What effect of tyre size on road speed?

[kw573]

Hello all,

Many will know that I am building a Diamond T 969 which I plan to use lots over the coming few decades. This will include some med/long distance trips. Because of this, and the mostly lack of parallel minor roads to use, I need to get a lot more than 40mph out of this truck.

One option that I have considered, and want to have another look at is fitting larger tyres. A long while back, I crunched some numbers on tyre sizes and was not impressed with the theoretical result, vis.,

For the 969,
9.00 x 20 tyres (dia. 1019mm); @ 2300rpm = 40mph
10.00 x 20 tyres (dia. 1055mm); @ 2300rpm = 41.5mph
11.00 x 20 tyres (dia. 1084mm); @ 2300rpm = 42.5mph

The diameters came from a tyre sizes chart at the local tyre shop.

The more I look at it, the less realistic these numbers look. What have I done wrong? If you have on-the-ground experience with this, please tell us about what you found out. Please.
Further, the 9.00 x 20 tyres look lost in the 969 guards. So I started to look more closely at period pics of the 969 with larger tyres.
Are these tyres (below)11.00 x 20? And if so, what rims is it using? The M1A1 uses 11.00 x 20s, and it's rims are around 2" wider but have a similar off-set. The disc (where the wheel nuts go) is about 3" outside the bead. But the front rim in the pic seems to have zero offset to the outside bead. Can't see for the rear wheels. Anyone know what rims they are?



I have other questions, but this is enough for now. Help appreciated.
Have a nice day.
Sam.

[Shane Hodby]

Hey Sam
There is a specific formula to use to work it out and I cant remember what it is but will try to find out for you and let you know when I do
Cheers
Shane

[40 Chevy]

Sam;
I have looked at that picture often and one possible explanation is this. The rear tires are 11-20 on standard rims and the front is a special offset wheel to keep the front and rear tires in line. There might be a spacer block, similiar to the ones used on a DUKW on the rear wheels. I do not think those tires are 11.00 -18 as used on the dukw, but might be 12.00-20. Anyways they offer superior traction in sand as compared to rear duals.

Just my two cents.

John G

[kw573]

Thanks Shane,
I would be interested in that formula. It would be a very hand bit of info.

thanks John
Yes, 12.00 x 20s is probably more likely than 11.00 x 20s. The rear rims do look like originals, well, a wider version with similar offset. To mount 12.00s properly would require a rim about 3" wider than for 9.00s.
However, that still leaves the front rims that seem to have zero offset. Surely they were not specially made, but were a take-off from another model vehicle.
Does anyone recognise these rims?

This pic shows a Diamond T that seems to have singles as original. Once again, the rims look the same as the pic above. Actually, now that I look at it, they could be standard off-set rims on the front.


Comments requested please.

Happy restorations.
Sam.

[Shane Hodby]

HI Sam
Have emailed you the calcs for tyre size etc
you maybe able to put them on here for others if they need to use it as I am not that good at putting things on here when I get the other one I will send it to you
Cheers
Shane

[kw573]

Wow, Shane, those tables are just the thing! Thanks heaps.

Hi all,
Shane emailed a pair of excel tables to me, which you enter the tyre sizes you have/want and it crunches the numbers, including speedo error!
Can someone tell me how to post a "live" excel document as I don't have an online place to put it. G503 Album only handles pics from what I can see.
Sam.

[Brett Nicholls]

Hi Sam,
The change in tyre size I think is the easiest way to go so far as increasing road speed. I have an Inter ACCO truck and originally could not drive it too far as the best speed I could get out of it was about 75km/h. It had 7.50x20's on it so I looked into it a little and changed them to 9.00x20. The truck can now do about 100-105km/h which made the truck very usable. First gear was not really useable before the change as the truck is 4x4 and first gear gave a very low ratio and no good for standing start unless loaded. once tyres were changed I was able to make use of first gear which was good. I was very surprised at the difference.
It would be interesting to look at the spread sheet. Maybe send the moderator an email to see if they can help get it up on the Gee.
Regards,
Brett.

[Jeff Gordon]

Can someone tell me how to post a "live" excel document as I don't have an online place to put it.

Sam I have overcome this before by taking a photo of the screen.
I am sure there is a better way but it worked for me.
I am keen on seeing it.

I was talking to Micheal H and he thinks you could put a strong truck gearbox in behind the transfer case but back to front.
If you were to use a Clark box and you changed it from direct to third which is normally 1.85:1, back to front would be 1:1.85 which is 0.54:1
You might not have the horsepower to push it. But the main box does not have to be in overdrive.
Just a thought.
Cheers

[kw573]

Hi Jeff,
If I just post a pic of the table, it won't be a "live" file. This table does the calculations when you enter different tyre sizes and transmission ratios. But I will try to put a set of numbers for the 969 in, see what speed it calculates, and post a pic of that.

Does anyone have a place on the net, like a photo host, that will handle excel documents, that can be URL'ed for all to access? Then anyone can do their own sums.

As for a reversed extra transmission, this was suggested on "Speed up a 969" a long while ago. Sgt. Hancock kindly suggested a "brownie box", a three speed joey box. Perfect! Except, there is not enough room in the chassis. Combine a short wheelbase, transfercase, (and "remote" at that) and hand brake and that leaves barely 3' (1000mm) of rear tailshaft to play with.

So, my current thinking is to put it all together original (except engine) and see how it goes. Then play with tyre sizes during the running in of the new truck. Another real possibility is to have a set of higher OD gears made for the gearbox, say from 0.78:1 down to 0.68:1. If there is space in the case.

Will I see you at Toowoomba Swap?

Have a nice day.
Sam.

[Shane Hodby]

Hi Sam
Changing the overdrive gears wont make that much difference as you are still running through a transfer case and then your diff ratio will keep you slow
You could put a a set of 34,000lb Rockwell diffs under it old ones out of a 70,s model truck they would look similar from the axle ends and just set them up to take the 6 rod set up you should be able to pick up a set from a truck wreckers or off a old farm truck that would solve all your problems a ratio around 4.5 to 5 would suit it
just another thought for you to consider

Cheers
Shane

[chevy43]

I have seen a couple of 969's here in the states with 1100x20 on the original rims. It looks good!

900x20s turn about 525 turns per mile and 1100x20s are about 480 turns per mile so you'd gain about 3-4 mph.

Put a detroit diesel 6v-53 engine in it and you get about 55 mph at 3,000 rpm or a Cummins 5.9. 3k rpm is pretty screeming fast for either but they will last a long time doing it. The Hercules is really spinning fast at 2,300.