Hello, does anybody know how to re-calibrate the speedo in a 1991 R2500 Suburban, rear axle has been changed to different ratio. Thanks

You'll need to take it to a transmission specialist. They might be able to correct the ratio by fitting either a different rim size, a different tyre size or both. One full rotation of a tyre gets shorter as the profile, (sidewall height), gets smaller. So the correct 'low profile' tyre could fix the problem for not too much money.

yes
you will have to have the ratio changed by a professional diff shop and then you will have to change the speedo sender to suit the different ratio
That is not too hard if you use the diffs from the same make of vehicle as sedans run the 3.08 and station wagons run the 3.55 or different ratios for petrol and diesel
By staying with the same manufacturer , the speedo ratio s are obtainable as well

Hello, you didn't say what gear ratio you have in your rear end. But you need a speedometer gear with more teeth. you should try the next one up 17 or 18 tooth. Hope this helps.

the Manual transmission is going to have a rear axle assembly gear ratio of 4.30 , and the automatic transmission axle will be a 4.88 ratio.
The transmissions have different ratios between the 1.6ltr , 2.0 ltr , 2.5ltr and the 2.7ltr engines.

While I am uncertain, sadly no diesel here in the USA (danged EPA) I would expectthe diesel rear axel to have a different ratio from the gasoline, as the diesels run slower. You should be able to manually turn the yoke on the rear-end and count the revolutions for the axel to make one full revolution. With this you will be able to determine the rear-eand gear ratios.

Best of luck!
Tom

the build sheet codes are affixed inside the glove box
It will have the letter G with two numbers after it as like G80. Some may have two letters and one number but unlikely so
The G code will determine the ratio of yours without ripping off the rear cover to count the teeth and divide what you have

It is possible, you just have to know what gear ratio you have in your Sierra. I had issues with my son's rear differential and did the Salvage Yard and picked one up for $250.00 with less then 45,000 miles. Remember new cars end up at Salvage Yards also.

Here are the Pro code for the rear ends to your Sierra and you can match one up at a Salvage Yard.
GQ1 -- AXLE REAR, STD RATIO
GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, LIMITED SLIP (POSI TRACTION)

HC4 -- AXLE REAR, 4.56 RATIO

Open Your glove box and look inside it. Should be a list of codes on the bottom.
Example: GU6 G80 According to that, you'll have a 3.42 posi

Good luck and keep me posted and call around Salvage Yards, there prices will differ from one to the next salvage yard and don't ask them for a posi rear end. The cost will you more, just search all the SUV's and look at the pro codes in the glove box. Hunt, most awd system SUV will have the G80 rear end.

Good luck and keep me posted.

I would call the dealer parts dept. they should be able to tell you what you need to know. there may be a difference in gear ratio for each engine/ trans combination. A salvage yard may be able to help you , they should have a computer system showing what rear is compatible. there should be a code on the axle housing itself.

camaro firebird and transam axles are what i would go for. remember lots of people change out the ratio just look for one with posi it will bolt right up if the ratio is wrong it will only affect speedo and fuel economy a little bit. this can also be changed easily with your ring and pinion out of the old one if need be.

check the fuses, mine did the same thing, I replaced the fuse in the fuse panel and it works great now.. easy fix