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Bakes

Better motorway cruising

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Having just covered 2000 miles in the sunshine of France I have returned with a need to improve the cruising experience.

So the question is how do I alter the gearing to reduce revs in top gear.

I have a late '71 6 with A type overdrive, I know it's possible to put in a foreign box but I like the overdrive and wouldn't like to lose it.

I have considered the pipe and slippers version with a burbling V8 but I'm just not old enough for that yet!

 

Any thoughts??

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Morning Bakes, what is your different ratio....3.45:1 gives good cruising.

Or maybe your overdrive isn't dropping the revs sufficiently.

Cheers.

Conrad.

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Bakes

 

Never had any 'cruising issues' with the standard set up of 3.45 diff and the 22% overdrive... Has anyone fitted the alternatively available (and non-standard) 28% OD?

 

Cheers

Ian

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My last 6 didnt have overdrive and was tiring on a motorway, but cuurent car with overdrive is pulling less than 3000 revs on the motorway.

 

Even with the big exhaust its perfectly ok.

 

What sort of revs are you cruising at ?

 

Steve

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I have gone for 28% overdrive for long tours in Europe. It works very well at higher speed but does bog down a little at lower speeds.This is more notable

when I drive my other factory standard (TR5)22% overdrive and that is sweet in any gear. So horses for courses.

Regards Harry TR5s Nutter

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I think I'm running about 2600-2700 at 70 (I think, speedo packed up before I managed to leave the country even though it had just been refurbed and returned as I wasn't happy with it. This being the case I can't double check right now) but if you're driving for 4-5 hours at 70-80 in 90 degrees of heat then being able to drop the revs to 2200 ish would be lovely.

It has to be said I want my 6 to be more of a grand tourer than the sports car it is.

Does the 28% overdrive make a significant difference? Harry, do you know what revs you're doing at 70?

In answer to Steve's question I'm fairly sure I have the standard 3.45 diff.

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Hi Bakes, your revs seem a little high to me, but not very high.

 

If you are revving at 2600 now for 70mph then a change from 22% to 28% od should reduce that to 2400 by my calculations......

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The simple way is to have a 27% or 28% overdrive fitted. (The normal is 24%) Take box and o/d as a single unit to a specialist like Overdrive Spares in Rugby and they will swap in a gear set for you without breaking the bank.

This is particularly useful for motorway cruising especially if you have a 3.7 diff. (UK spec is 3.45:1, USA was 3.7:1 as I recall)

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Tyre section. If you are on 55 section then changing to 70 will drop rpm by 9% and 2700 falls to 2460rpm

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Peter

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I seem to recall one of the two well known OD specialists telling me that the A-type was available in 33% for some other make. Have a word with them and see what they say. Certainly changing to 28% will give you a useful drop in revs.

 

Cheers, Richard

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Tyre section. If you are on 55 section then changing to 70 will drop rpm by 9% and 2700 falls to 2460rpm

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Peter

 

OP says at 70mph his engine is turning over at 2600-2700 rpm. This is correct. 70mph should be 2600 revs or so, assuming 3.45 diff and standard tyres. His numbers therefore suggest he is not on low profile tyres.

 

I think the tiring thing is the exhaust note. The engine itself should be more or less inaudible at almost any cruising speed, but the exhaust is the variable thing here. Some exhausts give the impression that the engine is working harder than it actually is. I find it difficult to believe that different gearing is needed in order to enjoy cruising at 70-80mph.

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OP says at 70mph his engine is turning over at 2600-2700 rpm. This is correct. 70mph should be 2600 revs or so, assuming 3.45 diff and standard tyres. His numbers therefore suggest he is not on low profile tyres.

 

 

Steve, Not necessarily...he could be on any tyre section. Changing tyre section wont affect the speedo reading: it will still read "70" at 2600rpm on any section tyre. However, if 70mph is independently measured, say by GPS, then the taller tyre will give that true speed at lower rpm.

Peter

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Oh yes, I see what you mean Peter.

I recently changed tyre section and was a bit puzzled...until I realised what was going on. Now I have no idea what speed I'm doing.

Peter

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Tyre section. If you are on 55 section then changing to 70 will drop rpm by 9% and 2700 falls to 2460rpm

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

Peter

I'm on 195 65 15 which as far as I can tell are pretty similar in diameter to the standard british spec tyre even though my alloys are slightly wider than my old wires, I could change for a 70 section tyre on the back although the set I have are only 2500 miles old...... I have changed my wheel barrow type sports exhaust for a less noisy option but it is stainless which is going to be noisier than the humble old standard systems. I guess I could look at some sound proofing. I will investigate having my overdrive converted to a 28% version and change the rear tyres I'm sure they will make a reasonable difference when done together.

But whilst the gearbox is out I really need to change my clutch and I guess I could get the flywheel lightened......

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Hi Bakes

I do many many miles cruising in France and agree 2600 rpm for GPS 70 sounds right.

My CR revs to 6000 if I would let it, I have a Rev limiter set at 5500.

Very tiring 4 hrs at 70/80mph! Hat's off! (literally !)

For me it's keeping HWMBO happy is paramount.

Our last trip to France was spoiled by exhaust fumes feeding back into the passenger side, I couldn't smell it.

When with wife, we have the side windows up and a breeze breaker on the roll over bar.

The boot seal is in good nick.

Phoenix Twin LHS sports exhaust.

I am at a loss what to do to stop the backwash.

Edited by Prefect

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Hi Bakes

I do many many miles cruising in France and agree 2600 rpm for GPS 70 sounds right.

My CR revs to 6000 if I would let it, I have a Rev limiter set at 5500.

Very tiring 4 hrs at 70/80mph! Hat's off! (literally !)

For me it's keeping HWMBO happy is paramount.

Our last trip to France was spoiled by exhaust fumes feeding back into the passenger side, I couldn't smell it.

When with wife, we have the side windows up and a breeze breaker on the roll over bar.

The boot seal is in good nick.

Phoenix Twin LHS sports exhaust.

I am at a loss what to do to stop the backwash.

I suffered a little with this when I had twin pipes out the back E type style, since I changed to a standard system I haven't had the same problem, I also drive with a breeze breaker and windows up on long runs.

Edited by Bakes

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Careful Steve, once you start.....

 

If you mean me, I fear it's too late.....

 

When I went through the heartache of an exhaust system swap (work done by Clive Manvers) I discussed with Clive at great length. I told him I'd got problems with fumes and was tiring of the noise. He said he was quite often doing twin back box back to standard conversions for these reasons. The standard system is never fumey apparently. I picked my 'converted' car up at 3pm on the Friday and by mid morning the following day Clive had died. He and his boys transformed my car and I never told him how pleased I am with it. Mine is stainless but has no 'ring' or metallic noise at all (although I expected some degree of this).

 

To the OP: the 195/65 tyres have a significantly smaller rolling circumference than 165/80 (standard) so tyres might help but I wouldn't just change the rears. Mine had 195/65s for years but I've gone to standard size now (and very pleased with them). I really do think the main cause of your issue is most likely to be the pitch of the exhaust note.

 

Cheers,

 

Steve

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Hi Bakes

I do many many miles cruising in France and agree 2600 rpm for GPS 70 sounds right.

My CR revs to 6000 if I would let it, I have a Rev limiter set at 5500.

Very tiring 4 hrs at 70/80mph! Hat's off! (literally !)

For me it's keeping HWMBO happy is paramount.

Our last trip to France was spoiled by exhaust fumes feeding back into the passenger side, I couldn't smell it.

When with wife, we have the side windows up and a breeze breaker on the roll over bar.

The boot seal is in good nick.

Phoenix Twin LHS sports exhaust.

I am at a loss what to do to stop the backwash.

http://www.tr-register.co.uk/forums/index.php?/topic/52233-tr6-exhaust-fumes-airflows-over-bodywork/

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Surely taller tyres are cheaper than new OD or Diff.

Just a bit worried about lack of grip!

Use GPS for a speedo until recalibration.

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"Just a bit worried about lack of grip! "

 

Then drive differently, you'll enjoy having the increase in handling quotient anyway.

 

Mick Richards

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My experience has always been that ear plugs make the whole long distance travelling experience much less tiresome.

Tim

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Was out yesterday, and at 70mph I was just under 2500 revs. Firestone tyres on standard size 165's.

 

Dave

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