Found out using a GPS speed tracker that I am a little bit off on the speedometer using 195/65-15’s I had previously checked the speedo and it was almost dead on with the old Vredesteins. Now at an indicated forty I am only going about 36-37 M PH. At 80MPH indicated it is only about 76 real MPH. Traffic was really flying in Northern Colorado today around 85 in a posted 75 zone. I did manage to hit 95 for a brief moment, but Angela started squealing about a ticket or some nonsense and was not looking at the GPS tracking gauge. I really wanted to see what an indicated 100 would actually be.
What affect will this have on my odometer. I believe it will not track up the true mileage and come up short?
BTW I was extremely hard pressed trying to get the Yokohama’s to squeal in the slightest on any of our fast hard curves in the mountains of Colorado. Put in almost 100 miles in 90 F plus heat with no hitches, except for that annoying slight gas smell because of the ethanol boiling over on hard curves. A good smell indeed.
The odo and the speedo are two separate mechanisms driven by the same signal (speedo cable from tailshaft). If your speedo and odo were accurate before the tire change, then the error in the odo will be out by the same ratio as your speedo.
The odo is direct driven, it just counts turns of the speedo cable. The speedo is a magnetic drag cup (i.e. indirect) so it may have been reading off with the old tires (the pivot oil oxidises, the magnets move).
If you can find a decent straight stretch of road you can check the odo calibration against the GPS too.
My speedo is around 7mph out , at 78 ish mph the sat nav says I am doing 70 , better then I thought it would be , 65 3.4 engine , 1968 gearbox with overdrive , and a Auto back axle , with 205/70R/15 tyres .
My ford Ranger is 5 mph out , so at 55 mph I’m doing 50 !
What thoughts might y’all have about changing the rear end ratio. I have the 3.77 with overdrive. I find that I am close to redline in first gear at a bit under 30 MPH. Same goes for running out of engine in second gear. Will I experience a huge decrease off the line in power if I go to the 3.54 or should I look into the Tremec 5 speed. Getting better fuel mileage would also be nice as my cat gobbles the gas.
First gear is only really there for pulling stumps and climbing walls. It’s never going to be an Olds 442, but changing either the diff or the tailshaft ratio will make it a bit quieter on the highway. As to gas mileage… It’s never going to be a VW Lupo, but you’ll gain some fuel efficiency at highway speeds.
I am in need of a differential rebuild as it has well over 100K on it without any major work. It is a bit noisy and I thought of going to a 3.54. Someone told me of using a 3.07 in their E type which made for easy cruising.
Any recommendations for a rebuild here in the USA, reply off line if necessary. Good and bad comments welcome
If you do the math you will find that going from a 3.77 to a 3.54 will
increase your speed at any given RPM by about 6.5%. That means that at 100
MPH with your current diff. you would instead be going 106.5 MPH. Your 30
MPH shift point would then be 32 MPH. I doubt if you would even notice, so
it certainly wouldn’t be worth the trouble and expense. If you want to see
a difference, go to the 3.07 rear for a 23% change and bringing your shift
point to 37 MPH. I put a 2.88 rear in my V-12 E-Type and now first gear is
actually usable.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
Way back when my ‘S’ was running I managed to wreck the diff. The car was an auto so had the 3.54 ratio. At the time the only replacement diff gears I could get were from an ‘E’ at 3.07 so I had these ground to fit, this would have been around the time that I changed the trans from the DG box to a 4.2 Man O/d (I had already blown the DG box up twice doing gymkhanas so didn’t bother doing it again) the overall effect of this was to drop the effective ratio to 2.88 From memory it was a bit sluggish off the line but cruised nicely at about 1800rpm for 60mph
Gerard,
A potential source for a rebuild in the US is Quantum Mechanics in Oxford CT. I think they are very competent and fairly priced. You’re right though, diff overhaul is a big job and won’t be cheap.
I had the speedometer rebuilt at the time we did a complete mechanical rebuild. I will be fine with having the accuracy off by a little bit. I just got a quote for a complete rebuild with removal and replacement of the entire axle for about $600 USD.
I fitted a 3.54 limited slip diff to replace the 3.77 in our Mk 2 3.4.
There’s quite a noticeable difference and not sure that I agree with Mike’s figures as I’d say the difference certainly feels greater at least, however in practice I find that: a) you can hang on to 3rd for longer when overtaking and get past quicker, b) as against this you might not have needed to change down at all before and c) the car feels happier with its longer legs - quieter, less engine wear presumably, and modern car gearing.
As to fuel consumption, I’ve not really checked this but doubt if you’d notice much difference. I once got 27.5 mpg (Imperial) on a 300 mile fast run averaging 62 mph, but that was just with me on board and with the old 3.77 axle. Every similar long run since the changeover seems to have been with 2 or 3 up and a ton of luggage, so can’t really compare and the car seems quite sensitive to added weight when it comes to mileage.
But give me an overdrive rather than a 5 speed any day !
