Rover P5 3 litre Ratios

[enigmas]

Could someone knowledgeable with the 3 litre cars list the final drive differential ratios used in the range of models both manual and automatic. I'm also interested in finding out the overdrive ratio (.6, .7, .8 or whatever it may be) for the manual cars fitted with electric overdrive.

[tonys]

MKI & MK11 manual 3.9:1
MKI, MK11 & MK111 manual o/drive 4.3:1
Overdrive ratio on all 0.77:1

DG auto 3.9:1
MKIII & P5B BW auto 3.54:1

I don't profess to be knowledgeable but I can read James Taylor's book

[enigmas]

Thanks Tony, it saves me a bit of research. I was primarily interested in the overdrive ratio of the P5 Rover's manual transmission for calculating a range of final overall ratios.

As an aside, for those interested who may have a manual trans with an overdrive fitted to their car, the final ratio in overdrive ends up 3.9 x .77 = 3.003 or 3:1.

If fitting a MK3 or V8 rear end gear set (3.5:1) to an earlier model 3 litre with a manual overdrive transmission then the final ratio comes out at 3.5 x .77 = 2.69 (virtually 2.7)

That's a tall rear end final ratio for a 3 litre 6 cyl car. Fitted with lower profile radial tyres (less rolling diameter) it could be a very effective highway ratio.

[MK IA Norway Viking]

I have an MK IA manual without overdrive.

At 60 mph it is revving too high for my taste.

I installed oversize tires (crossply 700 x 15) and that relaxed the driving substantially.

It has the advantage that the speedometer is now showing true speed.

The downside is that the car sits high above the ground however the wheel-arches are "properly" filled so optically speaking there are pros and cons to having oversized tires. 

My conclusion is that the benefits outweigh the disadvantage.

That said, I will transfer the oversized wheels to my 3 ltr auto, to have more relaxed cruising, and fit overdrive to the manual, which will have standard 205 x 75 x 15 radial tires.


Tire dimension 185 x 80 (or 85, if available) x 15 would be ideal for a manual shift car that HAVE overdrive installed.

[enigmas]

I've always been interested in gearing for my cars Norway Viking and there are several ways to go about it. Fit taller wheels (they roll further per each full rotation) fit a lower numerical set of rear end gears, fit an overdrive if possible and if an automatic ideally fit a trans with a lockup torque convertor incorporating an overdrive. Some of these options are possible with a standard P5 and the range of available Rover transmissions and some are not.

I'm actually looking for more performance by virtue of less space between the ratios, whilst still maintaining a viable top end ratio on my Mk3 P5 coupe. They are heavy cars and 3 ratios don't provide the best optimal gearing with a tall rear gear set. The rear gears in my P5 are 2.9:1. The BorgWarner rear end gears were custom grafted to the original Rover housing many moons ago when I first built the car as its been powered by a P76 V8 for the last 30 years.

I have a ZF 4 speed automatic aside that will be fitted to the car. The overdrive ratio when in lockup 4th is .73. When coupled with a 3.9 ratio rear gear set the final drive ratio will be 2.84.
The benefit being that the car will accelerate quicker between each ratio and with less effort.

The fully rebuilt P76 engine going into the car is standard apart from a full rebalance and some mild head work. There's nothing radical, the car will just be more throttle responsive due to the improved ratios.

[dmaxwell]

When I finally started working on my P5, the 3L engine was replaced with a Chevy 4.3L V6 with an aftermarket 5 speed transmission. I also replaced the diff with a 3:54 from a Range Rover. After messing with the speedometer gear on the transmission, the speedometer is within 5% along with the odometer. Having the 3:54 diff and the Chevy engine, the car now has enough power to go up hills and gets up to speed faster than before. I did have to modify the front of the driveshaft to fit the American transmission but over the last year, I've taken the car from California to Oregon to Wisconsin and back with no problems, averaging 20MPG. All in all, I'm very happy with the result!

David
California

[enigmas]

Obviously a lot of work David and you have the result you want...so why not post some photos of your car and the work carried out.

[MK IA Norway Viking]

Having picked up my 3.5 and travelled 1250 km on highways, I can see how much of an improvement the ZF 4-speed autobox will deliver. The V8 is not making much noise at 3000 RPM - however the body does, creating a lot of wind-noise inside the car subduing anything else - still the lower RPM that a ZF box deliver will be highly advantageous. 

Naturally, making a conversion will compromise the historical aspect of the P5B. Since my kickdown is not working, it seems that some action is needed in any event.




What would a budget figure be to convert to the ZF box ? Thank you.

[break23m]

I own a 3 Litre MK2 automatic - three gears, but on normal driving the car uses only the top two. Thanks to a lockup in top it gives a nice throttle response and useful engine-braking down hill. But it uses more revs than I like on the highway (100 km/h). Is there a simple way to get a longer final drive? 

[enigmas]

I presume your car runs a DG automatic so is fitted with a 3.9:1 ratio rear gear set. You can swap the centre gear set (complete unit) from a Mk3 to obtain a 3.5:1 final drive ratio. It will then have longer legs for quieter highway cruising but will lose some acceleration due to the taller final gear ratio

[break23m]

Thank you, I will consider this. For faster acceleration I can engage first gear when necessary.

[MK IA Norway Viking]

Good afternoon - I believe your specialist handling the engine could do this rebuild / conversion for you at the same time as he establish cause of losing enigne oil/cure the problem.

Now that the engine is out would be the ideal occasion.

Note that the 3.5 ltr is using the autobox BW 35. Later editions of the 3.5 ltr. are using improved editions of the BW 35 autobox. Perhaps your specialist could supply and fit the 3.5 ltr. autobox to your MK II - or even a ZF gearbox (which has 4 speed an dbetter suited to modern motoring). 

Making these alterations would mean your car is not historically correct any longer, however keeping the original gearbox will enable you to roll back the changes so as to make your car original again.

The other option will be to fit larger tires.

I have fitted crossply to my car (700x15) that are oversize (original dimension on the first edition of P5 in 1959 is 630x15) - result is that the speedometer show true velocity and the engine runs significantly quieter, engine RPM is reduced by approx 10%.

[break23m]

I do like the automatic of the MK2 with it's quirky behaviour, I have learned how to make the most of it. It also has a lockup on third, I want to keep that. My specialist gives me the option to install a second hand differential of a P5b, 3.54 ratio. That will give it longer legs. Not a visible modification and more or less keeping the car historically correct. 4 hours of work. 

I did consider higher tyres, but I could not find Michelin XAS tyres. These are radial tyres with dimension very close to the original spec. I wouldn't dare to ride on crossply tyres in modern traffic. So now the car is standing on Michelin XVS, slightly lower and slightly wider. They are new and a bit expensive, not going to change them now. 

[djm16]

I also have a 3 litre auto with DG box. Mine registers 2,500rpm at 50mph, so around 3,000 rpm at 100kph. I can't say the engine noise bothers me, in fact it is pretty much inaudible.

[break23m]

The car is back from the workshop. The longer final drive doesn't make a big difference in sound, but the speedometer is now correct and the shift to top is at a slightly higher speed. And it's nice to know I can drive it above 100km/h without making to much revs. I'm happy with the small changes it brings.