Speedo under-reads

[garcormack]

Dear all

I have a 3.8 which was brought back from the US some years ago. The speedo under-reads by c 20-25%. What could cause this? And what can I do to correct the reading?

Many thanks

Gary

[cactusman]

Could just be a faulty Speedo if the odometer is correct. If the odometer is similarly out chances are the rear diff has been changed from 3.54:1 to 3.07:1 meaning the gearing has changed. As the tachometer is driven from the gear box output shaft the Speedo will under read as shaft speed will be lower per mph. Solution would be a rebuild of the Speedo and a calibration at the same time. Try e.g. speedograph Richfield who should be able to refurbish the instrument.

[garcormack]

Thanks Julian

The odometer under-reads as well so I suspect you are right that the rear diff was changed when the car was brought back from the US. The change you mention would give a 15% error. I think I am seeing more than that - the speedo will not be 100% accurate anyway. Thank you for the suggestion of speedograph Richfield. Luckily not too far from me

Regards

Gary

[christopher storey]

Gary : before sending it off anywhere, first check what the speedo is for. The codes on the front of the speedo such as 6322/05 etc will tell you. They are in the knowledge base somewhere , but if you post on this thread what is on the front of yours, ( and also the turns per mile such as 1312, 1216 or 1120 ) someone will tell you

The other problem which sometimes arises is that someone has used the wrong angle drive at the gearbox end. Don't take this as gospel, but I think that the original used an 14:11 gearing in the drive. If this has been substituted with a 1:1 , you may get the same underread even if the speedo and axle are matched

Edit : here is heuer's original posting on the subject


I found the table below in the Jag-Lovers archive detailing speedometer serial number, turns per mile, axle ratio and tyres. You will see Jaguar produced a wide range of speedos for various applications. The list is missing some TPM figures so if you have one of the listed speedometers and can provide the TPM figure (will be on the right hand side of the needle boss) I will include them.

Series 1 3.8 - All fitted with 6.40x15 RS5 tyres

Number; TPM; Diff; Calibration

SN 6322/00 ; 1216; 3.31; mph
SN 6322/01 ; 760; 3.31; kph
SN 6322/02 ; 760; 3.31; kph (Germany/Italy)
SN 6322/03 ;1312; 3.54; mph
SN 6322/04 ; 820; 3.54; kph
SN 6322/05 ; 820; 3.54; kph (Germany/Italy)
SN 6322/06 ;1086; 2.93 ; mph
SN 6322/07 ; 678; 2.93 ; kph
SN 6322/08 ; 678; 2.93 ; kph (Germany/Italy)
SN 6322/09 ;1120; 3.07; mph
SN 6322/10 ; 700; 3.07; kph
SN 6322/11 ; 700; 3.07; kph (Germany/Italy)

Series 1 4.2 - various tyres as specified

Number; TPM; Diff; Calibration;Tyre

SN 6322/00A; 1216; 3.31; mph; 6.40x15 RS5
SN 6322/02A; 760; 3.31; kph; 6.40x15 RS5
SN 6322/03A; xxxx; 3.31; mph; 6.40x15 SP41
SN 6322/05A; xxxx;3.54; kph; 6.40x15 SP41HR
SN 6322/09A; 1120; 3.07; mph; 6.40x15 RS5
SN 6322/11A; 700; 3.07; kph; 6.40x15 RS5
SN 6322/13; xxxx; 3.31; kph; 6.40x15 SP41
SN 6322/14; xxxx; 3.54; mph; 6.40x15 SP41HR
SN 6322/27; xxxx; 3.31; mph; 6.40x15 SP41HR
SN 6322/28; xxxx; 3.31; kph; 6.40x15 SP41HR
SN 6322/29; xxxx; 3.54; mph; 6.40x15
SN 6322/30; xxxx; 3.54; kph; 6.40x15
SN 6322/31; 1020; 2.88; mph; 6.40x15
SN 6322/32; 638; 2.88; kph; 6.40x15
SN 6322/33; xxxx; 3.07; mph; 6.40x15 SP41
SN 6322/34; xxxx; 3.07; kph; 6.40x15 SP41
SN 6322/35; xxxx; 3.31; mph; 185x15 SP41
SN 6322/36; xxxx; 3.31; kph; 185x15 SP41
SN 6322/37; xxxx; 3.54; mph; 185x15 SP41
SN 6322/38; xxxx; 3.54; kph; 185x15 SP41
SN 6322/39; xxxx; 2.88; mph; 185x15 SP41HR
SN 6322/40; xxxx; 2.88; kph; 185x15 SP41HR
SN 6322/41; xxxx; 3.07; mph; 185x15 SP41
SN 6322/42; xxxx; 3.07; kph; 185x15 SP41

[garcormack]

Christopher

Many thanks - very helpful. I have a speedo marked 6322/03 1312 so meant to work with a 3.54:1 diff. This probably explains the under-read, but I take your point about the angle drive.

Regards

Gary

[abowie]

Christopher

Many thanks - very helpful. I have a speedo marked 6322/03 1312 so meant to work with a 3.54:1 diff. This probably explains the under-read, but I take your point about the angle drive.

Regards

Gary

Richfields will be able to correct this for you.

[Sarthe72]

20 years ago the angle drive on my car broke and I had no idea that there were two types. With a wrong reading we researched and realised we needed a 1:1.25, not a 1:1. Problem solved.