Final drive ratio and sppedo accuracy

[tjasper]

Hi all,

Got a Griff 500 - early 1994 one with PAS and the T5 'box - and I'd like to know what ratio the final drive is. I need an accurate figure so that I can work out the gearing - the speedo seems to read somewhat optimistically!!

That nice Mr Heath's book gives the gear ratios and mentions the rear diffs, but no sign of actual gearing. Anyone got any ideas?

Anyone else's speedo read about 10% fast?

YellowShed

[GreenV8S]

quote:Hi all,

Got a Griff 500 - early 1994 one with PAS and the T5 'box - and I'd like to know what ratio the final drive is. I need an accurate figure so that I can work out the gearing - the speedo seems to read somewhat optimistically!!

That nice Mr Heath's book gives the gear ratios and mentions the rear diffs, but no sign of actual gearing. Anyone got any ideas?

Anyone else's speedo read about 10% fast?

YellowShed

Your best bet is likely to be to time it over a known distance at constant speed and work out the actual speed from that. The 100m markers along the side of motorways are ideal for this.

[simpo one]

'The 100m markers along the side of motorways are ideal for this.'

Better still, the white squares in the middle of carriageways that Plod uses to calibrate its evil little tax-toys. I don't know exactly how far apart they are but your trip meter will tell you if it's 1/10 of a mile etc.

[shpub]

3.31:1

There's a spreadsheet on my website that does the calcs for you as well!

Steve
www.tvrbooks.co.uk

[trefor]

quote:
'The 100m markers along the side of motorways are ideal for this.'

Better still, the white squares in the middle of carriageways that Plod uses to calibrate its evil little tax-toys. I don't know exactly how far apart they are but your trip meter will tell you if it's 1/10 of a mile etc.

But if the speedo's out the trip will be too!

[simont]

I've been told that for a 500 in standard trim 4000rpm in 4th gear equates to 90 mph.

Mine reads 95-98 mph so thats about 10% out

Simon

[CK Griff 500]

I always deduct 10 miles from what the speedo is showing, not much help I know, perhaps I have been lucky only 3 points in 6 years.

[tjasper]

Steve, all,

I'd guessed it was probably 3.31:1. That means my speedo is really 10% fast. That means 100mph on the speedo is actually only 91mph - boo! Still it's safer ref Plod and scares the passenger more!

I have fitted the 245/45R16 tyres, and now see from shpub's book that the correct tyre was 235/50R16? for a PAS Griff from '94. Even using Steve's spreadsheet, the speedo gearing is 6% optimistic. That's more than I thought TVR would have used. Heigh-ho!

I've used the markers by the roadside on motorways for a long time to check the calibration of my speedo's on various cars. Time taken to pass four posts after the start divided into 900 gives average speed for that 1/4mile (there are 16 to a mile).

YellowShed

[Dave Marett]

Checked mine out with a GPS whilst in Germany,

90 on the speedo showed 91 on the GPS

140 on the speedo showed 139 on the GPS

Is it possible that my speedo is spot on..!?

I know GPS is an average speed over distance but when I was cruising at 80 my mates' speedos were all showing 85+.

Dave
4.3 Griffith

[eharding]

Depends on your GPS - I believe some of them (Garmin?) use doppler-based math rather than purely time over distance to measure speed - and are hence pretty accurate (+/- 1mph-ish)