Determining Differential & Speedometer Ratios

[JamesWilson]

I recently took my Rochdale Olympic for an extended drive and used my SatNav.

I noticed that the speedometer would read ~80 MPH while the SatNav only showed a speed of ~68 MPH. I'm pretty sure the SatNav is correct.

The Rocjdale is a kit car made up of components from varius sources and I really want to reconcile the Speedometer with the true speed.

The car has a 1275 BMC A engine with a 4-speed MG Midget transmission. I believe the differential is a 3.7:1 but cannot be sure of that.

I propose driving it at top gear (a 1:1 ratio through the gearbox) and note the RPM, the Speedometer indicated speed and the true speed from the SatNav.

There's a formula somewhere for determining speed given engine RPM, tire size and gearing, etc. Can anyone direct me to it? I reckon I cna use that to determine what differential is in the car.

But how then do I fix the Speedometer? I don't want to change out any drive gears in the gearbox... Swaping the guage itself would be do-able. But I recal having seen a gizmo that allows a up- or down- gearing that could be fit to the drive cable. Can anyone direct me to those or a vendor?

TIA

James

 

[JPSmit]

you might want to repost this in the Spridget forum. I just did a quick search and came up with two tidbits.

1. apparently the issue is not speedometer but odometer as according to one poster, 60's speedos tend to read at least 5% high.

2. In this post, https://tinyurl.com/5zasr2 Kim just used the GPS and moved the needle till it was about right.

 

[JamesWilson]

Thanks for the link.

 

[PAUL161]

I recently took my Rochdale Olympic for an extended drive and used my SatNav.

I noticed that the speedometer would read ~80 MPH while the SatNav only showed a speed of ~68 MPH. I'm pretty sure the SatNav is correct.

The Rocjdale is a kit car made up of components from varius sources and I really want to reconcile the Speedometer with the true speed.

The car has a 1275 BMC A engine with a 4-speed MG Midget transmission. I believe the differential is a 3.7:1 but cannot be sure of that.

I propose driving it at top gear (a 1:1 ratio through the gearbox) and note the RPM, the Speedometer indicated speed and the true speed from the SatNav.

There's a formula somewhere for determining speed given engine RPM, tire size and gearing, etc. Can anyone direct me to it? I reckon I cna use that to determine what differential is in the car.

But how then do I fix the Speedometer? I don't want to change out any drive gears in the gearbox... Swaping the guage itself would be do-able. But I recal having seen a gizmo that allows a up- or down- gearing that could be fit to the drive cable. Can anyone direct me to those or a vendor?

TIA

James

<span style="font-size: 12pt"><span style="font-weight: bold">James, What's a Rochdale Olympic?</span> </span>

<span style="font-size: 12pt"><span style="font-weight: bold">Photos please! </span></span>

 

[JamesWilson]

Here it is, with added scenery:

on Skye

In the Highlands


and back home, loitering in the driveway

At rest....

 

[ecurie_ecosse]

Didn't know you had one of those,James.If I had,I'd have been begging to borrow it in 2007,when we met up.

Stuart.

 

[mccalebr]

Lovely...any photos of the front?

Ray

 

[JamesWilson]

Didn't know you had one of those,James.If I had,I'd have been begging to borrow it in 2007,when we met up.

Stuart.

I only got it in March. I'd seen it and another about 5 years before; but already had the Big Healey project and wasn't about to take on another car; and it wasn't then for sale.

Since seeing it I then did a little background research and decided I'd kind of like to get one, if it wasn't too bad. About a year ago I joined the Owner's Club and bought all the old club magazines and really found out more about them.

Then in January I thought my other projects ('67 Healey, '72 Jensen-Healey) were coming along and I might look for one very casually, not really expecting to find one any time soon because there aren't too many around.

I visited the fellow who was most active in Scotland and found he was selling his. I took a look and decided it was pretty nice and too good an opportunity to pass, so I bought it. This was a couple years earlier than I thought I would, but what the heck ... I didn't have the money... or any place for it... or any time if it needed any... so I bought it without a second thought.

Basic Data:

It's a KIT car.

Built by: Rochdale Motor Panels, Rochdale, England.

Built from: 1960- 1967, with one made as late as 1972/3

Numbers made: 250 Phase I (like mine) and 150 Phase II- the main difference being the Ph II's had a rear hatch. Although mine is a Ph I someone, sometime added a hatch to it. Ph I had front torsion bar suspension from Wolesley/Riley; Ph II from Spitfire.

Of the 400 built there are estimates that around 100 still survive in some form and that between 20 and 30 are on the road. I believe there are 3 or 4 known to survive in the USA.

Engine: Mine has a 1275 BMC "A" and 4 speed from a '60s Midget; others came with Riley 1.5 or Ford Engines. I'd guess I get about 65 HP from the engine which is stock so far as I know apart from an unleaded head conversion.

The most distinctive feature is the car's Fiberglass Monocoque Chasis/Body; like the Lotus Elite Mk. I. There is no steel frame- the only structural iron is a front cross-frame that the suspension and engine bolt to, and which is glassed-into the bodywork.

The car is extremely light and it's performance was excellent by comparison to contemporary sportscaars like the MGB and TR4.

It's a lot of fun. And I'm getting to do a few "little" jobs on it. (New hoses, tinkering to make it fit me better, etc.)

It's a great little sportscar.

The company just stopped making them- guess it was too much trouble and not profitable enough. Such a sahme really... But they didn't suffer from being produced until obsolete.