I am now able to report on changes to the front tyre pressure on my GV.

Using Vmaxxer's method of 42PSI on the front tryres (but retaining maker's advice about rears at 35PSI), there has been an improvement of 4.5mpg. in the brim-brim figure. The computed readout figure was in error by 2.5mpg. high. No change in driver, no change in journeys.
The steering is noticeably lighter.
The initial acceleration is better, what I call the "snap" acceleration.
All the tyres are running at the same temperature by hand-test. This is about blood heat in current ambients.

I see this as a substantial improvement.

Leedsman.

 

Good results!
Question: why you keep rears at 35PSi if I might ask?

Although I can imagine with no passengers or cargo in the back you could keep the pressure somewhat lower in the rear, but in my situation I need the rear pressure as high as front because I frequently ride with (sometimes) heavy cargo in the back.

 

Answer to your question, "why keep the rear tyres at 35PSI"?
I never carry anything heavy in back of the vehicle.
I look at the "bulge" in the tyre sidewall and it's only modest.
Tyre temperatures (by hand) are the same all round now, and reasonable.


Therefore I'm concluding 35PSI is about right for the loading on the rear tyres, for my circumstances. The "bulge" in the front tyres is about the same as the rears, so I'm thinking your running pressure of 42PSI is about right there. I've also looked at many other vehicles and they all have that bulge about the same in front as in back. Radial-ply tyres of course.

If I was carrying heavy loads and the rear tyres were bulging unusually, I would up the pressure to 42PSI. This could happen as I'm thinking of moving house before very long. The GV's sheer size, carrying capacity, and seat adaptability was a main reason I bought it in the 1st. place. The GV has not proved to be any real problem, not to me at any rate.
The fuel consumption is now far better than a Transit van would be, so is the acceleration.

Leedsman.

 

Tyres on UK roads [tar or asphalt] have a 0.03 rolling resistance coefficient at about 1500 kg, rolling resistance or rolling friction is a different thing to do with weight in the vehicle which resists the pulling force of the engine. Our GV's are 2,242 kg @ 0.03. Leedsmans figure of ± 5mpg is in line with my experiences over 50+ years with both cross & radial ply tyres its just common sense. However weight is spread via the axles to the road surface, having one axle of weight distribution at 45psi and another at +50psi must induce drag and reduce the MPG with any vehicle of any weight particularly our 2.25 ton bus.

 

When I swapped out my GV wheels for a set in 18's I played around with the pressures for a while, trying to figure out what worked best. At first I was running the front at 44 psi (cold) and the rears at 40psi. However, although this was great for light steering and improved MPG, I found it was really bad in wet conditions under heavy braking or acceleration. So with a little bit of experimentation I found 40 up front works best. Dropping below this traction in the wet is again poor.

As for the rear, I find that 36 (without seats) to 38 PSi works best. any more and the ABS is kicking in under heavy wet braking.

Now, when I fitted snow tires I had to rethink it all again. 40 was too hight as again wet braking and traction were effected! 37 was the sweet spot!

 

Valuable observations Vonhofinvule, I was wondering about wet grip myself. I will report if anything transpires in the next few months.

Leedsman.

 

Just thought I should add that I am using 17" rims with Michelin winter tires (Year round) 38 PSI Front and 36 Rear seems to be about right... Running higher in the front tends to accelerate wear in the tread centre - also my speedo is much more accurate with the 225/55R17 Alpins