Cross, bias & radial-ply tyres: gearing & speedo cal

[NASkeet]

At sometime in the furure, I shall be upgrading the instrumentation, plus wheel & tyre combination, of my British specification, 1973 VW "1600" Type 2 Westfalia Continental campervan.

Originally, the vehicles came equipped with factory-fitted, 7.00-14 8PR cross-ply tyres, which were replaced in the late-1970s, with Michelin XZX, 185 SR14 Reinforced radial-ply tyres; as specified in the official 1973 VW Type 2 owners' handbook, published by Volkswagen, in August 1972.

According to my general tyre information source, the majority of cross-ply tyres, had an aspect ratio of 95%, but some of the later, more refined cross-ply tyres, manufactured the 1970s, had an aspect ratio of 77%.

This and other tyre information sources, including some wheel & tyre conference proceedings, dating from circa 1983, variously describe standard-profile radial tyres, as having an aspect ratio of either 80%, 82% or 83%. My calculations for 7.00-14 and 185 SR14 tyres, result in predicted external tyrecircumferences, as follows:


7.00-14 (77% aspect ratio), external tyre circumference = 1977·3 mm (i.e. 77·85 inches)
7.00-14 (95% aspect ratio), external tyre circumference = 2178·4 mm (i.e. 85·77 inches)

185 SR14 (80% aspect ratio), external tyre circumference = 2047·0 mm (i.e. 80·59 inches)
185 SR14 (82% aspect ratio), external tyre circumference = 2070·3 mm (i.e. 81·51 inches)
185 SR14 (83% aspect ratio), external tyre circumference = 2081·9 mm (i.e. 81·97 inches)


Sometime ago, I measured my nearly-new, Michelin XZX, 185 R14 Reinforced, spare tyre, having circa 6 mm tread depth (new tyres have circa 7~8 mm tread depth) and found it to have an external circumference of 2040 ± 1 mm, which considering the modest tread wear, is consistent with an aspect ratio of 80%.

Assuming the original 7.00-14 cross-ply tyres were of 95% aspect ratio, I predict that the tyre change from cross-ply to radial-ply type, would have resulted in a 6·03% decrease in external tyre circumference, which would alter both speedometer calibration and overall effective engine gearing.

Noting that the 1971~79 VW 1600 Type 2 engine, reaches its maximum power of 50 DIN horsepower, at 4000 rpm, a vehicle fitted with 185/80 R14 tyres, would have optimal gearing with half-worn tyres, which facilitates the greatest maximum attainable road speed, of circa 68 mph (as quoted by Volkswagen, in the owners' handbook), for the non-high-top, fixed-roof van or microbus). With new or fully-worn tyres, the vehicle would be overgeared and undergeared respectively, by about 1%, resulting in a modest reduction in the maximum attainable road speed.

From similar considerations, use of a factory-stock 1971~79 VW 1600 Type 2, 50 DIN horsepower engine, with a factory-stock 1976~79 VW 2000 Type 2 transaxle, in conjunction with 185/80 R14 tyres, would result in significant undergearing and hence a substantial reduction in the maximum attainable road speed!

In my future-projects box, is a dual range, 15~150 mph & 20~240 km/h, 80 mm diameter, accessory VDO Cockpit speedometer (VDO part No. 120·020·142·014), with trip counter, having a calibration of 790 revs/mile. I am contemplating using this, as part of my future upgraded instrumentation, in a home-made, full-width (i.e. from glove box edge to driver's ventilation outlet) instrument binnacle, incorporating two, 60 mm diameter, 8-segment warning-light clusters (providing 16 warning lights,with colour filters and emblems of my choice!) and upto twelve additional 52 mm diameter gauges, including a VDO Cockpit fuel gauge (VDO part No. 301·272·052·001), which seems to be compatible with my existing dip-pipe fuel-gauge sender.

Assuming the 790 revs/mile were exact and noting that 1 mile = 1·6093 km, this would require an external tyre circumference of 2037 mm (i.e. 80·2") for maximum accuracy of speed & distance measurement; corresponding closely to a partially-worn 185 R14 tyre, with an 80% aspect ratio. This is also remarkably close, to the predicted 2026·3 mm (i.e. 79·78") external tyre circumference, of a 165 R15 tyre, with 80% aspect ratio, which I suspect is the stock tyre size, for the VW Type 1, 3 & 4 cars. Note that a barely legal, worn tyre (i.e. 1·6 mm tread depth, in Great Britain & Europe), has an external tyre circumference, of circa 2% less than a new tyre, with 7~8 mm tead depth.

If any were compatible, with my 1973 VW Type 2's dip-pipe, fuel-gauge sender and tyre size, I would alternatively consider using a VW Type 1 Beetle's combined speedometer & fuel gauge, whose larger diameter (thought to be circa 110 mm!?!) and 90 mph or 100 mph upper speed limit, would make it easier to read, but ideally it needs to have both mph & km/h scales, for driving in Great Britain & Europe.

Using the larger diameter, combined speedometer & fuel gauge, would reduce the number of possible gauges by one, but a speedometer, plus a total of eleven gauges, will probably be sufficient for my needs!

At the moment, I am ignorant of both the stock VW Type 1 Beetle's and 1968~79 VW Type 2's speedometer calibrations in revs/mile. Does anyone know their specification?

In Great Britain, the accepted legal tolerance on speedometer calibration, is said to be 10%, but I have yet to establish whether this means ± 5% or ± 10% or – 0% & +10%, which will influence by how much one can legally modify external tyre circumference, from the factory-stock specification. In some territories, this is specified as ± 2%, without any reference to speedometer calibration.

Regards.

Nigel A. Skeet

[MedicTed]

Nigel, you are definitely overthinking the tire thing. 2% difference is nothing. You'll find that variation in vehicle to vehicle. Get the tires you want and enjoy.
_________________
Ted Wojton
70 VW Bus Westfalia camper
2003 GMC Sierra

[pig-pen]

I read somewhere or I have a memory from somewhere that VW SPEEDOS measure a mile with 800 wheel revolutions, and a KM with 500, it all seems pretty approximate anyway.

Eg on a 50s bug (sorry this is my reference) the wheel circumference would be approx 77.8" then it would actually be 814 revolutions to the mile, and 506 for KM

VW simplified this to 800 / 500 so it is already some 2% under on the dial...
_________________
1954 L227 Type 113
1964 L31 Type 265

[pig-pen]

So...

with that in mind... and anything close to 2000mm circumference being more accurate, then you could go for...

175 / 80 / 14 - circumference of 1997mm

or

185 / 75 / 14 - circumference of 1989mm
_________________
1954 L227 Type 113
1964 L31 Type 265

[SGKent]

Actually you can calibrate the speed of the bus accurately by the size of the splats on the windshields from bugs if you know what kind they are.

If the speedo was off 10% - 20% I'd be concerned but that low of a percentage now like 2% it seems like you spending time recreating the wheel. A speedo shop could probably calibrate it for you. When these buses were new, here in the states AAA used to have mobile dyno setups where you could go to one and they would tell you exactly where your speedo was at three or four different speeds. I don't recall the error on my 1971 being off by more than a mile or two at any speed when using radial tires. Unless you have a 1% tire pressure gauge you won't even get your tire pressure right, and that will affect the rolling diameter too. Also as the tire wears that will change the rolling diameter.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[pig-pen]

Couldn't agree more.

I think my point is any tyre around 2000mm circumference is pretty much the standard tyre size that the OP posted, and works with the speedo as set by VW, so the tyre sizes I suggested work better than those 185 ones that the OP was mentioning.

185 / 75 / 14 is what they need.
_________________
1954 L227 Type 113
1964 L31 Type 265

[richparker]

Interesting that you say the 185/75/14 is the correct size. I could be wrong but I thought the aspect of the 185/14c Vantra Was 80 with a 25.5” diameter and the aspect of the vantra 195/14c was 82 with a 26.5” diameter. I’m currently running 27x8.5 Grabber ATX, aspect unknown, but the diameter is 27” and they state the revs per mile is 769. I would think the Vantras would have lest revs per mile b/c of the smaller diameter. 🤷‍♂️

My bus is using the OG speedometer with a new cable, I’ve checked the speed with an app on my phone and those radar machines on the side of the road (we have a lot of them here). They seem to match, So that’s good enough for me.
_________________
__________
’71 Westy build
Adventure thread
’65 Deluxe Build
’63 Deluxe Build

[Wildthings]

I have found that the factory calibration for the speedo tend to make it read 4% or so high. 195r14 or 27x8.5r14's will bring it pretty much spot on.

185r14 and 195r14 will have an aspect ration in the 80-82% range.

[NASkeet]

[cdennisg]

[Alan Brase]

There is NO REASON to use any non C rated tire on a type 2 (or Vanagon-same wheel as the later bays)
This means usually 185R-14C. There are currently about 10 different brands out there and I've had good results with several.
They are about 25.5 inches in diameter. Which puts them right in the ballpark.
Most important, they are rated about 1900lb at 65psi, so they can carry the rear weight of a bus quite well at 45-50psi and will have VERY stiff sidewalls.
185/75 is NOT the right tire, because it is a passenger tire and not nearly stiff enough.
_________________
Al Brase
Projects: 67 sunroof bug, 67 Porsche 912 Targa, 70 Westy
Dec 1955 Single Cab pickup WANT 15" BUS RIMS dated 8/55, thru 12/55
To New owners: 1969 doublecab, 1971 Dormobile
Vanagons:
80 P27 Westy JUL 1979, 3rd oldest known US
83 1.6TD Vanagon, 87 Wolfie Westy daily driver, swap meet home

[pig-pen]

[richparker]

[SGKent]

stock size Hankooks on my 1977 are dead on. When speedo reads 60 I am doing 60. I've timed between surveyed mile markers, and it is 60 seconds at 60 MPH.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[NASkeet]

[cdennisg]

[NASkeet]

[cdennisg]

[NASkeet]

[cdennisg]