Wheels / Speedo / GPS Speed Question

Radar1968

Radar1968

Member
Hi All,

I’m really new to the campervan scene and I have purchased a second hand T28 with a Revolutions Rico conversion.
The van has Kensington wheels fitted with 275/40R/20 106V XL tyres, presumably from when it was converted. The suspension is higher to accommodate.
I’ve noticed on a journey where my speed is constant that the speedo is approx 5% slower than the speed being displayed by my iPhone TomTom GPS. I get 40 on the dial ( with cruise enabled ) and 42 on the GPS.
So I guess I have a couple of questions.
Is this something I should worry about and that I need to get attended to? If so how? Should I be worried that the mileage is also out by up to 5%?
I assume I cannot correct with different tyres and would need to change time whole set of wheels to resolve?
I sort of understand the physics of the tyre circumference etc but I’m unclear as to whether it’s just a margin that is ‘normal’ ( my Ford Fiesta is approx 7.5% the other way. 30 on the dial 28 on GPS ) or if there is a calibration that needs setting.
I’m based in Gloucestershire I’d I need to take it to a specialist.
Any help and advice gratefully received.
Radar

70924203606__9531715D-635D-4C0E-8A60-6FCA9402C2B4.jpeg
 
So this is quite a common scenario
The bottom line is that that the tyres are a little to big and this increase in rolling circumference is whats causing the difference between measured and actual

What options do you have ?
You could fit tyres with a lower profile to reduce the rolling circumference (but making the tyre thinner can affect comfort over bumps as less tyre to absorb movement)
You could also change wheels and tyres to a smaller size to reduce circumference with out reducing tyre profile

There is a function that can be programmed in the van to tell the vehicle that a larger circumference tyre is in play but unfortunately its adjusting a hidden value so not an exact science and this change is a one way ticket and once increased it cannot be decreased so use with caution !!
Going this route would require software/lead to perform so you would need a garage or a local member with diagnostic kit
 
Thanks for the info. It’s kind of what I thought.
The tyres will need changing soon anyway. The van had sat around previously and they are starting to perish a bit.
My main concern initially was legality and whether this was damaging anything.
I thought about 275/35 instead which should correct the issue if the maths is correct in the posts I’ve looked at. I’m just concerned about getting them fitted and then not being correct. Plus the ride comfort, though the 275/40s aren’t exactly luxurious. It’s a hard but wallowy ride already.
275 also seems very wide ( and expensive ) but I guess I’m stuck with those due to the wheels.
Any suggestions re tyres sizes I could use is greatly appreciated. I’m assuming front and rear wheels are the same width but I don’t know.
Also be interested to know if anyone recommends just ditching the wheels completely and replacing. I’m not adverse to that but then it may change the look and the ride height / suspension are there to accommodate the current sized wheels.
 
Thats the tyre size i run on my van and the speedo is pretty exact, if im super accurate with it then i would say its still about half a mph over
Personally i think its best to keep it under reading as it should be so i intend to keep the existing tyre and adjust the van setting slightly to bring the speed correction over slightly and get it back a bit closer to where it should be (dont want the wife blaming me when she gets a speeding ticket !)
 
Thanks again for your time and advice.
So you’d suggest I get 275/35s fitted? I’d imagine they are going to make very little difference to the comfort and still look OK with ride height aesthetics and suspension.
If I went for new 19 inch wheels, forgetting the width for now, what sort of profile would I need to look at for the tyres? I assume I could get 19 inch wheels and tyres fitted that would fill the same physical area in the arch as the wheels currently on the van to keep visually the same appeal?
 
The 35 profile should pretty much erase the over inaccuracy in the speedo and get you very close to actual road speed
Not sure about 19s as not a wheel size i have ever run but as the wheel gets smaller you have the option for more rubber but if your happy with the comfort of the 20s then i guess its a moot point
 
Thanks again.
The comfort is a moot point as I have nothing to compare it to but it does feel hard over bumps. Could be tyre inflation as I have no reference for that and they are inflated to 52psi as I received it. Is there a way of calculating the correct psi or do you know?
I’m not sure of the costs relating to swapping tyres against swapping the whole lot though I’m suspecting it will but quite a lot less.
The tyres as they are run at £150 each or their abouts. I’d imagine the 35s will be in the same ballpark.
I think I’ll mull this over for a while before I do anything though I suspect tyres are the only option. I just hope they don’t alter the aesthetics too noticeably.
I’ll just have to keep an eye on my speed in the mean time and as long as I’m not breaking any laws or putting myself at risk then I can live with that for a while.
 
I thought so myself but I have nothing to base anything on. It’s as I received them.
Would you suggest I drop it down to around 40 and see what happens?
 
My speedo is also out due to running BFG's, by approx 10%. Yes you are correct, this would affect the mileage displayed too

IMG_5034.jpg
 
Hi I have a problem that I cant read the speedo in sunlight on a Citroen so I purchased a GPS speedo which is very clear and accurate regardless of tyres. I tried it out on our t6 with standard tyres and a Ford C max and found all three vehicles reading a higher speed than the gps speed.
not expensive simply plugs in and sticks onto the van switches to KPH for when in Europe

mickw
 
Having checked the spec on the tyre wall, 52psi was beyond the max inflation for the tyre . Max was 50. I’d been running at 50+ since I had it based on how I received it. Not sure why this was done but perhaps max’s out on the basis of van weight. Which seems naive. Obvious these are cold pressures on my drive.
I’ve dropped the pressure to 43 which seems to tie in with charts I’ve managed to find.
I’ll see how this goes in terms of comfort as the tyre was basically 20% over inflated. Not sure there will be any difference but we’ll see.
After that I’ll look at 275/35 prices etc and see if a swap out will help.
Any other suggestions or comments though are still welcome.
 
Mine overreads by about the same, also on 20's.

It came from the convertor with 50psi in the tyres, and wasn't a great ride. I dropped them down to 40psi all round and it's much better.
 
Thanks. I’m hoping for the same result.
I think they just whack it up to the max because of the weight etc without actually checking.
I’ll see if I can tell any difference with the ride. 20% over is a big margin.
Changing to 275/35 should correct the overreads though may affect ride. It’s only a 5% change but better to be on the nail when looking at the speedo and not over. Luckily the tyres need a look at soon I think.
Happy to live with overreads for now assuming it’s not illegal and that no damage is being done.
 
Alied said:
@Pauly according to @mmi post [Guide] [T6_modified] T6 Speedo Adjustment with VCDS if you have a T6.1 then it is irreversible but on a T6 you change the value without any harm.
Click to expand...
I’d rather not get anything done that involves changing things like that. I’ve been living with KPH on the dash display just because of this. Drives me nuts but there you go.
My background is IT and changing anything in firmware is always a risk. Happy for others to take it and I’m sure things are fine, I don’t judge, but having had the issues I’ve had with my van since I bought it I suspect the thing would explode if I got someone to VCDS it!
 
Check out this guy on YouTube > Urban Ark overland >Big wheels and tyres on your van, side effects.
he goes over this and is a quite interesting watch.
 
I’ve dropped the pressures to 43 and the ride is less harsh.
Speedo still 2mph under against GPS at most speeds, certainly 30/40 at 50+ it’s 2-3 mph so I think it’s roughly 4% under.
I’m concerned because this is basically illegal. You can be over on the speedo against GPS (actual) by up to 10% but not under at all.
So I have a dilemma. I could fit 275/35 and swallow that expense to see if that gets me closer to actual. The circumference is approx 4% smaller so should get me down to match the speedo. That is the most immediate option though £600+
I could swap the wheels and tyres to get a smaller size with more rubber for a more comfortable ride and lower the circumference to better match speedo. But if I do this what wheel and tyre do I go for? No idea what was factory fitted so no idea what van thinks it has fitted.
Third option is VCDS which I have no idea of costs or where in my area (Gloucestershire) I could go to get it done and how effective that will be.
I suppose I could just leave it and risk problems but not sure where it leaves me with insurance? How can a van be sold like this where it’s under in the speedo and that’s not allowed?
Views?
 
Radar1968 said:
Third option is VCDS which I have no idea of costs or where in my area (Gloucestershire) I could go to get it done and how effective that will be.
I suppose I could just leave it and risk problems but not sure where it leaves me with insurance? How can a van be sold like this where it’s under in the speedo and that’s not allowed?
Views?
Click to expand...

Personally, that's the direction I would take, I'm sure there will be someone reasonably close to you that can make the adjustment. For me it was an annoyance, and because i drive several different vehicles I didn't want to get a speeding ticket if i forget my van was going slightly faster than the gauge shows.
 
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