Speedo reading too high...
Discussion
With my 17 inch rears sporting 255/40/17 inch tyres The Speedo over reads approximately 5% from about 50 miles an hour upwards. Fine for getting through speed cameras etc.
My 18 inch rear track wheels with 265/35/18 inch allow the Speedo to read spot-on.
10% is a bit much and if necessary I would get it re-calibrated if nothing else is amiss.
My 18 inch rear track wheels with 265/35/18 inch allow the Speedo to read spot-on.
10% is a bit much and if necessary I would get it re-calibrated if nothing else is amiss.
cp81 said:
Original wheels and standard tyres.... car has had nut and bolt restoration....
When you say original tyres, as a pre facelift car, you probably should have 225/55r16 on the rear, rather than the later 225/50 spec or possibly 245/45r16 . Which do you have fitted and what did the original handbook say? If the tyres have changed that would explain some of the over reading.Edited by SMB on Tuesday 27th August 16:59
The 50 profile being a percentage of the width of the tyre suggests the 235 tyre would have a slightly larger circumstance than the 225 so would show itself as higher gearing meaning your speedo would under read.
70 mph on the speedo would be say 75 actual mph on the road if my maths is correct.
Bigger tyre.
I’ve gone from
225/40/ 17 which keeps the tyre size close to original and keeps gearing roughly the same
235/45/17 which using Rainsport are quite a bit larger circumference and gearing is slightly higher, probably only a few hundred revs but enough for me to have noticed, feels slightly longer legged.
Basically slightly lower revs at 70 mph, my fuel economy is impressive too
70 mph on the speedo would be say 75 actual mph on the road if my maths is correct.
Bigger tyre.
I’ve gone from
225/40/ 17 which keeps the tyre size close to original and keeps gearing roughly the same
235/45/17 which using Rainsport are quite a bit larger circumference and gearing is slightly higher, probably only a few hundred revs but enough for me to have noticed, feels slightly longer legged.
Basically slightly lower revs at 70 mph, my fuel economy is impressive too
For the gkn series one cars I expect what calibration Tvr did was done using the more common 225/55 16 size. And not changed when they produced the 500 version until the btr diff came along as the difference is minimal, but you have gone to th later size now that’s even smaller.
If that is so the difference between the calibrated tyre and your fitment is circa 3% on top of what was probably already a likely upto 10% in the Tvr figures ( as legally allowed)
I don’t think changing the sensor gap would fix the issue s it’s obviously counting all the pulses, if it’s more than the calculated 13% error then you probably need the speedo head calibrated. Could your car have had a replacement speedo as well, ? It was fairly common for them to fail in cars from that era ( my 95 car had a replacement). The replacement ,ent may never have been calibrated to the car.
If that is so the difference between the calibrated tyre and your fitment is circa 3% on top of what was probably already a likely upto 10% in the Tvr figures ( as legally allowed)
I don’t think changing the sensor gap would fix the issue s it’s obviously counting all the pulses, if it’s more than the calculated 13% error then you probably need the speedo head calibrated. Could your car have had a replacement speedo as well, ? It was fairly common for them to fail in cars from that era ( my 95 car had a replacement). The replacement ,ent may never have been calibrated to the car.
I have the same problem. So just a random thought, what would happen if you removed one of the fingers on the prop-shaft ring. ie it was counting less pulses per rotation. I assume it would read a slower speed. But by how much ?
I wonder also if rather than removing a finger (drastic) if you could blank one of the gaps off in the fingers using a piece of metal. This would also reduce the pulse per rotation.
I may try this.
I wonder also if rather than removing a finger (drastic) if you could blank one of the gaps off in the fingers using a piece of metal. This would also reduce the pulse per rotation.
I may try this.
My TVR is spot on.
My Saab is within 1mph at all speeds.
My bloody Nissan reads 10% fast and is driving me mad, constantly having to multiply by 90% to work out what speed I am doing.
Is it easy and affordable to get any speedo recalibrated, or would it be simpler to buy 10% bigger tyres when these wear out?
My Saab is within 1mph at all speeds.
My bloody Nissan reads 10% fast and is driving me mad, constantly having to multiply by 90% to work out what speed I am doing.
Is it easy and affordable to get any speedo recalibrated, or would it be simpler to buy 10% bigger tyres when these wear out?
I use the Sat Nav MPH most of the time anyway plus it warns of the dreaded little boxes 
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