P38A What are the tallest tires you can put on a standard P38 16 inch rims ?

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RangeRoller dt said:
I had 32s (“) on my p38. They won’t fit on as standard - no chance don’t even try it. Trust me It gears down and speedo reads wrong. You get used to it as it’s only few mph here and there until you get well over 100.
Don’t help your mpg i promise.
Any scrubbing will flag on your MOTs - you can adjust the stops but you then need to go around the moon to turn at the end of the steet, your spare won’t fit and the standard bottle Jack is now useless. If your EAS drops you could be in serious trouble.
Obviously this is a stupendous size, you may get away with a little bit but it’s the same principleView attachment 340476
Click to expand...
Yes I don't think I would cope with worsened steering 😂 it already has the turning circle of a limo .
 
You talk about ride and fuel economy and yet you want to increase weight by fitting unnecessary steering gear and silly tyres both of which will do nothing for fuel economy. The ride on the P38 as standard is among the best I have experienced and I have had many different cars from a Roller down.
 
RangeRoller dt said:
I had 32s (“) on my p38. They won’t fit on as standard - no chance don’t even try it. Trust me It gears down and speedo reads wrong. You get used to it as it’s only few mph here and there until you get well over 100.
Don’t help your mpg i promise.
Any scrubbing will flag on your MOTs - you can adjust the stops but you then need to go around the moon to turn at the end of the steet, your spare won’t fit and the standard bottle Jack is now useless. If your EAS drops you could be in serious trouble.
Obviously this is a stupendous size, you may get away with a little bit but it’s the same principleView attachment 340476
Click to expand...
Im going to have to disagree with this.
We have a P38 on stock air suspension thats on 235/85R16 and they fit. They do touch very slightly but not nearly enough to warrant trimming.
On the rear axle under full compression they touch slightly on the top of the wheel arch liner. On the front axle under full compression and full lock they will touch the wheel arch liner at about the 2 oclock position.

They do affect the gearing though. On stock gearing we did get slightly better fuel consumption when highway cruising, the cars ideal cruising speed did increase though. Definitely worse on town driving.
On the highway when going up hills and stuff you have to fight the car to not downshift into 3rd gear.

We recently changed the diff ratios to 4.12 which has basically put the whole cars gear ratio back to stock with the bigger tyres and it performs almost identical to stock now.

If you do off roading, change to 235/85R16 and if you feel like it the ratios too, its worth it.
If you dont do off roading dont its not worth the trouble.
 
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LR-hobbs said:
Yes I don't think I would cope with worsened steering 😂 it already has the turning circle of a limo .
Click to expand...
There is almost no diffrence in steering and turning circle.
High speed cornering is worse but are you going to race tracks in your P38?

There is a huge diffrence in off road performance between stock and the 235/85R16
 
Datatek said:
No
Click to expand...
Incorrect
There are two potentiometers on the back of the gauge cluster that are used to calibrate the speedo. They where used on the police cars to get an accurate speedo.
They only calibrate the speedo itself though, so your distance measurements will be off.
 
kermit_rr said:
Good point
If the standard is 255/65R16 and that's what the car expects then the speedo will read ~60 at ~65mph
Click to expand...
The speedo over reads with stock tyres, so you can go bigger to get a more accurate speedo.... however, such things just aren't checked anyway. And with GPS on your phone, it is very easy to see what is what and adapt.
 
LR-hobbs said:
Hello all
Just wondering wether anyone has experience in fitting tall tires to a standard P38 , I found some I liked however the sizes were 235, 85, r16 , are these to tall and going to rub?


Cheers
👍
Click to expand...
What are you wanting to use the p38 for? Is it for off roading? And do you still have air suspension?

The biggest issue is, if you are on air still and the system drops to the bumps stops. Bigger tyres will rub massively and you may not be able to steer. Which could leave you stranded, if you are off road on a lane this could be horrendous to deal with.

And not all air suspension fixes are easy, I had an air spring separate under articulation, the plastic piston came out of the rubber bladder. So no reset tool would be able to get it back to normal ride height, being that it was a physical failure of an air spring.

I run 255/70R16's on my p38. Which ironically makes the speedo run 100% accurate. But on the bump stops moving the vehicle did damage the lugs on the tyres. Anything taller and you'd probably stand no chance to move it on the bump stops.
 
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Post by Montyjohn
 
FEI said:
Incorrect
There are two potentiometers on the back of the gauge cluster that are used to calibrate the speedo. They where used on the police cars to get an accurate speedo.
They only calibrate the speedo itself though, so your distance measurements will be off.
Click to expand...
I take it the two potentiometers you are talking about are the tiny ones I have circled in the photo below. Good luck trying to tweak those without a calibrated test rig.
IMG_0761a.JPG
 

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I cant remember which ones exactly but two of the ones ive circled in red are labeled VR3 and VR4.
VR4 changes the speedo offset.
VR3 changes the slope.

This bit i havent confirmed but VR1 adjusts temp gauge offset VR2 fuel gauge offset, VR5 and VR6 adjust the offset and slope for the tacho.

Just drive a steady speed check what your GPS shows, check what the speedo says and just adjust the pots little by little, if you find a nice piece of road itll take you 30min to adjust it within a reasonable tolerance.
 

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FEI said:
I cant remember which ones exactly but two of the ones ive circled in red are labeled VR3 and VR4.
VR4 changes the speedo offset.
VR3 changes the slope.

This bit i havent confirmed but VR1 adjusts temp gauge offset VR2 fuel gauge offset, VR5 and VR6 adjust the offset and slope for the tacho.

Just drive a steady speed check what your GPS shows, check what the speedo says and just adjust the pots little by little, if you find a nice piece of road itll take you 30min to adjust it within a reasonable tolerance.
Click to expand...
Have you tried doing it? Sounds a bit hit and miss to me depending on how sensitive the multiturn pots are. Might be interesting to build a test rig one day when I have time.
 
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