Theoretically yes, but you have to take into account tyre growth.

Do you mean tyre wear ? Going from 7-8mm of tread when new down to totally bald should only affect the speedo reading by 2-3%.

Or are you talking about the tyres getting bigger at speed compared to rest ? Any idea how much bigger they get ?

all speedo's are made to under read because they would rather u thought u were doing 60 ur only doing 56 rather than tought u were doing 60 but doing 65 and then u get caught and claim tht u were not breaking the limit according to ur speedo!!

Glenn,

Good point - Isn't the UK spec WRX (and presumably the later classic Turbo) geared to almost 25 mph per 1,000 rpm in fifth?

If so, then that means that, for example, 4,000 rpm equates to 100 mph. So, why does my GPS insist that 4,000 rpm in top is only 91 mph on my private track? ;) That means gearing of only 22.75 mph per 1,000 rpm.

So, is it my GPS that's pessimistic (or optimistic, depending on which side of the magistrates you're sat on....)

According to my Blue-i my speedo is accurate to within 1 MPH.This is the same at all speeds up to 130, over that I don't want to look too long !!

Tyre size 225 x 40 x 18 on MY01

Gaz

Dont worry peaps it's as mentioned once or twice totally normal :rolleyes: & trying to calculate by revs etc is totally futile when u have a GPS system that tells u accurately how fast it is moving (again as mentioned presuming we're talking consistent speeds not just after accelerating etc) & not the sum of the square of the RPM multiplide by the aperature of a flake of rubber 10 microns from my gob shyte ;) lol
Just seems a bit OTT it over reads for good legal reasons & as long as u no exactly wot speed u can "get away with" ;) it's all good init :D
Si
PS u can always pull needle off & stick it a bit further round but i woodnt recommend it i broke 2 rev counters doing white dial conversion & probably added a few % on the speedo but better safe than sorry (pre roadangel days;))

According to my snooper, my scoob's speedo seems to be about 10% out. When i had my astra, my handheld gps showed the speedo to be out by a consistent 4mph at 40mph and higher.

John.

Harry_Boy,

The only reason I can think of for the discrepancy in the speed calculated from your engine revs and that from the GPS is that the diameter of wheel plus tyre is less than it should be (assuming of course that the GPS *is* the more accurate of the two).

The diameter should be (W + T) where W comes from the wheel and T from the tyre. Assuming standard tyres of size 205x50R16 then W is just the wheel size (i.e. 16") and T is twice the tyre sidewall height. So T = 2 * (50 / 100) * 205. Giving T = 205mm.

So the total width of wheel plus tyre should be 61.14cm. Time to get your tape measure out and check what it really is. I haven't got one here with me at work but I can believe that it might be a centimeter or two out. Still can't see how it could be 10% out though.

My speedo is definitely under reading, I reckon about 7mph at 70mph indicated so 10% is about right, although I would have liked it to be more accurate than this. I don't think my previous wrx was this far out though.

Fitting bigger wheels (as long as it results in an overall wheel + tyre diameter increase ) should improve accuracy as the speedo works off how fast the drivetrain is turning not how much ground you are covering. Larger diameter means more ground covered so you're going faster.

Therefore, solution to an under registering speedo is fit bigger wheels !!

Cheers Ken.

So, with an over-reading speedo, I should really be fitting castors to my WRX :) :) :)

Or Skis.... :)

Here's another one...
I moved up to 17" wheels on my Forester and put on 215/50 x 17. This is about 7% bigger rolling circumference than std.
Now with Tomtom I am getting 130 mph on the speedo but only 111 mph on the GPS.
If I already have 10% overread on the speedo then I could be 17% out!!
Wondered why my trip meter showed 136 miles when my route planner shows it should be 123 miles.
Will have to find if there is a gearbox speed sensor to reduce the overread.

Nick
1997 Forester S/tb

Interesting 'characteristic' Nick. Hope it doesn't encourage you to speed too much! :)

Thanks Glenn - it's just the inconsistency between the gearing, the speedo and the GPS that seems so peculiar..... :(

[Edited by Harry_Boy - 6/11/2003 9:54:43 AM]