On road diameter of 900x20 tyre

My AT4-690 bus is fitted with the wrong size tyres (8.25x20) In order to use an online calculator to calculate the speed difference of 900x20 tyres I need someone to measure the actual ground to centre of hub radius on a vehicle fitted with 900x20's. I can then double that figure to use as the true diameter in the calculator.

From the Tyre and Rim Association Standards Manual (all tyres sold in Australia must comply "closely" with these measurements.)

Diameter of new tyres loaded at the correct tyre pressure

If you put the figures on the calculation table the relationship between the two will be the same so long as you measure both either loaded or unloaded.

8.25x20 Road Pattern 992mm Traction pattern 1063mm
9.00x20 Road Pattern 1039mm Traction Pattern 1099mm

PS Find one of the many speed calculators that use nominated tyre size not inches/mm they use rolling diameter so the work is done for you.

Lang

page 7 has the info you require..

infrastructure.gov.au/roads/vehicle_regu.../files/VSB2_1991.pdf

Actual diameter Olympic trojan x ply 995 mm ; 39 1/8 in

Actual Diameter Michelin case (radial) bandag cap 1015 mm ; 40 in

both about 50%.

Loaded radius is a bit tricky atm coz truck parked on sopft sand and carby playing up so cannot move, but probably 1/2 inch in it.

Vic's Government chart has the instant answer. Turns per kilometre are the ultimate way of measuring with no diameter calculations required and much more accurate because of large numbers and obviously a loaded vehicle.

9.00 do 12% less turns for the same distance than 8.25 therefore, your revs are going to be 12% slower with the bigger tyres for the same speed, or the speed is going to be 12% faster for the same revs.

Lang

But it will be slower to accelerate or climb a hill.

Additionally it may even be less economical.

Certainly more gear shifting will be required.

You can't have your cake and eat it too!

That's a little bit of over generalisation there Zuffen ... it's all relative.
On here were working with essentially "retired workers"
As such they no longer pull full loads, so don't always need the gearing they did in the workforce.

I'll use my Terrier as an example. Standard tyres were 30 - 34 inch diameter, 80 - 90 kph at 3000 (ish) rpm.
With 9.00 x 20s (40 inch diameter) she jogs along at 100 - 120 kph
Fully loaded, she would drink about 8 - 10 mpg, now as a club / fun vehicle she's cut back to about 15 mpg
Sure ... I gotta get a lower gear to climb a hill, but only because top gear is now effectively an overdrive.
First gear is now always needed to move off, whereas on the little skate wheels second, even third gear could be used when unloaded.


... so ... I'm piggin' out on cake!

The only thing that will change is you will be doing the same things one gear lower because you have basically put a higher ratio diff on the back.

As mentioned you will now take off in first with exactly the same acceleration you used to have taking off in second. Your second last gear will be giving you about the same speed, revs and pulling power you used to have in top. You now have the bonus overdrive and assuming the engine can pull that gear on the flat you will be doing lower revs or have higher speed for the same revs giving you better mileage.

You will just have to get used to changing from the newly discovered overdrive top sooner than you would have previously going up hill. You will also lose your creeper gear low.

Lang

Me old Yanky Kenworth Swami Book sez:













cya

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Lang wrote: You will also lose your creeper gear low.

,sure, but as we've already established ... don't need it no more.

My Terrier was my everyday work truck;- used it to cart my spray gear around.
Now gone from mostly carrying/towing 5-6 tonnes, to occasionally towing/carrying up to two tonnes.
Not missing that low gear at all, and much nicer to drive overall.