running larger rear wheels to improve cruising

[weeboll]

when I first got my van it had 185 r15 91s on the back, these were changed to r14's all round but I often wonder if putting them back on would be better as it would lengthen the gear ratios so make for better cruising on the motorways. running 40's and a few other enhancements I'm not bothered about the sacrifice in power if the benefit is less noise and sitting at a lower rpm.

has anyone else got 'big' rear wheels on a relatively stock camper?

 

[70-CA-Panel]

I've got BF Goodrich Baja Champions All-Terrain TA light truck tires all around on my bus, been getting two new to rotate around since 1998.

I don't have any other bus or experience driving other busses to compare to though...
My speedo is out by a little due to the large tires, but ground clearance is awesome.
Surprisingly the handling is excellent too, I used to cruise on mountainous gravel road as well.
It's been awhile since I drove the bus now, but hopefully within a month.

Let me know if you have specific questions.

Cheers.

Andy

 

[Trikky2]

It can make a significant difference. 10% is quite achievable.

 

[mcvw]

Hi,

I have 195/65/15's Goodyear's on the back of my bus which I believe is the same proportion/rolling radius as the original stock tyre/14" wheel combo.

Seems to cruise no problem - recently drove from Derby to Surrey with no issues.

Cheers,


Mike

 

[Alex VW Heritage]

I went from 195/65/15 to 205/70/15's when i changed my rear wheels, made almost a 10mph difference for the same RPM.

Thsi is something is which is so often overlooked!

 

[sam@bugeyeddesigns]

I've got 195/80-15

Made a huge difference

 

[mcvw]

I don't know if this is correct, but isn't it advised to keep the same rolling-ratio on the rear - due to the original cooling capability of the aircooled engine?

Therefore, if you change the ratio (via tyre/wheel combo), you might actually be achieving a higher cruising speed - but a lower RPM - and as such the cooling-fan wouldn't be performing to the same abilities. Over time you could possibly even be damaging your engine :?

I was led to believe that changing the gearbox (and therefore final drive figures) was the best way to improve cruising ability - and also maintain optimum cooling performance?

:?:

 

[Alex VW Heritage]

I was led to believe that changing the gearbox (and therefore final drive figures) was the best way to improve cruising ability - and also maintain optimum cooling performance?

:?:

By changing the rear tyre size it has the same effect as changing the 4th gear or the R&P,

Your correct though VW flat four like 3400rpm as a cruising RPM to cool the engine.

I now sit at 70mph at 3400RPM unlike the 60mph it was before for the same RPM.

 

[weeboll]

one of my worries has been cooling but I rarely get above 90 (on my gauge) sitting at 60 so not overly concerned, taking a hilly route concerns me more as i'll more likely have to drop gear to climb hills but then that should balance the cooling again.

back on the r15's go for next year, bit late to get them shotblasted and reshod befiore heading down to Weymouth later today!

thanks for the advice guys

 

[Graham L]

I was led to believe that changing the gearbox (and therefore final drive figures) was the best way to improve cruising ability - and also maintain optimum cooling performance?

:?:

By changing the rear tyre size it has the same effect as changing the 4th gear or the R&P,

Your correct though VW flat four like 3400rpm as a cruising RPM to cool the engine.

I now sit at 70mph at 3400RPM unlike the 60mph it was before for the same RPM.

Agree with Alex on this, no difference between changing gear ratios and tyre sizes, as long as the end result is that your cruising speed falls at approx 3500rpm then you will have no issues.

More of an issue is getting larger tyres on the back of your bay especially if you have a flat rear arch 68-70, Ego had his rear axle tube shortened so he could run larger tyres on the rims hes running so its not completely straightforward as wheel offset comes into play, I definitely would not get 205's on the back of mine but might squeeze 195's.

 

[Jolly45]

Sorry to hijack,
I've been looking at this as well, I've got 185/75 r14 on the back and need to get the revs down a bit, so was looking at putting 15's on and running 195/75 Yokohama R818 van tires.
A stock set-up (185/82r14 with 0.82 4th gear and a 5.375 FD) will give you a true 59.5mph at 3400 running 205/70r15 at 3400 gives you 60.4mph according to the calculator I just used. :msn4: I know your speedo will always read over and if you got smaller (than stock) front tires it's even further out, so I assume you guys are running different 'boxes or final drive ratios or something to get 70 @3400

Link to the calculator, just in case I've got it all wrong :lol:

http://wahiduddin.net/calc/calc_speed_rpm.htm" onclick="window.open(this.href);return false;

 

[Graham L]

Sorry to hijack,
I've been looking at this as well, I've got 185/75 r14 on the back and need to get the revs down a bit, so was looking at putting 15's on and running 195/75 Yokohama R818 van tires.
A stock set-up (185/82r14 with 0.82 4th gear and a 5.375 FD) will give you a true 59.5mph at 3400 running 205/70r15 at 3400 gives you 60.4mph according to the calculator I just used. :msn4: I know your speedo will always read over and if you got smaller (than stock) front tires it's even further out, so I assume you guys are running different 'boxes or final drive ratios or something to get 70 @3400

Link to the calculator, just in case I've got it all wrong :lol:

http://wahiduddin.net/calc/calc_speed_rpm.htm" onclick="window.open(this.href);return false;

Using the miata tyre size calculator you are only look at a 1.4% difference between the 2 sizes you've quoted above, hence why the difference in speed isn't that great.