Tyre sizes

[mojobaby]

As you probably know, R4's are designed to use 135/70 tyres.

A few months ago I needed two new front tyres. The tyre shop only had 145/80 tyres and I would have had to wait a few days to get the correct ones, so I just bought them without thinking it through.

On a 135/70 tyre, the width is 135mm and the height of the tyre wall is 70% of 135. Or 94.5mm
On a 145/80 tyre, the width is 145mm and the height of the tyre wall is 80% of 145. Or 116mm.

I started thinking that with the bigger tyres, I would go further on a tank of petrol! But what actually happened, was that I had to change down to 3rd gear to get to the top of hills. I thought that there was something wrong with my engine-it seemed to lack power.

Yesterday I changed the 145 tyres to the back and put the 135 tyres on the front. When I went to town today, the difference was incredible and I could get to the top of the hills in 4th gear again, even with 75Kg of cement and sand in the back.

I now realise that more torque is required from the engine to turn the bigger tyres. Perhaps that's why my drive shafts were damaged?

 

[mr-reno-139]

I've never seen any R4 with 135 70 13 tyres only 135 13 or some markets and vans 145 13
Standard profile is 80 % so the 80 bit wasn't on older tyres but seems to be on most new standard profile tyres now
Would of thought your engine is running with regarded ignition timing to make such a big difference

 

[petak]

Standard tire on R4 is 135 R13 what would actually be 135/80/13. Except F4 and F6 (145 R13 or 145/80/13)
Your tire 135/70/13 was from the bigging smaller than original and engine could turn it easier.
If you want to change tire to wider you should kept hight, and circumference of a circle in some +/- 2,5% difference

So if you go to 145 tire closest to original is 145/75/13. Difference in hight is 0,27% or 1,5mm, and circumference 4,71mm bigger. But not sure is it possible to get that tire.
145/80/13 is a lot bigger. Hight +2,93% or 16mm, circumference 50,27mm, (this could be one of the reasons why F4 and F6 has different ratios in gearbox - not only reason)
145/70/13 is smaller. Hight -2,38% or -13mm, circumference -40,84mm

155/70/13 is closest to 135/80/13 that you can find on market. It is bigger 0,18% or 1mm, circumference 3,14mm. (that 1 mm you can rub down if you just once slide front tires )

If your old tires were 135/70/13 best option for you would be (if you can not find 135/80/13) 145/70/13.
This tire would be bigger that 135/70/13 and closer to original setting and you should be able to get up hill in 4th gear as you did before.

R4 is small car with small engine power and he is very sensitive to this small changes, unlike new cars where you can not see difference that drastically

But i am aware that it is problem to get tires in dimensions that R4 needs.

I have made my own tire calculator in excel but there are few good ones on line

This one can show you difference if you change whole wheel (tire and rim)
http://www.rimsntires.com/specspro.jsp

And this one is only calculating tire difference
http://www.tyresave.co.uk/tyresize.html

 

[mr-reno-139]

Well explained petak
It was late last night when I replied with a brief explanation
If I wasn't so TYRED I would of gone more in depth ☺
Think 135 70 13 are from Citroën and small peugeot models

 

[mojobaby]

Thanks Petak for that information. My restoration car has 135/80/13 tyres so I have changed them to my daily drive.
So I now have 135/80 in front and 145/80 at the back.
According to my calculation, circumference of the 135/80 is 1,72meter and the 145/80 is 1,77 meter.
But the 135/70 have a circumference of only 1,63 meters. A big difference!!

I suppose it's like trying to pedal a bicycle with a big wheel. More effort is required than with a smaller wheel. More strain on the engine!!
I have made life more difficult for myself because I must now consider what size spare tyre I should carry

 

[angel]

145/80/13 was always an option on R4 Berline, so they should not make such a difference in overall gearing. It has never been that obvious to me (but I had always been going from 135/80 to 145/80).