Recalibrating regearing speedo when smaller tyres are fitted

[Nick Woods]

I'm planning on changing my 15" prisoner wheels wth 195/50 Goodyear Eagle F1's for 13" wheels and 185/60 A021R's.

 

I know that the new tyre diameter will be a bit smaller and that this will cause the speedo to read incorrectly.

 

I found this post here but having spoken to Caterham's parts department they didnt seem to know a great deal about it.

 

Can someone enlighten me as to exactly what parts I might need and where to get them from ?. The gearbox will be out of the car as its being rebuilt following my crash (at long last the insurers are going to fix it rather than write it off although it was a close thing for a few days)

 

The car is a 1.6K supersport with a 5-speed gearbox. i dont know what the diff ratio is but I guess its 3.92:1 as this is the standard ratio on a current Roadsport according to Caterhams website. The dimensions of a 185/60 A021 are 558mm diameter and 1702mm rolling radius.

 

The alternative is to get the speedo recalibrated so its not the end of the world if the gearing cant be changed.

 

Nick

P8MRA - The green one with red wings. Which is now bent ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️

[julians]

Not 100% about this, but I think the speedo can be recalibrated electronically by yourself (if its one of the ones with the digital odometer).

 

I believe caterham use a rebadged CAI (caerbont automtive instruments) unit, if so the speedo can be calibrated by setting a series of dip switches inside the unit.

 

Have a look here.

 

http://www.greengauges.com/help/faq4.asp

 

But then again I could be talking complete bishop as I had the original mechanical(non digital odo) speedo which I have replaced witha CAI one.

 

However I do know that the oil pressure and water temp guages are CAI, so it would make sense for them to use CAI for everything.

 

Edited by - julians on 25 Oct 2002 14:23:26

[Nick Woods]

Unfortunately I have the all-mechanical speedo like your old one so I dont think its adjustable. Thanks anyway though

 

Nick

P8MRA - The green one with red wings.

Which is now bent ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ 🙆🏻 ☹️ but is being fixed

[Sheds Moderator]

If it helps then Construction and Use Regs permit recalibration by sticky labels, Tippex, whatever. I know this isn't very pretty but it might get you out of jail. You can establish the difference in gearing with the old 2 Pi R sums for the two sets of wheels and readjust it accordingly. I understand also that the various boy racer mags do cut out and stick on speedo backs which you can use for recalibration having done your mathematics.

[Jaseb]

Picked up a leaflet at the fest for Speedy cables who offer speedo re-calibrations, their instrument division is in Swansea apparently on 01639 732213, haven't used them and don't know who has but it might be worth a go?!

 

Buckingham Massive doing it in flares...

[Jason Plato]

The alternative is to fit a bike speedo for £15 , with all the extra features that brings and just use the std speedo as a guide for oil changes .

 

Dave

 

[Nigel Riches]

Nick , if your gearbox is a Ford item this following is from the Tony Weale book, hope it may be useful:-Axle ratio = 3.92:1, drive gear (on gearbox output shaft), 7 teeth, driven gear, (to speedo drive) 24 teeth, Ford finis code 6011058, and the colour would be black, under all the oily filth.

The foregoing is for a speedo calibrated at 1000 turns per mile, except the de dion, with 3.62 axle calibrated at 1100 turns per mile, and equates to a speed of 16.5 mph/1000 rpm, in 4th gear, or, 20.2 mph/1000 rpm for the de dion, in 5th gear, it really is a combination of the tyre revolutions per mile to the speedo drive ratio, also a long while back, probably in the archive now, I posted a chart about tyre size, rolling radius and such, but it was back in the dim darks, may be worth a search, speedo cal can be corrected without to much expense or aggro if the maths is carried out before the gearbox disappears down the tunnel. Have fun Nigel.

[jerrypike]

agree with dave, cycle computer will be more accurate and should give you the option of calibrating for both sets of wheels so you can always have an acurate speed indication whichever set of wheels you are using

jerry

[NickD]

Your gearbox probably has the 24-tooth speedo gear. Replacing this with the 25-tooth gear gives a 4% change and is a reasonable match for the 185/60 x 13 tyres. Parts required are the gear p/n 6011057 (£12) and a retainer p/n 1425662 (£3), both available from a Ford dealer.

[V7 SLR]

I have a 6 speed with 3.62. My speedo is MILES out. At 56mph (true reading on rolling road) my speedo reads 70mph.

 

I have the red plastic speedo drive gear. I understand this is the correct one (as recommended) but for some reason the drive gear for larger wheels is more accurate. Can this be true?

 

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