio compared to the smaller wheel with the 3.23 ratio to achieve the same speed i.e. if for the smaller wheel arrangement at say 30 mph the engine turns at 1000 rpm then with the larger wheel arrangement it will have to run at 1210 rpm.

I think this is correct and hope it helps.

MartinW

PS Potentially better acceleration but lower top speed for the 25.6 tyre with the 5.25 ratio diff.

Edited By Martin W on 02/07/2018 23:15:05

Edited By Martin W on 02/07/2018 23:18:35

James Alford02/07/2018 23:35:14
341 forum posts
72 photos
Martin.

Thank you for the prompt reply.

If I understand your reply correctly, am I right in thinking that with the larger wheel, the car should accelerate more rapidly, but have a lower top end than with the smaller wheel: well, at least in theory?

James.
Martin W02/07/2018 23:46:20
785 forum posts
29 photos

James

That's the way I see it from the method I used to determine the difference between the two set ups. The driven shaft will have to turn faster to maintain the same speed with the 5.25 ratio and 25.6 OD tyre. It would be similar to running in a lower gear all the time.

Cheers

MartinW

Martin W03/07/2018 00:14:52
785 forum posts
29 photos

Double post

Edited By Martin W on 03/07/2018 00:15:34

James Alford03/07/2018 07:32:51
341 forum posts
72 photos
Martin.

Thank you for the confirmation. This is what my logic told me, but my late night calculations did not confirm it.

High revving for the slightly better acceleration will be fine in preference to under revving and struggling to pull in top gear.

Regards,

James.

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