I've never fully understood why makers 'overtyre' cars - but it does happen. A typical example was Vauxhall who supplied V rated tyres on things like 1.8 Cavaliers.
I don't know what the book speed of 1.8 Cav's are, but I've driven some that can top 125mph!!
My old 2.0 Cav (manual) was capable of 135mph. On one occasion it went off the clock (142mph). What let it down was the handling - or rather lack of it!!
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DD
Don't always believe the speedo. Book figures are:
1.8 - 114
2.0 8v - 123
2.0 16v - 135
The leasing company our company used usually insisted that replacement tyres were H rated rather than V rated. I assume they'd checked with the insurers.
Regards
John S
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What are T rated tyres safe for? My 309 1.4 speedo showed 123 on a long downhill stretch on the motorway recently, assuming this was say 115 was this too fast for the tyres?
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T = 118miles/hr
Don't forget this isn't a point at which the tyre will instantly fail, merely a recommended maximum.
Regards
John S
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John S,
Don't always believe the book figures. They are generally quoted minimum speed. All the Vauxhalls I have driven have always gone faster than the book speed.
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DD
Yes the book figures are flat road etc, but often the makers data are not matched by proper road test data. Yes, on a downhill stretch with the wind in the right direction you may be able to better the quoted maximum, but you can bet the speedo is overestimating what you're actually achieving.
One useful check is to look at the gearing in top. If the car is long legged, the maximum may be below revs for max power. On some shorter geared cars, it's at or above max power. In the first case they may find it easier to exceed the theoretical maximum, but in the latter as the power curve is dropping, it's a fair bet they won't, and certainly not on the flat. Most non-SRi vauxhalls are in the former category, SRi in the latter. All my Vauxhalls have been overeading by up to 4 miles an hour at 70 (checked on a measured mile). However, the tacho was apparently more accurate than the speedo as the indicated revs at 70 were equivalent to the actual speed.
Regards
John S
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John S.
Car I was referring to was a 91 Cavalier 2.0 CDi, 8v engine, manual gearbox, 115 bhp.
Difference between the SRi gearbox and the CDi gearbox is that the CDi gearbox 4th gear is the SRi's 5th gear.
Rev counter seemed to match speedo throughout in top gear.
ie, 31mph per 1000rpm.
66, 2000
93, 3000, ect, ect.
When the Cav speed went off the clock (speedo range to 140mph) I was doing 4,500 rpm. Speedo was reading ITRO 142mph. Seemed pretty linear to me.
Yes, I agree, the speedo was probably overeading, but not by much.
Strange thing was that the SRi was supposed to be quicker because it had the close ratio gearbox and 130bhp. However, I could hold the CDi in each gear longer, and the CDi engine had more torque than the SRi. I not only had a higher top speed, the acceleration was greater as well. As I said earlier, the handling was the only disapointment - until I uprated it with gas struts all round. Best Vauxhall I ever owned, wolf in sheeps clothing.
To get back on subject, it was fitted with 91H speed rated tyres. Handbook said that 91V's should have been fitted, but the local dealership and independant tyre gagare both said that the 91H's would be more than adaquate.
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Dave
Yes, the 2.0l 8v Cavaliers were a useful motorway cruiser, with a effectively an overdrive top gear. I had a '92 1.8 for while. Bit slow on acceleration, but a decent cruiser when it got there.
Sad thing is I still have the Cavalier brochure, and that gives the following data:
2.0 8v GL/CD etc 4th 21.2 miles an hour per 1000 revs, 5th 26.6
SRI 4th 16.7 miles an hour per 1000 revs, 5th 21.2
As you say, CD 4th is the SRi 5th, and the fact you could only reach 4,500 revs in top confirms that it is over-geared, whilst the SRi is quite short geared. My experience of 2 litre Vectras is that the SRi was noticeable sharper in 5th than the 'normal' car. On Vectras the non-SRi cars were not so high geared as the non-SRi Cavalier.
That said, your 31 miles/hr per 1000 is a bit of an over estimate. The indicated 4,500 in top multiplied by the brochure figure of 26.6 equates to about 120, which is to all intents and purposes the quoted 123 miles/hr maximum.
Regards
John S
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>> That said, your 31 miles/hr per 1000 is a bit ofan over estimate. The indicated 4,500 in top multiplied by the brochure figure of 26.6 equates to about 120, which is to all intents and purposes the quoted 123 miles/hr maximum.
Not saying you're wrong John. All I know when I was belting down the A34 at 2am at an indicated speed of 142mph, my mate in his 2litre Sierra couldn't keep up. His speedo was reading 120mph.
I guess that's maybe why manufactuers have now steered away from cable driven speedos running from the gearbox, in preference for the electronic ones running from a sensor from one of the wheels. More accurate I imagine.
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Blimey fellas....
Friend's just bought a half tidy 1994 Cavalier 2.0GLS 16V, sounds like that'll be making Mark's Ferrari look a bit slow.
John...no need to go on when you've got higher than the cistern.
;-)
David W
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When my 309 speedo showed 123 I had to laugh HOWEVER I don't think it was far off because the rev counter was slightly redlining (6200 rpm) and on the flat it will indicate 110, which is a few mph more than the book speed (107 i think) but not 10% inaccurate or whatever. Its a 1.4 (75 bhp, ish)
Bearing in mind the engine probably performs better than that of a new, 'test' car, how much will the speedo actually over-read?
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