280i FHC Final Drive Ratios

[v8s4me]

My Owner's Handbook states the speed in 4th at 1000rpm to be 22.18mph (actually is says 22.18rpm, but we'll let that go ). Today I was able to take the car for its first proper run and at a steady 3000rpm the indicated speed was 58mph. OK the speedo may be misreading but I'm fairly sure the indicated speed was about right and certainly not in the high 60's.

Any suggestions?

Apart from a couple of niggles, a headlamp fault and the tailgate shifting and chipping some paint on the lip of the opening, there were no significant issues. The headlamp is a bit of a puzzle because I had all the lights working fine yesterday.

Oh and a query over which way the heater lever goes for hot. Mine seems to be opposite to what the colours and legend on the panel says. When I say "hot" I mean "warmish".

[phillpot]

v8s4me said:

Any suggestions?

Got a Sat-Nav you can compare speedo with?



Could always jack the back end up and work out the final drive ratio, measure rolling circumference of tyre and do some sums (probably a chart somewhere to be found with a bit of Googling)

[Wedg1e]

phillpot said:

Got a Sat-Nav you can compare speedo with?



Could always jack the back end up and work out the final drive ratio, measure rolling circumference of tyre and do some sums (probably a chart somewhere to be found with a bit of Googling)

The diff should have a plate held by one of the bolt heads, on which is marked the final drive ratio.
On my car with 225/55 tyres the speedo was within 2mph of my Garmin all the way to 100mph; with the correct 50-profile tyres it over-reads by nearly 5mph.

[v8s4me]

I can't see any plates but checking 10 rotations of the rear wheel to 15 rotations of the prop shaft implies a ratio on 1.5:1. That can't be right, can it?

Sat-nav results:-

60mph = 3100rpm

3000rpm = 57mph

Using this calculator and inputing
3000rpm, 22.75" (jacked up & 57mph gives a ratio of 3.564:1. Which is pretty much what the handbook says (3.54:1).

So my rotations experiment must be wrong. But it does mean that to cruise at a steady 80mph (European auto routes obviously!) the poor old lump will be thrashing along at 4200rpm. Better order in some more sound deadening felt

Edited by v8s4me on Monday 7th November 11:36

Edited by v8s4me on Monday 7th November 22:58

[Wedg1e]

v8s4me said:

I can't see any plates but checking 10 rotations of the rear wheel to 15 rotations of the prop shaft implies a ratio on 1.5:1. That can't be right, can it?

Sat-nav results:-

60mph = 3100rpm

3000rpm = 57mph

Using this calculator and imputing
3000rpm, 22.75" (jacked up & 57mph gives a ratio of 3.564:1. Which is pretty much what the handbook says (3.54:1).

So my rotations experiment must be wrong. But it does mean that to cruise at a steady 80mph (European auto routes obviously!) the poor old lump will be thrashing along at 4200rpm. Better order in some more sound deadening felt

Yeah it is a bit annoying to see 3000rpm at 80 (my car has the 3.31:1 diff) when the company Connect is doing about 2400 at the same speed and Nursy's Skoda is barely ticking over at about 2000rpm/ 80mph!

[Tasmin200]

Mine seems to be really low geared as well but then it started as a two litre so I don't know what final drive it has. It also still has a four cylinder rev counter in it so over reads by 1.5! Makes sums difficult when giving it the berries.
I'll need to get all that stuff sorted out when it goes back on the road.

[RCK974X]

Officially, 4 speed 2.8 had a rear diff ratio of 3.07. 4 speed is 22.2 mph per 1k. The 5 speeder used a lower 3.54, for 24.5mph per 1k in 5th.

If you don't turn both output flanges (tyres) together in sync, you won't get a true reading. 1.5:1 sounds like one half, which I think is what you get with one flange locked.

And of course, someone could have changed the diff - they are all externally identical... if you have a good look there was originally a little metal tag through one of the banjo bolts with the ratio stamped on it. (again, it might not be there anymore...)

Edited by RCK974X on Wednesday 9th November 19:56

[RCK974X]

Heater - I do know from kit car days that the various heater valves come in both 'pull for open (= hot)' and 'pull to close (= cold)' varieties, so it's possible it's been changed for the wrong type....

My heater works well for hot air, but the flow is very poor - the design for the dash vents is awful and allows leaks everywhere...and I did flush out my heater matrix.

Edited by RCK974X on Wednesday 9th November 19:54

[RCK974X]

200 officially had a 3.44 (19.1 mph per 1k)

There is a formula I can give you which estimates tyre turns per mile, which works pretty well, and then you can calculate what speed you should expect with different tyre sizes/specs and diff ratios.