This LRO reader emailed LRO.firstname.lastname@example.org looking for advice on tyre changes and speedometer readings.
I’ve changed my Series IIA’s tyres from 6.00-16s to 7.50 R16s. The handbook lists 7.50s as an optional size but doesn’t say if they affect the speedo reading. Can you confirm, please?
Graham Richards, Chesterfield, Derbyshire
Yes, they will. The speedo will still be recording the number of times your wheels (or, more accurately, output shaft) turn, but the tyre circumference is larger so you’ll travel further for each revolution. A speedo isn’t allowed to under-read, so check your actual speed using a Sat Nav or other GPS device and compare it to what is shown.
The gearing is marked on the face, either under the mileage or around the bottom outside edge. For 6.00 tyres it’s 1536 and for 7.50 tyres it’s 1408.
There are sometimes other numbers but these are by far the most common.
This workshop advice appeared in the September 2015 issue of LRO. Back issues are available to download on digital devices here.