TheSamba.com :: View topic - Largest tyre sizes, to fit rear spare-wheel well?

Mon Sep 03, 2007 7:00 am

As I have mentioned in various posts of mine in the past, I am contemplating the future retro-fitment of 16 inch wheels & tyres, to my British specification, 1973 VW "1600" Type 2 Westfalia Continental campervan.

The tyre size, closest to that of the 185 R14C, is the 195/65 R16C, which is readily available in Great Britain, but I am also considering the practicality of using either 205/65 R16C, 215/65 R16C or 225/65 R16C.

There is also the possibility of using 195/60 R16 on the front and 215/55 R16 on the rear; these tyre sizes having external circumferences, which are 1·76% and 1·33% smaller respectively, than the 185 R14C tyre.

I would wish to have one or two matching spare wheels, to ensure predictable handling characteristics, so an important consideration will be what tyre sizes (i.e. section width, aspect ratio, tread width & external diameter) will fit readily, in the engine-deck recessed, spare-wheel well.

Even if one used a non matching spare wheel, the normal day-to-day road wheel, still has to be stored somewhere, if it is necessary to substitute the spare.

So, the big question is, what size tyres will fit in the spare-wheel well?

Regards.

Nigel A. Skeet

Mon Sep 03, 2007 10:15 am

I think I have a fairly large spare that the PO bought for the bus. I'll check the size on it when I get out to the bus later today.

Tue Sep 04, 2007 7:54 pm

The spare that's sitting in my spare tire well right now is a 27x8.5 R14, which works out to be about 0.3 inches larger than a tire sized 215/75/14

it fits right in, and the hard-shell spare tire cover fits right over it.

This site will let you figure the size of the tires, so all you will have to do is take a measurement of the spare tire well and see what all will fit.

http://www.s-series.org/htm/calc/tiresizeconv.htm

Sun Sep 09, 2007 5:23 am

splitty_smile wrote: The spare that's sitting in my spare tire well right now is a 27x8.5 R14, which works out to be about 0.3 inches larger than a tire sized 215/75/14

it fits right in, and the hard-shell spare tire cover fits right over it.

This site will let you figure the size of the tires, so all you will have to do is take a measurement of the spare tire well and see what all will fit.

http://www.s-series.org/htm/calc/tiresizeconv.htm

I thought Imperial measurement tyre sizes, like 27x8.5 R14, had been obsolete for decades!?!

A 215/75 R14 tyre, has a nominal section width of 215 mm, but actual section widths are reputed to be quite variable, between different tyre brands; an overall variation of upto 25 mm, being relatively common.

The rear load compartment of my 1973 VW "1600" Type 2, is somewhat cluttered at the moment, but I was able to take a rough measurement of the spare-wheel well, which appears to be circa 210~215 mm. Hence, a 205/65 R16C tyre, would probably be the maximum possible, nominal section-width, which would fit in the spare-wheel well, but I suspect that 195/65 R16C, would be more realistic!

A local aquaintance of mine, has a recent-model, Ford Tourneo (minibus version of the Ford Transit panel van), equipped with 205/65 R16C light-truck tyres. He has a part-worn spare tyre, in his garage, which wasn't mounted on a wheel rim, so I was able to measure it with my steel tape measure and tread-depth gauge, as follows:

External circumference = 2093 ± 1 mm
Tread depth = 5½ ± ¼ mm
Tread width = 165 ± 1 mm

Nominal section width = 205 mm
Actual section width = 210 ± 1 mm

His new, 205/65 R16C light-truck, front tyres, had a tread depth of 9½ ± ¼ mm, so bearing in mind the measurement tolerances, I estimate that a new tyre, would have an external circumference of 2118 ± 4 mm, which just about brackets, my theoretically calculated value of 2113·9 mm.

Regards.

Nigel A. Skeet

Sun Sep 09, 2007 4:00 pm

NASkeet wrote:

I thought Imperial measurement tyre sizes, like 27x8.5 R14, had been obsolete for decades!?!

We still use them quite a bit in the States. Almost all of our truck tires and off-road tires are measured this way. The tire I have for a spare is a very beefy All-Terrain Tire.

Thu Dec 17, 2020 8:27 am

I have used a circle-theorem pertaining to intersecting chords of a circle, to determine the radius of curvature of the spare-wheel well, which to a close approximation, is probably of circle-segment cross-section.

This is a technique I can also use to determine the required radius of curvature for the new replacement plywood gauge-panel [to accommodate the 100 mm speedometer, 100 mm tachometer, 100 mm three-gauge cluster, four 52 mm gauges and a 60 mm 8-segment warning-light cluster] to go in the substitute Triumph Dolomite HL style instrument panel that I fitted to my 1974 Triumph Toledo 1300 in 1984.

Had I studied traditional mathematics for GCE “O” Level, rather than SMP modern mathematics, I would have encountered circle theorems way back in the early-1970s, but as things turned out, I didn’t encounter them until I started giving private tuition in GCSE Higher Tier Mathematics from the mid-1990s onward!

According to the theorem, if two chords AB and CD intersect at a point E inside or outside the circle, then the product of lengths AE x EB = CE x ED.

If I measure lengthwise horizontally across the length of the spare-wheel well, this will give chord-length AB = 630 mm, via mid-point E, such that AE = EB = ½ x 630 mm = 315 mm.

Measuring vertically downward from mid-point E, to the bottom of the wheel well at point C, gives the length CE = 194 mm, forming part of chord CD = CE + ED, which is a diameter of the circle; recalling that a radius or diameter bisects a chord at a right angle.

Manipulation of the relationship gives ED = AE x EB / CE. Further manipulation gives the wheel-well diameter CD = CE + ED = CE + AE x EB / CE.

Recall that AE = EB = 315 mm and CE = 194 mm

Hence, wheel-well diameter CD = 194 + 315 x 315 / 194 = 705 mm

The dimensions AE and CE were probably measured to better than ± 1 mm, so the wheel-well’s cross-sectional diameter would probably lie in the range 700~711 mm

CDMAX = (194 + 1) + (315 + ½) x (315 + ½) / (194 – 1) = 710¾ mm

CDMIN = (194 - 1) + (315 - ½) x (315 - ½) / (194 + 1) = 700¼ mm

215/65 R1C tyre – predicted external diameter = 686 mm

225/65 R1C tyre – predicted external diameter = 699 mm

Comparison of the calculated spare-wheel-well diameter with the calculated external tyre diameters, suggests that a 215/65 R16C tyre should fit comfortably in the spare-wheel-well, provided the spare-wheel-well can be made wide enough; and might possibly even accommodate a 225/65 R1C tyre.

According to my measurement with a metric tape measure, the spare-wheel well is 214 mm (i.e. 165 mm + 49 mm) wide, so I’m a little surprised that a Mercedes C-Class 7J x 16 inch wheel with 205/55 R16 tyre didn’t fit!?!

Just to be on the safe side, I shall measure the external circumference (from which I can infer external diameter & radius) of the two 215/65 R16C tyres on Vauxhall Movano wheels that I possess, and create a corrugated-cardboard template to try for size in the spare-wheel well.

Thu Dec 17, 2020 8:41 am

I can't say my 27/8.5r14's fit the well perfectly, but they do drop down into the well and the cover with effort will go over them. Since a 225/70r14 is essentially the same size it should fit about the same.

Fri Dec 18, 2020 12:09 pm

Wildthings wrote: I can't say my 27/8.5r14's fit the well perfectly, but they do drop down into the well and the cover with effort will go over them. Since a 225/70r14 is essentially the same size it should fit about the same.

The actual section-width of a tyre between the inboard & outboard surfaces of the sidewalls, depends on both the nominal section width (i.e. 225 mm for a 225/70 R14) and the wheel-rim width. The rule of thumb, is that the actual section width increases by circa 1/4 inch for each 1/2 inch increase in wheel-rim width.