Axle Ratio's

[slider]

i was under my 7 V8 and i looked at the markings on the rear diff and they read, RKX 0100(if i remember rightly), does any body know what ratio this is i.e 3:08.1 or higher/lower any help would be greatly appreciated[:p]. my speedo reads 62mph at a sat nav reading of 70mph!and revs are 3'800rpm which seems a bit high to me or have i just got used to modern cars till i got my 7 V8 last year[xx(]at 100mph(on a circuit of course!)on the speedo the GPS sat nav says i am doing 114mph. the tyre size is 195x60x14. HELP!![?]

[FI Spyder]

Seems awefully high. With 3.90 diff I'm doing 60mph at 3,000 rpm. Haven't checked with GPS but speedo is pretty much dead on acording to radar display boards. At 70 mph I think I'm running about 3300 to 3400 rpm. Don't often do that as that's well over speed limit on most of the roads I drive. Not sure how much your tires will make off hand but your rpm is way high. Definately not a 3.08.

The diff's are stamped with two letters indicating ratio. The stampings are fine and not deep so are hard to see. (Assuming the crown and pinion are not changed.)


TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra

[Beans]

Cruising @ 75 Mph with a 3.45:1 diff and original size tyres ...



Number is stamped to the underside of the axle casing;
3.9:1 CH / Red
3.45:1 CL / Yellow
3.08:1 CM / Green-white
Colours refer to a dab of paint that was put on in the factory for (quick?) identification.

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[RJS]

I went through this with my car, and in the end I have a speedo that is very close.

First you need to figured out how far off the speedo is by a % using a GPS. Once you know that, change out your speedo drive gear with that same % change in the number of teeth.

Now you need to know what speedo drive gear you have. As I remember it takes about 5-10 minutes to pull it out and see what you have. They have a color code, but you can also just count the teeth.

http://www.victoriabritish.com/icatalog/tr/full.aspx?Page=72

Rob

[jeffremj]

You probably have 2 errors, one is the calibration of the tach - check it against a strobe or similar. If tach 'fairly' close, then you would seem to have a 3.9:1 - 21mph/1000rpm in 5th gear (3800x21/1000=79.8mph although not 70mph!). Note, 195/60x14 is roughly similar to 185/70x13 (standard) and 205/50x15 wrt rolling radius, so this has a limited affect on the maths. Regarding speedo, you need to 'speed up' the feed to it, so choose a drive gear with 2 less teeth - each tooth gives around a 5% difference.

[slider]

thanks folks, guess i will just have to get down and dirty soon to sort it as it is doing my head in [:(]

[busheytrader]

Hi Slider,

I have a sneaking suspicion that you may have a std 3.9:1 rear axle and a red speedo gear for a lower ratio axle. IIRC from Grinnal's brochure, 3,750 rpm with a 3:08.1 rear axle should equate to ~100mph. With a red speedo gear fitted to a std 3.9 rear axle a std speedo would overead by approx 10%.

When I first converted my 7 to a V8 I ran it for a while with the std 3.9:1 rear axle. 1st gear became useless and the remaining gears were far too close together. 5th gear felt like 3rd in the pre-modified car but at least the speedo read correctly.

When I changed the rear axle to TR8 3.08:1 the speedo under read by ~21%. As a very rough guide an indicated 30mph = 40. 60mph = 80

The black (23 tooth)gear at the gearbox end of the speedo cable needed changing. The nearest generally available gear was the red (22 tooth) one which was fitted but the speedo still under read by ~10%. I didn't find out until I stormed past most cars on the M4 at an indicated 70mph. It was closer to 80mph. In the end the guys on the forum here advised that the hard to find orange (20 tooth) gear was the necessary one for a correct speedo reading. What's more Howard on the forum had one for sale and was relatively local - Top Man. (If I'd had the original TR8 gearbox and axle the different gearbox ouput worm would have suited the red gear)

So the orange gear works for my speedo but I'm unaware if there's anymore of them out there. The best option may be a recalibration of the speedo as recommended by Robsport etc.

Page 40 of the Roger Williams How to Improve your TR7/8 shows the various gears for different combinations.

Personally, I'd like to try out a 2.84:1 rear axle on my V8 as I think it could do with a taller ratio and has more than enough torque to cope in the lower gears.



Adam










TR7 V8 DHC Jaguar Solent Blue. 9.35cr Range Rover V8, Holley 390cfm, JWR Dual Port, 214 Cam, Lumention, Tubular Manifolds, S/S Single Pipe Exh, 3.08 Rear, 200lb Spax & PolyBushes all round, Anti- Dive, Strut-Top Roller Bearings, Capri Vented Discs & Calipers, Braided Hoses, 4 Speed Rear Cylinders, Uprated Master Cylinder & Servo, AT 14" 5 Spokes or Maestro Turbo 15" Alloys, Cruise Lights, S/S Heater Pipes, Replacement Fuel Tank. No Door Stickers. Mine since July 1986, V8 from 1991