Bedford diff gears

I own a 1978 bedford pmc bus and i am looking for information on diff gears my bus has spring brakes its a bedford diff and im wondering if the crown wheel and pinion from a 16" wheel truck will fit in my 20" wheel bus

Short answer is most likely no.

What is the issue?

My bus has 6.89 gears and a 354 Perkins and top speed is about 70kms i put my trailer on and its 57/60 kms from what info i have about her she was a Geelong city bus

If just pulling a trailer knocks 10-15 kmh off your speed there is no way you are going to pull a higher top gear. Poor little Perkins only has 120hp so you can't ask too much of it.

What about just going up a size in tyres to 10.00. If it has 8.25 just 9.00 might get you another few knots.

Lang

11r22.5 tyres on it

My 72 Comair has about a 5.78 diff with a 0.78 overdrive gearbox on 8.25 x20's.
It travels at about 92kms @ 2200. It's good for >110kmh. Powered by an Isuzu 6BG1 turbo

If you're slowing down with a trailer, then a longer diff is not going to help.

A 5.78 crown wheel and pinion is about $700 new.

Thankyou all looks like a repower might be the only way to gain more speed

Just had a thought.

Do you know what revs you are doing at 70 kmh?

Perhaps the 11.00 tyres have overcooked the operation and you are being limited by too high a gearing not allowing the engine to achieve its revs. The big drop in performance with the trailer also points to you being gear limited not power limited. I think a 6354 will run nicely on 2,800rpm - are you getting that at cruise?

Lang

Yes its doing 2700 rpm just off the rev limiter the trip we took was through very hilly terrain i did gain 5 kms by going up to 11r22.5 from 9.00 20s it has a standard 5 speed box not an overdrive one

Something going on that is not right.

Just by maths - these figures are not open to discussion as they are pure gearing calculations and nothing to do with power or torque having the ability to pull the gearing.

With a 6.89 diff and the Perkins on the limiter at 2,800rpm.

8.25 tyres will give you 74kmh
9.00 tyres will give you 79kmh
11.00 tyres will give you 84kmh

This does not mean the vehicle will go that fast if the engine does not have the power/torque to pull the gearing but in that case you will not be able to reach 2,800rpm. Is the speed on the dash or is it GPS. Check your Garmin as the old speedo might be underselling you.

All above figures are for radials, old cross ply because they have higher sidewalls will produce a bit higher speed

Lang