Speed and revs, 60mph = 4100 revs!

[david.coan]

Recently i got my 1968 US series 1.5 import back on the road here in the UK after conversion to SU and RHD. The issue i have is the very high gearing, i took it on a first long motorway drive at the weekend and i could not believe how bad the gearing was. The handbook table says for a 3.54 US diff that 60 mph is around 2789 revs in 4th. But with a Satnav/gps saying i am doing 60 mph in 4th the revs were at 4100. 70mph was about my top speed without worrying and suffering the engine noise, 80 mph would be way beyond my reach. I have 205/70 tyres which i think could have a small impact, maybe 3%, but what else could be going on? I assumed and did think the 4 speed gearbox and rear diff are original, ideas welcome, i can only think the diff ratio is not 3.54 but even higher?
I worked out my mpg to do these 110 miles to be about 15mpg which i assumed was down to having such very high revs over most of the journey. My long term aim had been to switch to a 3.07 diff, but now i am thinking i need this more urgently.

[mgcjag]

Hi David...first thing is to work out exactly what diff you have....easy....if its a 3.54 then turn the i/p 3.54 turns and the rear wheel should turn once......next you need to ensure the rev counter is reading correct.....also check what gearbox you actually have.....Steve

[Tom W]

If it’s a 4 speed gearbox, then 4th will be 1:1. Tyres won’t make a difference to the speed the speedo reads vs any given RPM, the change is downstream of the speedo drive. They will, however make the speedo ever so slightly inaccurate. There’s also 2 ratios of speedo drive, but if this were wrong then the speedo would be more inaccurate.

If someone has swapped the diff for something even lower than 3.54, then your speedo would be wrong compared to the GPS, unless the speedo was also swapped. E-type speedos read to 160mph, saloon speedos have a lower max speed. What does your speedo read to?

My car used to have a 3.54 diff. It did feel very low geared on the motorway, I was always fishing for a 5th gear that wasn’t there. It would do above 70mph though.

Your fuel economy isn’t great, but that could be down to other factors besides just gearing. The setup of the carbs, ignition, and general condition of the engine, as well as driving style will have a bigger influence than gearing alone. My FHC with triple Webers and a 2.88 diff managed about 19mpg on a run.

[christopher storey]

The most likely culprit is the tachometer which I assume , this being a 1968 car, is probably an ignition impulse type ( it will have RVC or RVI on its front face ) and not an earlier one driven by a generator on the back of the inlet camshaft . The impulse tachos are notorious for becoming grossly inaccurate as the electronic components age - particularly the capacitors - but the first thing to look at is the potentiometer - usually accessible through a hole with a rubber bung in the back of the case of the instrument, and twiddling that to see if it makes a difference . The whole job is made easier if first you can borrow an independent tacho source as a basis of comparison - many multimeters have this function

[david.coan]

Thanks for the suggestions, i did the input turns measuring suggestion and found one wheel turn equals something like 4 turns of the prop shaft, hard to be precise but maybe slightly more than 4 turns. So I think my culprit is the wrong diff, I did not know there was a diff lower geared than 3.54.
Speedo is original according to my e type originality book with it’s serial number id suggesting it is set for a 3.54 diff, but I did not trust the speedo anyway hence I used the gps as my speed measure. I believe the Rev counter is there about for accuracy as I can hear the engine at 70mph measured by the gps is really noisy due to the sustained high revs.
I think changing the ratio in the diff is way beyond me and my facilities in my garage, so I need an idea about best parts source and likely labour times if anyone has an idea it would be appreciated

[Mich7920]

Hi david,

There is one adress used my many forumer :
AJS / Alan Slawson
ajseng@hotmail.co.uk
Never heared something bad about his work !

Mich

[Herzeg]

David

As per other posters, I had a 3.54 diff and could easily get 100+ plus out of the car although very highly geared. Now I have a 3.07 and it's much better so would recommend it, I think mine was done by AJS.

Why don't you get someone in a modern car to drive at 70 in front of you and see what your instruments say?

John

[Heuer]

You can get accurate figures by using this on line calculator: https://wahiduddin.net/calc/calc_speed_rpm.htm

Input you gear and diff ratios along with tyre size and it will immediately tell you the expected revs for any speed.

[mgcjag]

Hi David...you,ve had recommendations for AJS..as far as im aware you have to deliver the diff to Alan.... but i assume that you looking for someone to do the whole job...The diff is part of the IRS so you need to consider what work you actually want done as an IRS can snow ball...check everything out...drive the car for a while then think about a diff change and/or IRS refurb.......Steve

[Tom W]

You have a couple of options with the diff. You can either swap the whole diff unit for one with a different ratio, or you can have yours rebuilt with a different crown wheel and pinion.

If your going for a complete swap, then you can either risk fitting the unit untouched, and hope it’s ok, or have it rebuilt before it’s fitted. If you’re not going to do the fitting yourself, then it’s probably wise to have the diff rebuilt, in case it turns out to be a dud. That way you don’t incur labour charges for removal and fitting twice.

In which case, you might as well consider having the diff you’ve got rebuilt. However, there are different variants of crownwheel carrier, to cover the different ratios. Not every ratio can be fitted to every carrier. So, you’ll need to know what ratio you have, before you know what it can be changed to.

Also, as it sounds like your diff isn’t original to the car, you’ll need to find out roughly what age/spec it is to find out how much can be rebuilt. Not all the bits seem to be available for every age/spec of diff. A specialist will be able to help here.

As for cost, the biggest variable will be how much you need/choose to do on the rear suspension. To swap the diff the irs needs to come out of the car.

[david.coan]

It is the whole job i will want doing. It is a good point that until the diff is out it wont known for sure that it can be rebuilt. Are second hand 3.07 complete diffs around or are they hens teeth now?
I am sure the temptation will be a bit of a refurb of the irs once its out too, so i am sure the bill will rise

[malcolm]

Everyone on here will tell you correctly that I am not a technical expert - however, your revs figure for 60mph sounds crazy to me. It's hard to imagine a diff of that nature being fitted. It would mean that hitting the red line would only give you about 75mph! Something has to be wrong. Are you sure it's not something silly like your clutch slipping? I would say tacho error, but from what you said about the noise, it sounds like you are revving high.

[mgcjag]

Hi David.....I think it would not be a bad idea befor you change the diff to have a ride in a car with a 3.07 diff..just as a passenger.....at 70mph in 4th they are not quiet cars and you can hear the car revs....unlike a modern vehicle..Im sure there,s someone local to you on here if you ask.....Steve

[malcolm]

Good point Steve. I'm not far away in Fleet, 17 miles from Reading station - google says 35 mins at the moment?
Happy to take you out in my 3.07 diff David, and also listen to yours.

[Heuer]

It might be useful to know if the speedo matches your diff. The turns per mile are printed on the face and are as follows:

3.54 - 1312
3.31 - 1216
3.07 - 1125
2.88 - 1058

[david.coan]

Speedo has 1216 n the face, which as you suggest says matched to a 3.31, but i believe all USA 1.5 cars would have original had 3.54, so maybe the speedo is not original, but in any event i used the gps to measure my speed which i regard as reasonably accurate. I went along the M4 in a 50 mile limit stretch near Reading with all the traffic matching around 50 mph through the average speed cameras, as was i, and with my gps saying too that i was doing 50 mph and the revs reading around 3500 and sounded like it too.
I dont get any sense of a slipping clutch, i accidentally got wheel spin changing up to second when i over revved on the change. But the killer fact is one turn of the rear wheel saw the prop shaft go around about 4 times, maybe a tad more. I read Jag did do a 4.09 diff but not for these e types, maybe i have that? I would say the feeling i have is 75mph would be my max, 80mph would be too much for the engine.
A ride along in a 3.07 sounds a great idea

[alfred]

Hey David, most if not all mechanic shops can connect a test lead to their diagnostics and give you the real revs. My tach was reading 1,000 high at 3,000 rpm and I thought it was reving high by the noise. Once you know the real revs you can decide what to do next.

Alfred

[malcolm]

I've replied to your PM David, in case you missed it (or in case I replied wrongly! )

[ChrisC]

I have a 3.54 in my FHC but coupled it with a 5 speed gearbox.

when sitting at 60mph my electronic rev counter says 2000rpm... pretty much the same as standard 4 speed gearbox coupled with a 3.07. Mind you 1st gear is near useless and I am htinking of refitting the original diff to give me a long distance cruiser.

[Philk]

I have a 3.54 in my FHC but coupled it with a 5 speed gearbox.

when sitting at 60mph my electronic rev counter says 2000rpm... pretty much the same as standard 4 speed gearbox coupled with a 3.07. Mind you 1st gear is near useless and I am htinking of refitting the original diff to give me a long distance cruiser.

Hi Chris

That sounds like a good idea. You'll then definitely benefit from lower noise/revs at higher speeds. We have the 3.07 with the standard Moss 4 speed box and that is pulling around 2,750 at 70mph. For yourself in fifth gear with a 3.07 you should be comfortably below that. First gear will also then be better for traffic jams!

Regards

Phil